1825.] of Claude- Louis Bert hollet. 171 



Proust's reasonings in this very controversy, vvhich is on so many 

 other accounts interesting, I shall make no apology for presentr 

 ing an outline of it to the reader. 



It is a matter extremely obvious to observation, that there ^re 

 certain proportions in which chemical substances combine by 

 preference with each other. Thus from all the various processes 

 by which it is possible to unite oxygen with iron, the result 

 (with the exception of a single more recently discovered oxide) 

 is constantly one of two combinations, the proportions of which 

 are unalterable. These are familiar in chemistry as the black 

 and the red oxides of iron ; each of them characterises and forms 

 the basis of a peculiar class of salts ; and no other oxide of iron 

 has a name, or is known. The general truth of a combination 

 by preference, it was impossible for Berthollet to deny ; but he 

 affirmed that wherever two bodies possess a reciprocal affinity 

 for each other, they ma i/' combine in an infinite variety of pro- 

 portions. He accounted, for the apparent preference by suppos- 

 ing that wherever it exists, it is a consequence of the interfpreqce 

 of some foreign principle, such as cohesion, elasticity, See. with 

 the simple operation of affinity. Thus, he alleged, when any 

 ordinary combustible, as hydrogen or sulphur, or any volatile 

 metal, as zinc or arsenic, is ignited, it is converted into vapour 

 by the elevated temperature, its cohesive attraction is over- 

 powered, it is at once placed in the situation the most favourable 

 for combining to saturation v.'ith oxygen, and the result is of 

 course an instantaneous absorption on their part of the greatest 

 portion of that air with which this process can ever unite them. 

 But, on the other hand, said he, when any of the more fixed 

 metals is ignited, as tin or lead, it undergoes a process of pro^ 

 gressive combination by imperceptible degrees of increase with 

 oxygen, forming compounds of every variety betvyeen zero and 

 that dose which constitutes the saturated oxide. 



The arguments of Berthollet were affirn^ed by his antagonist 

 to be wholly without foundation, and his experin;pnts were pro- 

 nounced to be either inaccurate or inconclusive. With respect 

 to the native oxides of iron, the instance quoted by Berthollet 

 which seemed the most forcibly to illustrate and support the 

 view of combination in infinite variety of proportion, Proust 

 took a true, and, at the same time, a most ingenious view. He 

 conceived that although a given mass of oxidized iron may be 

 found, the constituents of which are resolvable into oxygen aqd 

 the metal in any proportions between the maxima and minirna 

 in which these substances are ever found united, yet in evefy 

 Kuch case the mass is composed of the black and the red oxidps, 

 mixed through each other in every various proportioij, an4 still 

 no atom of oxygen is combined with anyone atqm of iron unless 

 in one or other of the proportions which make the black or the 

 r^^ pxide. And the mode in which lit: l>}Qyed all oxidized iron 



