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heavy particles are deposited in the first zoo feet of flue, but the fume is being discharged from the stack, after having traveled a much greater distance. Preliminary tests were made on these gases a t a point about 1 5 0 feet from the stack base. The temperatures varied from 100' C. to 150' C. During these preliminary tests, the ore charges upon the sintering machines were varied and checked against the assays of the dust recovered electrically, during the day, in order to ascertain how the different charges influenced the loss of lead. Assays of recovered dust varied from 20 per cent. t o 45 per cent. lead during this period. Even a t this distance from the roasters, considerable silver and an appreciable amount of gold were found in each sample. The dust recovered was dry and could easily be handled, as i t was so dense and compact that a little draft did not blow it away. The smeltcr is now proceeding with a n installation on a largcr scale and it is expected t h a t considerable values will be recovered. Furthermore, this installation may make i t possible to push the roaster department to a greater tonnage, due to the ability to collect any materials carried in suspension by the gases. In some roasters, a fairly large percentage of sulphur trioxide is produced. Apparatus can easily be designed t o handle this character of gas successfully, without undue deterioration. Another recent application of these processes is in the removal of tar and suspended carbon from illuminating gas. Mr. Walter A. Schmidt has directed the work upon the gas manufactured from crude petroleum a t one of the gas plants near San Francisco. A complete clearance of all visible particles was obtained. I n the eastern part of the United States, the work of Prof. A. H. White, of the University of Michigan, should be mentioned. Under the auspices of the Michigan Gas Association, Prof. White has conducted extensive investigations, primarily upon the removal of tar from gas manufactured from soft coal. The results were so gratifying that negotiations have been instituted for a larger installation, for this purpose. The tar is collected upon the collecting electrodes and runs off into sealed chambers. This field promises t o be a very wide one. Several months ago, prior to the organization of the Research Corporation, Dr. F. G. Cottrell made arrangements with Prof. R. K. Duncan, of the University of Pittsburgh, whereby the latter arranged to use the Cottrell processes for conducting investigations upon the smoke problem of the city of Pittsburgh. Dr. Strong, Dr. Benner and others have been of assistance in this important problem. Many other problems of a nature similar to some of those mentioned above have been brought before us and an effort will be made to adapt the processes to them a t the earliest possible date. There are many places adjacent to New York City wherein considerable values may be recovered and a t the same time, improve the atmosphere, a t least in so far as suspended particles and obnoxious fumes are concerned. Much more of the details of the processes themselves or their application to specific problems might
Dec.,
1912
be given, but i t would prolong this paper unnecessarily. I sincerely trust that the above general survey may bc of value as well as interest to the chemists and cngineers. NoTE.-Mr. Bradley explained in detail the construction of the small apparatus prepared for demonstrating the Cottrell processes. The first test was upon the precipitation of cement dust, which was blown through a pipe 6 inches in diameter. The effect of the high tension current was to collect all of the dust within the pipe. A test was next made upon somc chcmical fumes. Compressed air was blown through bottles containing concentrated ammonia and concentrated hydrochloric acid, separately, and thc chemically laden gases mixed in a chamber just prior to being introduced into the electric treater. I3cnsc fumes of ammonium chloride were thus obtained and the effect of turning the current on and off was demonstrated. Samples of the various matcrials, which were collected by the processes in large scale opcrations, were upon the desk for inspection. THE CONTROL OF TEMPERATURE IN THE OPERATIONS OF ANALYTICAL CHEMISTRY.' BY THEODORE W. RICHARDS.
The control of temperature is a very important question in the work of the analytical chemist. The reason is a t least three-fold. I n t h e first place, temperature affects greatly the speed of all chemical reactions which are generally accelerated to extents varying perhaps seven t o twelve per cent. by each degree's rise in temperature. I n the second place, temperature affects the final equilibrium attained by many reacting systems and therefore influences both the yield and the composition of the products dealt with by the analyst. In the third place, accurate physical measurements, t o which the quantitative experimenter must frequently resort-such as weighing the measurement of the volumes of gases and liquids, and the determinations of calorimetric or electrical magnitudes-demand considerable control of temperature if any accuracy is sought. Clearly, the subject is too large for the brief ten minutes to be devoted t o it; but a few words may be able t o point out the more vital features. Let us begin with the control of temperatures near t h a t of the room. I n the first place, it is clear that every chemical laboratory may advantageously have a thermostat attachment t o its heating arrangements. For years I have used a commercial contrivance which, when operating properly, has kept my laboratory a t 2 0 O C. within half a degree, greatly to my satisfaction. I n order t o attain any such constancy, the air of the room must be efficiently agitated by means of a n electric fan; just as any other form of thermostat should be adequately stirred. Of course, when a n operation affected by currents of air is undertaken, the fan must be temporarily stopped. Entirely within this room, surrounded by glass walls and without any outside windows, is built a
'
Paper presented at the Eighth International Congress of Applied Chemistry, New York, September, 1912.
Dec., 1912
T H E JOC7R.YAL OF I - Y D C S T R I A L rl-1-D E,YGI-\*EERI,\.G
balance-room, which resembles a huge balance case ; and this remains constant for long periods within one- or tu-o-tenths of a degree, because the somewhat larger fluctuations of the outer room do not quickly pass through the glass walls. The plan has worked so excellently t h a t all balance-rooms in the new Wolcott Gibbs Memorial Laboratory a t Harvard are t o be built in this way entirely within other rooms. A very suitable, sensitive, and easily constructed thermostat attachment for regulating the temperature of a room is the sealed hydrogen manometer, with electrical contact.’ This consists of a large sealed bulb containing hydrogen, which gas is arranged t o support a column of mercury having a n electrical contact a t its upper end-the affair is a combination of a gas thermometer and a barometer. The rapid heat conduction in hydrogen makes this gas especially suitable for the purpose. We have used the device for many years with great profit, and by its aid, have been able to keep a cellar laboratory. which is protected b y double windows, within 0 . I degree for weeks a t a time. The electrical current, made and broken a t the mercury surface in the top of the manometer by the fluctuations of the expanding or contracting hydrogen, may be made to operate a relay which in turn regulates the heating apparatus, whether this uses steam, hot air, gas or electricity. A form in which the current running through the mercury in the manometer itself is used for heating has recently been described,z but I should be afraid t h a t this might not be able t o maintain quite as accurate a constancy as a n apparatus using a weaker current in its regulating manometer. Often the yet more accurate regulation of the temperature of small objects is needed; of course the Ostwald toluene regulator in a bath of water or oil gives the most convenient thermostat for such purposes. Of late we have used, on account of its cleanliness and safety, only electricity as a means of supplying heat t o this b a t h ; the regulator is arranged so as t o make and break a feeble electrical circuit which operates a relay through which in turn runs the stronger heating current. The chief, probably the only disadvantage of electrical heating, is the possible leakage of electricity. If it is used for electrochemical work, one must guard against stray electromotive forces b y efficiently grounding the thermostat. The description of a n arrangement of this kind making possible the maintenance of the temperature within 0 . O O I O or 0 . 0 0 2 O for days is perhaps not out of place. This has been used a t Harvard for many years; i t was briefly described several years ago and has been used independently by others.? The thermostat consists of a large can which may be as large a s 7 0 centimeters in diameter and 7 0 or more high. The can is covered on the outside with felt and may have its surface protected with oil if evaporation is t o be prevented. Within this can is immersed a regulator similar in principle t o the toluene thermostat regulator of Ostwald. The receptacle 1 Regand and Fouillard, Z . a.iss. M i k r o s c o p . , 20, 138 (1903); also Richards and Mark, Proc. A m . A c a d . , 41, 119 (19051. 2Bonsfield. Chens. S e w s , 105, 13 (1912). 3 Richards, Carnegie Inst. XXTashington, P u b . 58, 22 (1906) ; 7 6 , 9 (1907).
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for the toluene is made with five large fingers having walls of moderately thin glass, and is arranged t o have a capacity of over half a liter. The mercurycolumn, raised and lowered by the expansion or contraction of this toluene, “makes” and “breaks” a feeble electrical circuit which governs through a relay the stronger current used for heating. The latter current passes through the relay and through a large insulated heating coil immersed deeply in the water of the thermostat. The “making” and “breaking” happen advantageously in a somewhat narrow tube, perhaps two millimeters in diameter, and the mercury in this tube should be protected from the laboratory air by an a t mosphere of pure hydrogen, supplied by a very small automatic hydrogen generator attached t o the apparatus. This device is very important if great constancy is sought; it constitutes the only unusual feature of our apparatus. The efficiency of this regulator, or indeed of any other, as a means of keeping the temperature constant depends greatly upon the agitation of the mater in the thermostat. This should be violently stirred by means of a rather powerful motor in order to keep the temperature constant throughout and t o effect a rapid exchange of heat between the bath and the toluene regulator. The degree of agitation usually employed is entirely inadequate. The temperature is obviously much more constant if the room containing the thermostat is allowed to vary but little in temperature ; it is advantageously kept perhaps a degree below the temperature of the bath. By running a thin pipe containing cool water around the inner circumference of the top of the bath, compensation for a higher temperature may be easily obtained. If care is taken and the glass toluene receptacle is sufficiently large, and is well seasoned so that it has assumed a reasonably constant volume, the thermostat mill keep constant within one- or two-thousandths of a degree for weeks. This arrangement makes no pretense t o novelty in principle, but in efficiency it probably exceeds most other forms because of the details needed in its construction. A very similar apparatus has been more recently described b y Hulett,I but he does not seem to have sought or attained quite the degree of precision which me have successfully employed. Very satisfactory thermostats may be made from baths filled with pure salts in the act of transition from a state of greater to a state of less hydration. Such mixtures keep a striking degree of constancy for a long time. Sodium sulphate is the best substance for this purpose. For details a paper by K. L Mark and the present author in Vol. 38 of the Proceedings of the American Academy of Arts and Sciences should be consulted. The control of low temperatures and of high temperatures has received so much general notice recently that this expert audience needs no detailed exposition of these topics. The employment of pure ice for maintaining a definite degree of coolness is knon7-n to every one, but the almost equally serviceable 1
Physical R e I ’ i r u , 32, 277 (1911’).
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use of ice mixed with solutions of definite concentrations for increasing coolness is perhaps less generally recognized. I For example, ice mixed with dilute hydrochloric acid containing 28.19 grams of hydrogen chloride per liter gives a perfectly constant temperature of -3.00°.* Of course, if heat is added, some of the ice melts, the solution becomes more dilute, and the temperature rises. This, however, may be easily prevented by enclosing the constant-temperature bath by another bath containing a mixture having the same freezing point. If a n air space (or still better the evacuated space of a Dewar vessel) is placed between the two baths, the inner one will maintain a n amazing degree of constancy for long periods of time, particularly if more dissolved substance from time to time is added t o the outer vessel as its ice melts. The lower limits of the temperature attained in this way are of course the cryohydric temperatures of the more soluble substances. Lower temperatures are customarily obtained by means of solid carbon dioxide with alcohol or ether, and still lower ones by liquid air and hydrogen. By boiling the liquefied gases under reduced pressure their temperature-range may be extended considerably. The details of working with these nom familiar substances need hardly find a place in this brief review. Acquaintance with the necessary technique is becoming more and more a n essential part of the complete chemist’s outfit, although analytical operations rarely demand low temperatures. Turning now t o higher temperatures more frequently employed by analytical chemists, wide limits must receive consideration. Open steam baths are essential, and are too well known t o need discussion. Thermostats of concentrated salt solutions or oil or paraffin, or of fused mixed sodium and potassium nitrate, may be used a t fairly high temperatures, if proper regulating devices are employed. For most purposes, however, air baths regulated either by thermostat control or by approximately constant steam, gas, or electrical heating are more commonly employed. If steam under constant pressure is available, it forms a very usual and convenient means of maintaining constant temperatures somewhat above 100’ of the needed in analytical laboratories. Gas, or more recently, electrical heating is more commonly used ; the latter has the great advantage of cleanliness. Other vapors also, such as toluene (IIO’), the xylenes (140’+), and aniline (184’) are often used with advantage. Air baths without thermostat attachments are usually arranged to maintain only a certain difference bet\%-een the room temperature and the higher temperature sought. This is all very well if the room remains constant in temperature, but sometimes laboratories change as much as I O ’ or 15’ during the day, and this change may have serious effects in some delicate operations when i t reappears in the heated air bath. For such operations a thermostat regulation of the temperature of the bath is almost essential. The forms of air bath which have been proposed This was suggested by RoIoff, Z p h y s Chew , 18, 572 (1895). Richards and Jackson, Proc A m Acad , 41, 473 (1906).
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are countless. Most of them lack the needful requisites of resistance t o corrosion and therefore of cleanliness. The utensils containing the substance t o be heated usually needs t o be protected by some form of shelter or roof, such as a watch glass, to keep out particles of impurity from the corroded walls of the oven. This is altogether unfortunate ; the air-bath used by analysts should be free from such defects, therefore it should be made of glass or porcelain.’ It is almost needless to point out t h a t for many purposes the products of combustion also should be rigidly excluded from the inside of the oven if gas is to be used a s a source of heat. Moreover, the air of the laboratory is often injurious to sensitive substances; such material should be heated in boats within tubes, even if only dried a t 100’. The well known “bottling apparatus,” so much used a t Harvard, makes this precaution easy.* For higher temperatures burners of various shapes are available, and more and more use is being made of the yet greater possibilities of electrical heating. Most of the gas burners are either modifications of the Bunsen burner or the blast lamp, and some of the former, especially the Meker burner, have been made of such efficiency as to replace for many purposes the use of air under pressure. To obtain a constant temperature with these burners, i t is necessary usually t o attach t o the gas supply a contrivance for giving i t constant pressure; otherwise the fluctuations are considerable. It is needless to point out that contrivances for preventing radiation, such a s clay cylinders and other forms of furnaces, greatly promote the constancy of temperature attained. Turning now to electric heating, we have not only the electric furnace so much used in technical work, but also, more suitable’ for analysts, a sort of electric muffle, a porcelain or other refractory tube wound with resistance wire, capable of producing and withstanding the high temperatures needed. For temperatures up to 1000’ the alloy called “nichrome” works very well as the material of the resistance wire; for higher temperatures platinum is necessary, but even t h a t is less refractory than one might wish. We have obtained excellent results with the type of furnace made in this fashion with tubes of pure silica as core. The maintenance of constant voltage and , even conditions of radiation provides the operator with a very fairly constant temperature, which is most conveniently estimated by the platinum-rhodium thermopile. I n summing up, i t may be said that attention has been called to the relative advantages and fields of usefulness of some of the more important methods of controlling temperatures between 3000 O and -250 ’. A few of the most essential details of execution have been suggested and emphasis has been placed upon the frequent importance of constancy of temperature and of the exclusion of outside impurity. HARVARD TNIVERSITY, CAMBRIDGE.NASS. 1 h simple, clean, and inexpensive electric oven is described in A m . C h e m . J o u r . , 22, 45 (18991. 2 See, for example, the Faraday lecture of 1911, Jour. Chem. Soc. Trans., 99, 1203 ( L o n d o n i
