An Historical Note on the Conservation of Mass
Robert D. Whitaker University of South Florida Tampa. Florida 33602
Histories of science usually ascribe the formulation of the law of conservation of matter in chemical reactions to Antoine Lavoisier a t the end of the 18th century. However, i t is often oointed out that this law was used imnlicitlv much earlier. Joseph Black. for examole, utilized the ~ r i n c i ~ l e throughout his quantitative experiments with t h i carbonates and oxides of magnesium and calcium. I n fact, he ascribed a n apparent lack of mass conservation to experimental error. The following equations illustrate one series of experiments MgCO, ---t MgO 'CO, (magnesia) (calcined magnesia) (air)
+
MgO MgSO,
+
+
H?SO, Na,CO,
a
MgSO,
aq
+
~
H,O
+
MgCO, Na,SO, (magnesia) Black heated magnesium carbonate (magnesia) to drive off carhon dioxide (air), and dissolved the magnesium oxide (calcined magnesia) in the minimum amount of sulfuric acid. By the addition of sodium carbonate he precipitated magnesium carbonate which he proceeded to collect, dry, and weigh. This weight was a little less than that of the original magnesium carbonate. He also noticed that the amount of sulfuric acid reauired t o dissolve the mamesium oxide was slightly less than that required to disrolve an amount of magnesium carbonate equal to that which had been heated to produce the oxide. His expeiiments were carried out using amounts in the range 3-10 g. The largest difference was about 9% hut most of his results checked to better than 1%,yet Black felt compelled to explain the difference. We read in his "Experiments upon Magnesia Alba, Quicklime and Some Other Alcaline Substances," published in 1755, the following words' -+
As in the separation of the volatile from the fixed parts of hodies, by means of heat, a small quantity of the latter is generally raised with the former; so the air and water, originally contained in the magnesia, and afterwards dissipated by the fire, seem to have carried off a small part of the fixed earth of this substance. This is probably the reason why calcined magnesia is saturated with a quantity of acid, somewhat less than what is required to dissolve it before calcination: and the same may he assigned as one cause which hinders us from restoring the whole of its original weight, by solution and precipitation. Clearly, Black relied on the conservation of mass even though he never explicitly stated it as a working principle. Likewise, Henry Cavendish and the Russian M. V. Lomonosov were other 18th century scientists who simply asPresented st the 3rd ACS Conference on Chemical EducationPenn State Priestley Conference, State College, Pennsylvania, July 30-Aup. 3,1974. Alembic Club Reprints, No. 1, E. & S. Livingstone Ltd., Edinburgh, 1963. p. 18. 'Alembic Club Reprints, No. 11, E. & S. Livingstone Ltd., Edinburgh, 1953, p. 14. Ref. (21, p. 17. 'Ref. (2), p. 35. 658 / Journal of ChemlcalEducation
sumed the principle without feeling compelled to state their assumption. We can conclude that by the middle of the 18th century, conservation of mass was a c c e ~ t e dbv some scientists as a valid idea for the interpretation of ex.. perimental results. Furthermore, any discreoan. apparent .. cies between observations and the principle required rationalization. Much earlier, however, a n explicit statement of conservation of mass as well as interesting deductions based on it are found in a series of essays hy the medical doctor, Jean Rey. The essays were titled, "On a n Inquiry Into the Cause Wherefore Tin and Lead Increase in Weight on Calcination," and were originally published in 1630 a t Bazas and re~ublishedin 1777 in Paris. Rev was born in 1582 or 1583 -a t ~ Bugue e in Dordogne. He received his M.D. from the University of Montpellier in 1609. We read in Essay V12
. .. weight is so closely united to the primary matter of the elements that they can never be deprived of it. The weight with which each portion of matter was endued at the cradle, will he carried by it to the grave. In whatever place, in whatever form, to whatever volume it may he reduced, the same weight always persists. He next offers logical proof that conversion of "earth" to "water" cannot be accompanied by any change in weight. His argument is the following2 Let there he taken a portion of earth which shall have in it the smallest possible weight, beyond which no weight can subsist: let this earth be converted into water by the means known and practiced by nature: it is evident that this water will have weight, since all water must have it, and this weight will either he greater than that of the earth, or less than it, or else equal to it. My opponents will not say that it is greater, for they profess the contrary, and I also am of their opinion: smaller it cannot he, since we took the smallest weight that can exist: there remains then only the case that the two are equal, whieh I undertook to prove. What is shown of this particle may be shown of two, three, or a very great number-in short, of all the element, which is composed of nothing else. The same proof may he entended to the conversion of water into air, of air into fire; and conversely, of the last of these into the first. The atomic nature of matter is clearly anticipated in Rey's argument, although the Aristotelian flavor is strong throughout. With this clear, albeit rather poetic, statement of mass conservation, the question immediately arises why this important principle was not incorporated into the interpretation of experiments of the 17th century. Rey himself provides a t least a partial answer to this question and the answer lies in the attitude of many scientists toward reliance on empirical data provided by instruments. Certainly, the use of a balance is necessary in the application of a mass conservation idea to actual experimental phenomena. We find in Essay VIII the following3 I . . . affirm that the examination of weights which is made by the balance differs greatly from that which is made by the reason. The latter is only employed by the judicious: the former can be practiced by the veriest clown. The latter is always exact: the former is seldom without deception.
Rey obviously puts his trust in his reason, and the principle of mass conservation arrived a t bv good Aristotelian iogic is not to be connected with sense-experience. The seeming non-conservation of mass attending many phenomena such as the burning of wood or calcination (oxidation) of metals is not a problem since the reason has supplied the proper answer regardless of deceptive observations. Finally, consider the following observation found in Essay XV4 For when we have no regard to the bulk or volume of the thing, if we examine its weight by the reason, I say that there is nothing which increases in heaviness hut by the addition of matter, nor which decreases in heaviness but by the subtraction of matt e r . . . But if we make the examination by the balance, we meet with a ease in which without addition or subtraction of matter the thing will appear heavier or lighter: namely by shrinking, or dilation.
Thus is i t possible to believe the principle of mass conservation s e t make no attempt to utilize this principle in any actuai experiment. If thehstrument gives faultydata, why try to obtain data using it? T o be sure, not all 17th century scientists took this view. It is probable, however, that this attitude was prevalent enough to explain, partly a t least. the seemine inconsistencv between the belief in a general principle suih as mass co&ervation and the lack of anv exolicit use of the orinciole in exoeriments. The confrontation of sense experience and reaaon was resolved in favor of reason in Rey's time. By the middle of the 18th century, contradictory observations with instruments such as the balance were not so liehtlv dismissed. and it has been shown how Black reconciled discrepancies between observation and fundamental principle in the same manner as we would today.
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Volume 52, Number 10, October 1975 / 659
