282 M. Pasteur on Aspartic and Malic Acids. 



weiglit. Crystallograpliy has pointed out the conditions of iso- 

 morphism ; and it is certainly a circumstance no less curious than 

 instructive^ to observe how a science which is concerned solely 

 with the study of form, has been able to render assistance to that 

 which is exclusively occupied with that of molecular phsenomena. 

 But the separation which has been made between these two 

 branches of investigation, in order to facilitate their comprehen- 

 sion, is merely an artificial one, and they are really very inti- 

 mately connected. The word isomorphism, in the sense in which 

 it is employed by chemists, expresses a triple analogy of proper- 

 ties, which are very frequently, though not invariably, presented 

 by substances having the same chemical formula, that is to say, 

 those in which the sole distinctive feature consists in the differ- 

 ent nature of their constituents, simple or complex, expressed by 

 their literal symbols. Among those substances having similar 

 formulae, a large number spontaneously aggregate when they 

 crystallize separately into geometrical solids, the forms of which 

 are identical with each other, or vary very slightly. They also 

 crystallize conjointly by their united action when they are dis- 

 solved together in all manner of proportions ; and then if the 

 operation is well conducted, the crystals which are formed pos- 

 sess a continuous structure and a homogeneous composition 

 throughout their entire mass, still presenting the same relations 

 of form with each other and the preceding crystals, except some 

 trifling inequalities in their angles ; as if the corj)uscles of differ- 

 ent kinds which constitute the mixtures, when they aggregate in 

 crystalline groups identical with each other and of heterogeneous 

 composition, were brought by their mutual reaction to take rela- 

 tive positions, differing very slightly from those which they would 

 have done when arranging themselves separately. It is not dif- 

 ficult to conceive in what manner the analogies so closely de- 

 pendent upon the molecular forces can have been of service to the 

 science of chemistry. It is these analogies which have afforded 

 to it the means of explaining and reducing to the comprehensive 

 law of definite combinations by simple multiples an immense 

 number of natural and artificial products, the complex and irre- 

 gular composition of which appeared to constitute an important 

 exception to that law. Moreover, inasmuch as compound bodies 

 must, according to this principle, be analogically related or op- 

 posite in character according as they may be isomorphous or not 

 with each other, very strong arguments have hence been deduced 

 for the pui'pose of distinguishing, by comparative examination, 

 those bodies which ought to be arranged in the same order or in 

 different orders of atomic combinations; which has fui'nished 

 theoiy with a new groundwoi'k and novel conditions of coordina- 

 tion. Finally, as always happens by the alliance of sciences, the 



