THE NEW CHEMISTRY, A DEVELOPMENT OF THE OLD. 119 
conception of binding power being extended to the elements, 
and being deepened and widened by laborious experimental 
researches, led to the general theory of valency, which included 
in itself the essential features of the older doctrine of equiva- 
lents.* 
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 
I parts of chemical compounds which existed as distinct chemical 
entities, i.e ., the molecules, must have a definite structure : that 
the parts (atoms) of the little systems 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 pro- 
duced ; and careful experiment has often succeeded in preparing 
all the different, theoretically possible, compounds. The differ- 
. ence of properties of such compounds, 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 5 or ‘ rational 9 formulas of modem 
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. VT e appear to be now fairly 
embarked in the prosecution of molecular dissection, and our 
chief guide is the theory 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 pro- 
duced had also a certain binding power, but this more indefinite 
than the atomic binding 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. 
TThat 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 important un- 
solved problems of general chemistry. 
Then, again, granting the existence of an inner structure to 
* See former paper, Jan. 1878. 
