4 oo THE POPULAR SCIENCE MONTHLY. 



researches, led to the general theory of valency, which included in 

 itself the essential features of the older doctrine of equivalents. 



Having thus gained the conception of a definite binding power as 

 applicable to elementary atoms or groups of atoms, it followed, as an 

 almost necessary deduction, that the smallest parts of chemical com- 

 pounds which existed as distinct chemical entities, i. e., the molecules, 

 must have a definite structure : that the parts (atoms) of the little sys- 

 tems must be arranged in accordance with the valencies, or binding 

 powers, of these parts. 



Hence, given the number of atoms in a molecule, and the valency 

 of each atom, it became possible to calculate the number of different 

 arrangements of these atoms which could be produced ; and careful 

 experiment has often succeeded in preparing all the different, theoreti- 

 cally possible, compounds. The difference of properties of such com- 

 pounds, i. e., of compounds the molecules of which are constituted 

 of the same number of the same atoms, but differently arranged, is 

 attempted to be indicated in the " structural " or " rational " formula? 

 of modern chemistry. 



Berzelius spoke of compounds composed of parts held together by 

 mysterious bonds : the idea survives in these structural formulae of to- 

 day, only we are now able to define what we mean by the smallest 

 part of a compound having a chemical existence, and we have gained 

 certain generalizations which enable us to trace with some degree of 

 accuracy the relationships which exist between the inner parts of these 

 smallest chemical wholes. We appear to be now fairly embarked in 

 the prosecution of molecular dissection, and our chief guide is the the- 

 ory of valency, itself a development of the dualistic chemistry. 



Each elementary atom, I have said, seems to have the power of 

 directly binding to itself a maximum number of other atoms ; but it 

 would further appear as if the groups of atoms thus produced had also 

 a certain binding power, but this more indefinite than the atomic bind- 

 ing power, and very variable under different physical conditions. This 

 atomic binding power appears to have a fixed maximum value, but not 

 always to reach the maximum. What is the exact way in which the 

 binding power or valency of the elementary atoms is influenced by 

 definite changes in physical conditions ? This is one of the most im- 

 portant unsolved problems of general chemistry. 



Then, again, granting the existence of an inner structure to the 

 molecule, granting that groups of atoms do exist in the molecular 

 building, does the fact, that in a certain reaction certain atoms are 

 withdrawn as a group, prove that these existed in the form of the 

 same group in the original molecule ? In other words, do our struc- 

 tural formulae express the relative collocation of atoms within the 

 molecule while the molecule is unacted on by extraneous force, or do 

 they merely roughly represent the condition of things when the mole- 

 cule is in a state of strain, because of the stress between its parts and 



