M. Pasteur on Aspartic and Malic Acids. 281 



formula as a complement of the definitiou in that case as well as 

 in all others. 



This problem becomes solvible by a very natural analogy, 

 when the substances under consideration, being placed in similar 

 circumstances, form combinations with other substances, the 

 products of which are constant and clearly defined, and do not 

 differ in their composition except with regard to the relative 

 masses of dissimilar matter which are respectively associated to 

 a given weight of the isomeric substances which are compared 

 together. The equivalents of weight, and consequently the 

 specific masses of the corpuscles, are then proportioned to these 

 relative quantities, which process gives the common factor by 

 which their symbolic formulie are to be multiplied in order to 

 retain them constantly in this proportion. 



It is thus, for instance, that the four isomeric compounds of 

 cyanogen with oxygen, which are called cyanic and cyanuric acids, 

 cyamelide, a neutral substance, and fulminic acid, are character- 

 ized by a distinct factor. The difference in the nomenclature 

 thus applied to these four bodies is evidently unassailable when 

 it is employed as a symbolic representation of the obseiTcd facts. 

 But its physical intei-pretation implies an inference, the accuracy 

 of which is only probable. It is, that in those experiments the 

 unequal composition of the products investigated should be 

 solely attributed and proportionate to the relative masses of the 

 isomeric corpuscles, the ingredients of which are combined 

 therein. Thus in such cases, also, chemists avail themselves of 

 every analogy calculated to confirm the jn-oportionality which 

 they assume. 



The study of organic substances, which has attained such a wide 

 extent, presents a great number of instances of isomerism, which 

 are far more difficult to define even symbolically than those above 

 mentioned, because the reactions which take place between them 

 and other substances almost always transform those which are 

 to be tested, and then only characterize them indirectly by means 

 of their products of decomposition, and, so to speak, only after 

 they have ceased to exist, which tends to extiuguisli the original 

 characters of dissimilarity, which would be a key to the isomerism. 

 In order to escape from these ambiguities, chemists have en- 

 deavoured, and successfully, to find among those sciences which 

 are connected with chemistry, auxiliary evidence, the application 

 of which would be exempt fnjiu any destructive influence. Ei'om 

 physics has Ik-cu derived the law of volumes, while the determi- 

 nation of tiie density of bodies in the state of vapour, as well 

 volatile as non-volatile, and the estimation of their specific heats, 

 liave afforded means of distinguishing different substances by 

 a new class of equivalents in addition to the equivalents of 



