JOSEPH BLACK'S INAUGURAL DISSERTATION. I1 Communicated by LEONARD DOBBIN* Gavelton, Fdadam, Blackshiels, Scotland
THE FIRST portion of Black's dissertation, presenting his general remarks on "The Acid Humour Arising F r m Food," was printed in the May issue of THISJOURNAL. The remainder of the dissertation, here subjoined, consists mainly of a detailed account of the experiments that Black curried out, prior to June, 1754, in his examinationof Magnesia Alba.
+ + + + + + ON MAGNESIA ALBA (CRUM BROWN'S TRANSLATION) Magnesia united to an acid. It is therefore not surprising that i t should be liberated from the acid on the addition of an alkaline liquor, hut both Glauber's salt and the muriatic salt give Magnesia in the same way, and yet they are said to consist of alkali united with acid. Whence then the Magnesia? It is indeed most true that they consist entirely of alkali and acid when they are pure, and that they then give no Magnesia on the addition of an alkaline liquor: but they are very rarely to be found pure in the market, and for this reason; for they are both prepared from sea-water, merely by the evaporation of the water by means of heat. The first salt to be separated in crystals is the muriatic: after no more of tfis can he obtained, there is found remaining a saline, dense, bitter liquor, to which they give the name of Bittern; out of this, Glauber's salt and the bitter cathartic salt are afterwards extracted. And here i t happens, that, as in all separation of salts by crystallization, we usually h d that the crystals are, none of them, pure, but have other salts mixed with them; because indeed the water which is quite necessary for the formation of saline crystals, is itself, in the case we are considering, impure and contains other salts. In this medley of salts, common or muriatic far exceeds the others in quantity: i t is besides more easily made to crystallize, and that a t a temperature which does not affect the others. Hence i t comes out purer than they and gives only a little Magnesia. Glauber's salt differs but little from the cathartic salt in readiness to crystallize: i t will therefore he the most impure, or have a great quantity of the latter mixed with it; and the salt-makers indeed take no pains to make it purer, provided that as much as possible i t exhibits the appearance of Glauber's salt, which the bitter salt itself, * Formerly Reader in Chemistry, University of Edinburgh: by very slow crystallization, assumes well enough. Secretary of the Alembic Club, Chemistry Department, King's It is obvious then that the Magnesia in all these Buildings, Edinburgh. salts is the same, and that i t does not matter much t Horr. Op. T. iv. p. 479. from which of them i t is obtained, if we regard its t Hoar. Op. T. iv. p. 500. properties only. But as the bitter salt is cheaper than ** See below. Exp. i.
THE MEDICAL Profession owes its knowledge of Magnesia to the celebrated Hoffmann. He is almost the only writer who deals with the subject, and he indeed fist gave the powder, with this name, a place before the public,t and, with the greatest accuracy, pointed out its powers: also the mode of preparing it, namely from the mother of nitre, or that saline liquid which remains of the nitrous solution after no more nitre can be obtained from i t by crystallizatibn. This powder, i t seems, had, already for some time, been prepared by certain private persons, and, under the name of "The Count of Palma's powder," acquired great fame throughout Italy, inasmuch as it, being destitute of taste and odour, could move the bowels without any inconvenience: but the method of preparing i t had been carefully concealed. Afterwards the great man, when he saw a saline liquor, similar to that nitrous one, prepared in a similar way from waters which yield common salt, suspected that this also would give Magnesia, nor did the result deceive his %ope; in this much easier way he made Magnesia in all reswcts most similar to that prepared as formerly.$ I have never happened to see the mother of nitre, or the Magnesia prepared from it, and have therefore been able to examine that alone which is obtained from the saline liquor remaining after the crystallization of common salt, from the bitter cathartic salt, from the more generally known Glauber's salt, and lastly from common salt. These all, on the addition of alkaline lye, give Magnesia, which is exactly the same from whichever of them i t may come; but seems to be different from both of the kinds described by Hoffmann.** The bitter cathartic salt, when subjected to experimental examination, is found to consist entirely of
Glauber's salt, and gives a much greater quantity of Magnesia than common salt, in preparing Magnesia it is to be preferred t o these two. The saline liquor itself, from which these salts are obtained, is quite suitable for this purpose; but as i t is not always at hand, Magnesia may be best prepared as follows. Take of the bitter cathartic salt and of potashes equal parts: let them be dissolved separately in a sufficient quantity of water, and filtered through bibulous paper; let them be mixed with very thorough agitation; then let them boil over the fire for a little: now let there be added twice the volume, or even more, of pure boiling water; when well mixed let them repose until the Magnesia has settled a t the bottom; let the superfluous water be poured off and new hot added; let i t again be poured off, and repeat this, with cold water, ten or twelve times, or even oftener, if you wish the very purest Magnesia, and therefore that most fit for chemical experiments; lastly dry it. In this way I obtained one pound and ten ounces of Magnesia from four pounds of the bitter cathartic salt. In this process the alkali draws the acid to itself with such force that i t compels it to abandon the Magnesia. This then, as i t is by itself insoluble in water, is necessarily precipitated. But a t the same time there is formed a new middle salt, from the alkali namely and the acid, having all the properties of vitriolated tartar: this is soluble with difficulty in water, and requires no small quantity of it in order that i t may be dissolved. A large part of i t therefore goes down with the Magnesia, owing t o the lack of the water required for its solution. Hence repeated washings first with hot and then with cold water are required for complete purification of the Magnesia; for the first dissolves the salt, the second washes it out. Indeed in the preparation of Magnesia as i t is usually sold, this point is not sufficiently attended to; for i t often contains some of the salt, and sometimes even has an alkaline taste. This powder prepared in the w+y just described, is quite free from taste: it quickly draws to itself any acid, changes it, and therefore also very promptly absorbs the acid humour in the stomach. But its great merit above the other absorbent powders is, that after the change of the acid, i t mildly loosens the bowels; whence, although it has no taste, yet it plays the part of an otherwise unpleasant purgative, and so carries off the impurities arising from the fermentation of food, which quickly turn new food into acid: Hoffmann rightly ascribes this its power to the middle salt formed from Magnesia united to the acid, and indeed experiment confirms this, for that I might be more certain about it, I saturated Magnesia with distilled vinegara product namely of fermentation, in all respects very similar to the acid derived from food: I dried up the salt thus made, and gave six drachms of i t dissolved in water, to be taken gradually, to an adult man, who, after drinking the third part of it, had four evacuations without the least inconvenience. I gave the rest to an active woman of rather full habit of body, and i t purged her ten times, so that this salt, tho&b mild
to the taste, seems yet to surpass other purgatives. It is therefore evident that Magnesia is a most serviceable remedy against acid humour in the stomach, and specially when children are tormented by this disease; for they often refuse almost every purgative, as being unpleasant: and yet Hoffmanu suspected that it was sometimes hurtful: "Nor yet" he says "can we avoid mentioning that we have sometimes observed an inconvenience from this powder of Magnesia, that i t leaves flatulencies and bitings in the lower part of the belly; if it is more frequently brought into use, and that the prima regio is exposed to corrosive juices produced, as is usual in hypochondriacs." All this is indeed in accordance with reason; and yet does not seem to make anything against Magnesia. For Magnesia only changes the acid in the stomach, it does not remove the cause of the disease, which is due to such a weakness of the digestive powers, that some kinds of food cannot be digested. The cure is therefore chiefly to be brought about by a suitable choice of food, and a t the same time by strengthening the organs that produce chyle: if these things are not attended to, the disease will increase day by day; Magnesia never completes the cure; on the contrary, by frequent purging, and so in a measure weakening the patient, i t makes the cause of the disease more unyielding. But it is doubtful if this is the complete explanation of Hoffmann's observation. Perhaps something is to be attributed to a certain specific weakening property, which our most sagacious Professor of Botany has observed in some middle salts, and which may also belong to that salt produced in the stomach from Magnesia. If that be so, there is need of caution in its administration in some cases; and it would not be out of place to mix 6 t h i t a little of some pleasant aromatic, which might excite the powers of life with a gentle and pleasing stimulus. Having discussed as shortly as I could the history and the actions of Magnesia, I may now in a few words glance a t its chemical properties. That the singular nature of this powder may be more clearly manifested, I have considered i t necessary to bring forward a few new experiments in reference to other absorbent powders, in addition to those which are given in detail about Magnesia. I have to confess that there was also another reason inducing me to investigate these various kinds of powders: the experiments of the celebrated Doctors Whytt and Alston, Professors in this University, have brought to light so much that is new and useful about quick-lime that this substance has now certainly become most worthy of the attention of all. I hoped, indeed, that I might find out a new quick-lime and lime-water, different from the common sort, and perhaps more powerful in dissolving the calculus. In this I was disappointed, but do not regret the labour. I have made no few experiments: many of them were new, and some even worthy enough of record: I therefore thought that i t would not be unpleasing to those of my fellow students who are fond of chemical ~ h i l o s o ~ hifv I were to ~ u b l i s bthe more remarkable 0; them: and so I hope t h k they will be fa-
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vourably received by such. For the result, whatever i t be, of a new experiment is not to be neglected; since the foundation of chemical science, so useful to medicine, and yet still so very imperfect, rests on experiments alone. Therefore whatever new thing makes its appearance should be sedulously gathered up and added to the common heap of experiments. In those that follow we see that four species of powders or earths are altogether different from one another, and yet chemists are wont to confuse them together under the common name of absorbents: physicians even use calcined hartshorn, and the other absorbent powders indiscriminately; although i t seems to be so useless that perhaps i t might be cast out of the materia medica without any detriment. Experiment I.-Magnesia is dissolved quickly and completely by vitriolic acid, and in their union a great abundance of air is expelled from them: when the mixture has attained saturation, a salt most similar to the bitter cathartic salt is produced. Here note that Magnesia most manifestly differs from the calcareous sort of powders or stones. All those substances may properly enough be called calcareous which, when strongly heated in the fire, are converted into quick-lime. Of this sort are, lime itself, marble, chalk, a kind of spar, the shells and stony concretions of animals, and lastly those things that are called lithophytes: all of these, if first pounded to a fine dust, do indeed become very hot when vitriolic acid is poured on them, but yet are scarcely dissolved by it, but, hiding the acid in themselves, lie a t the bottom of the vessel in the form of a white powder. For the same reason, if the acid of vitriol be dropped on any of them dissolved by means of the acid of nitre or the acid of salt, or vinegar, the vitriolic expels the weaker acid, and, uniting with the powder, falls with it in a similar manner to the bottom: but this does not happen to solution of Magnesia, for it, in union with the acid of vitriol, dissolves very readily in water: Hence i t appears that Hoffmann's two preparations of Magnesia are different from tlPat of which we here speak, for he says* of solutions of both of them, as of the saline liquors from which they are prepared, that they throw down a white powder when mixed with oil of vitriol: now I have very often mixed oil of vitriol with the saline liquor which in this country is prepared from sea-water; yet it always remained quite clear. The explanation of the dissimilarity seems easy enough if we consider only his first kind, the nitrous; for this has a quite different origin. But in the case of the second it is more difficult; and perhaps i t is to be looked for in this, that Hoffmam obtained his saline liquor from the water of springs yielding common salt, and that such water may be different from sea-water. Experiment 11.-Treated with the acid of nitre, Magnesia behaves in a similar way to that described under Experiment I. The solution imparts to the acid a golden colour, a sharp, bitter, nitrous taste, and bv ~ water a t the fire,. aives crvs, centlv - , e v a ~ o r a t"i nthe 'Hoaa. Op. T. iv. p. 480 and 500.
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tals, which retain their form if kept in dry air, but in moist air are resolved into a liquor. A piece of paper immersed in this liquor and then dried does not deflagrate in the fire like nitre. It is to be noted that calcareous powders and stones dissolved in the acid of nitre cannot in any way be made to form crystals. Experiment 111.-Magnesia also becomes very hot and is quickly dissolved when thrown into the acid of salt: but the solution does not form crystals; i t has a sharp muriatic taste. Experiment 1V.-It behaves also in altogether the same way when mixed with distilled vinegar, and gives it a not unpleasant taste: it does not form crystals, but, when the water is driven off, there is left a saline matter, very tough, very like glue, difficult to dry; when cold i t becomes hard and fragile. Calcareous powders and stones, joined to this acid, yield a salt pretty readily giving striated crystals, it has not the least tenaaty. Experiment V.-men dissolved in all these ways, i t is easily recovered by the addition of fixed or volatile alkali. Experiment V1.-If Magnesia is put into a hot solution of sal ammoniac i t drives the volatile alkali out of it, and this easily betrays itself by its peculiar odour. If the mixture is gently warmed for some time, the Magnesia is completely dissolved, unless more is put in than suffices for the expulsion of all the a!kali, and the saturation of the acid. It can be recovered again by the addition of any fixed, or even volatile alkali. This experiment directly contradicts the preceding. For here the Magnesia seems to have a more powerful attraction than the volatile Hlkali for acids, while there the volatile alkali has a more powerful attraction than the Magnesia. And the result of this experiment is the same, whether the Magnesia has been prepared by means of fixed or of volatile alkali, or, finally, by means of the alkali itself which bas been obtained in the next experiment. This indeed beats me, and I did not understand why the very alkali which Magnesia drove out from the acid does not, on the spot, again drive out the Magnesia. I surmised, indeed, that the alkali underwent some new changes in distillation, which might be the cause of this difference; for i t can never be prepared pure withont distillation: and this idea did not seem to be very incongruous, as from the experiments of the celebrated Hales, it absorbs air during distillation, and is therefore really changed.7 With this idea, I prepared, from volatile alkali and the acid of salt, a sal ammoniac similar to the common kind: to see if Magnesia would be unable to decompose this compound: but everything occurred with i t as in Experiment VI. Experiment VI1.-One part of Magnesia and two parts of sal ammoniac thoroughly mixed and distilled produce volatile alkaline spirit and salt, quite similar to those which are separated from sal ammoniac by means - of fixed alkali. t Analysis of the air, exper. lii.
Experiment VII1.-Let a drachm of any calcareou? powder be dissolved by means of nitrous or mutiatic acid, care being taken that no acidity remains; add a drachm of Magnesia, and let the vessel remain in boiling water for about four and twenty hours; then dilute with ten volumes of water and iilter through bibulous paper. The powder then remaining on the paper will be almost all calcareous; but the liquid which ha+ passed through will give, on the addition of alkaline liquor, true Magnesia, mixed with a little calcareous powder. Magnesia, then, takes acid away from calcareous powder: but as a much longer time is required for the complete separation of the calcareous powder from the acid, it is not wholly accomplished in this experiment, but both a little Magnesia remains undissolved and the liquor still contains some calcareous powder. Experiment 1X.-Magnesia, placed in a solution of mercurial corrosive sublimate, is seen to take on quickly a golden colour, and is gradually changed into a powder of a brick-red colour: this red powder, subjected to chemical examination, turns out to be mercurial. Experiment X.-Let there be well mixed one drachm of quick-lime, three drachms one sauple of Magnesia: let them then be placed in a flask of four ounces capacity; this, filled with water and well stopped, is to be left for two hours in hot water and then cooled. The water will have no taste of lime, will not in any way trouble solution of mercurial sublimate, nor leave any residue when evaporated a t a gentle heat. Having often tried this action of Magnesia, varying the quantity of quicklime added to the same amount of Magnesia, I have found that what has just now been given is the maximum with which the experiment will succeed: but this must vary according to the greater or less power of the quick-lime. Experiment XI.-I put three drachms one scruple of Magnesia reduced to a fine powder into a flask of nearly four ounces capacity; this I filled .with lime-water, closely stopped, and then warmed in hot water; the lime-water soon completely lost all taste, afid was converted into pure water; this I then poured off, added new, left it a t the same temperature, and repeated this operation, until I had poured in seven pounds fourteen ounces of lime-water; then indeed lime-water newly poured on no longer lost its taste. The well dried Magnesia then weighed four drachms one scruple fifteen grains. Experiment XI1.-Magnesia prepared in the way given in exper. xi well mixed with an equal part of sal ammoniac and distilled, gave a spirit and salt similar to those prepared in exper. vii and not caustic. Experiment XII1.-An ounce of Magnesia was heated, in a crucible covered with a lid, for about an hour, a t a temperature sufficient for the fusion of copper: when the crucible had been cooled the Magnesia weixhed three drachms one scruple. w e may call this Magnesia usta. Experiment X1V.-Magnesia usta (exper. xiii) is slowly- dissolved by all acids, and with them forms
salts quite similar to those described in experiments i i i and v : but what is chiefly to be noted, the solution takes place most quietly, and without the very least ebullition. Experiment XV.-The same put into a solution of mercurial corrosive sublimate, with the aid of a gentle heat, slowly throws down the mercury, in the form of a black powder; which, however, when rubbed on paper stains it like rust. Experiment XV1.-Magnesia usta expels volatile alkali from solution of sal ammoniac, as is described in the fifth* experiment. Experiment XVI1.-Yet it does not seem to trouble calcareous powder dissolved in acids. Experiment XVII1.-Nor does it produce any change in lime-water. Experiment X1X.-A drachm of Magnesia usta was gently heated with an ounce of water, in a closed flask for several hours; the water was then passed through bibulous paper, it was quite devoid of taste, and neither made solution of mercurial sublimate in any way turbid nor did it change the colour of syrup of violets: the Magnesia itself, when well dried, weighed one drachm ten grains: mixed with acids it did not effervesce, nor did it produce any effect on lime-water. We see from the immediately preceding experiments that quite a notable change is produced in Magnesia by burning: yet very different from that which calcareous powders and stones undergo, when so treated. The explanation of this for a long time seemed to me very obscure, and, that I might understand it, I made many experiments, of which the following seem to bring some light. Experiment XX.-I put three.o&es of Magnesia into a glass retort, with a receiver attached, and placed on sand: I then applied fire, which was gradually increased, until the Magnesia was just obscurely red hot. When everything was again cold, the Magnesia weighed one ounce three drachms and a half: it still effervesced strongly when put into acids, although hot so much as before. In the receiver I found a little whitish water, weighing five drachms: this scarcely smelled of spirit of hartshorn, changed the colour of violets to green, slightly troubled the solutions of mercurial sublimate and of silver: yet when mixed with acids showed very little action. We saw (exper. xiii) that Magnesia lost a very large part of its weight when heated. By this experiment, therefore, I wished to know what it was that it had lost. In this case the heat applied was much less than that required for its complete burning, and so i t still effervesced pretty strongly with acids, and I could not avoid this, for I had not the proper apparatus for the purpose: I was therefore forced to leave the experiment imperfect, until an opportunity should occur of carrying i t out more properly. The weight of the liquid caught in the receiver was not thc half of that lost; what then is it that has dis* (This is no doubt a misprint for sixth.)
appeared? Perhaps some water, but more seems to be air, a great deal of air must have remained with the Magnesia, for, after the loss of so much, it still gives off air on the addition of acids. The qualities of the water may perhaps be a little due to Magnesia carried over by the heat along with the air and water. drachms of Magnesia burnt Experiment =I.-Two in the way described in exper. xiii were reduced to two scruples twelve grains; when-put into spirit of vitriol they quietly dissolved. Magnesia was again liberated from this by the addition of solution of fixed alkaline salt; of which, to do this, a large quantity is required; i t was then well washed and dried. The weight had now increased again to one drachm fifty grains, i t acted on acids with great ebullition and energy; put into solution of mercurial sublimate i t formed a red powder; it precipitated calcareous powders from acids and rendered lime-water insipid: in Short i t had now recov&ed all the qualities which i t had lost in burning. What is i t then that the Magnesia lost in this experiment and again recovered, which when i t was present gave so many qualities, which its removal took away? Perhaps air: for the consideration of the twentieth experiment makes it very probable that the greatest part of the weight lost in burning consisted of air going away: and this indeed is codinned by the fact that, after burning, no air is given off by the action of acids. Is i t not also likely that the air is again restored t o the Magnesia from the alkali, a t the time when i t removed i t from the acid: for i t is proved by the experiments of the ingenious Hales that fixed alkali certainly abounds in air,* of which i t emits a great deal when i t unites with a pure acid: in this case i t certainly unites with an acid, though not with a pure acid, and indeed the middle salt formed by their union is in quantity, properties, in every way the same, as if the pure acid had been added: but yet only a very little air comes off: is i t not then that the air is driven out of the alkali by the acid and joins itself to the Magnesia? Experiment XXI1.-I put two &adms of sal tartari and an ounce of water together inm a large flask furnished with a long and slender neck: the flask with the salt and the water weighed two ounces twodrachms: I then gradually dropped in oil of vitriol diluted with water to complete saturation of the salt, which was attained when 1 had poured in two drachms two scruples and three grains of the acid; the flask now weighed two ounces four and fifteen the loss of weight was therefore one scruple and eight of this a very small part might be water, or something similar; the rest air. These things misled the celebrated Homberg, when he endeavoured to find the proportion of solid and liquid in different acids;? he attacked the question by saturating pure alkaline salt with each of the acids, and observing what the salt gained from a deiinite quantity of each acid; hut his calculation was necessarily erroue-
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Analysis of$x air, exper. l u i v and xcix. t Mem. de 1'Aced. do scienc., vol. i, p. 52.
ous, as the alkali loses no small part of its weight in the saturation. Experiment XXII1.-I repeated the last experiment with two drachms of Magnesia in the place of the sal tartari, and used the same spirit of vitriol: of this four drachms one scruple and seven grains completed the solution. The flask, with the Magnesia and the water gave, most exactly weighed, two ounces two drachms, before the addition of the acid of vitriol; and after the solution of the Magnesia, two ounces five drachms two scruples and eleven grains. Therefore there had been lost by effervescence, one scruple and sixteen grains. Experiment XX1V.-I did all the same things over again so far, with two drachms of Magnesia reduced by effective burning to two scruples and twelve grains: four drachms one scruple and two grains of the same acid completed the solution: it lost only two grains during the solution, which may indeed consist of water. I took care first to dilute the oil of vitriol used in this experiment with water, so that i t should not excite a sudden violent action and heat in the flask, while i t was mixing with the water, and so dissipate a part of this. I also chose a capacious flask, from those to wit in which Florentine oil is imported into this country. Hence, the vapours and droplets of water, ejected by the effervescing liquids, and wandering for a while in the hollow of the flask, were soon collected, like dew, on its sides, so that not a droplet ascended as far as the neck, which, when the experiment was ended, was quite dry. This experiment seems to show why Magnesia usta, when united to adds, produces salts similar to those which it produced before the burning: for Magnesia usta has lost its air before.the'encounter with the acid; but Magnesia non usta loses it in the encounter itself. From a consideration of the foregoing experiments i t seems probable that the prodigious noise which gold, when i t has first been dissolved in aqua regia, and then restored, in the form of a powder by means of an alkaline solution, gives out when brdught to the fire, is due to the air, transferred from the alkali into the gold, and suddenly liberated by the force of the fire: for certainly air can lie hid in metallic calces: seeing that it has been expelled, by the distinguished Hales, from minium itself.$ ON
p O m E R S AND STONES
. Experiment XXV.-I dissolved an ounce of chalk in spirit of salt; and again separated it, from the acid, by means of fixed alkali, and washed away all salt by repeatedly pouring on and pouring off water. The chalk then, on chemical examination, did not seem to have been changed in any way, inasmuch as it effervesced strongly with acid of vitriol, but yet was not dissolved by it; burnt in a sufficient fire, it became true quicklime, which gave lime.water, and drove out a spirit of very sharp odour from sal ammoniac. BY this experiment I wished to know. whether chalk could acquire the properties of Magnesia: or if Mag-
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t Analysis of the air, p. 192.
nesia owed these properties to a previous union with acid, and then separation from it: but chalk is nothing changed by this process. Experiment XXV1.-Let any calcareous substance be put, in fine powder, into a hot solution of alum; there will be produced immediately a great commotion and ejection of air; but yet the powder will not dissolve; i t will be seen rather to increase in quantity. Let the addition of the ~ o w d e rbe re~eated.until no more action is excited. ?he liquid now has very little taste of alum, and hardly any of lime; on the addition of an alkaline liquid it gives no coagulum, as solution of alum does, but is only slightly troubled. The calcareous powder, in this experiment, unites with the acid, and therefore turns out the earth of the alnm; but the calcareous powder also, united to the acid follows this earth to the bottom; for the acid of alum is similar to the vitriolic; hence, united with calcareous matter, it forms a powder scarcely soluble in water: the slight cloud formed on the addition of alkaline liquor comes from a very small quantity of calcareous solution. EXPERIMENTS ON CALCINED HARTSHORN
Experiment XXVI1.-Calcmed hartshorn put into spirit of vitriol, absorbs the acid, and gradually falls down into a very white powder: yet i t does not dissolve, nor does i t make the acid effervesce. Experiment XXVII1.-It dissolves slowly in t8e acids of salt and of nitre, with a very slight effervescence. After the acid seems to be already saturated you may add as much as you please of the fine powder and warm i t for several days with a gentle heat, yet i t will not completely lose its acidity; but it will retain both its acrid-acid taste, and still turn infusion of violets red. Experiment XX1X.-Distilled vinegar warmed over this powder, neither produces effervescence, nor dissolves any perceptible quantity of it. The vinegar when poured off has almost the same taste as before, and gives only a slight cloudiness on theaddition of
alkaline liquor. It does not, therefore seem to have any value in medicine; for i t is retained among the powders which alone absorb acid; while yet it is quite unsuitable for this business, if i t does not absorb the acid formed by fermentation. Experiment XXX.-Calcined hartshorn suifers no change when burnt in the most vehement fire; nor does it acquire any of the properties of quick-lime. EXPERIMENTS ON THE EARTH OF ALUM
I dropped gradually solution of pure sal alkali into solution of alum; a quite conspicuous effervescence a t once arose, and the earth of alum showed itself in the form of a coagulum. This when well washed was used in the following experiments. Experiment XXX1.-The earth of alum is dissolved by all acids, but indeed very slowly, unless heat is applied. The liquor which is formed from vitriolic acid united to the earth of alum clearly resembles solution of alum; that from the acid of nitre, or from the acid of common salt, is more harsh; that from distilled vinegar has a certain sweetness, mixed with astringency: none of them can be brought so far that the acid in them is perfectly changed: but they retain always a sub-acid taste, and the power of changing the colour of infusion of violets to red; in this they agree with solution of alum. This earth seems to adhere to acids with only a slender attraction and for solution requires a large quantity of them, and when united to them changes them less than other absorbent earths do. The earth of alum, dissoIved in distilled vinegar gives a new kind of alum, which, as it.is milder than the common kind, and perhaps more penetrating, may sometimes be conveniently preferred to i t in making up remedies. Experiment XXXI1.-The earth of alnm exposed for an hour to a very strong fie, sufFers no change, except that it loses about a fifth part of& weight, and afterwards dissolves with greater difficulty in acids.
