366 J. W. Langley—Chemical Affinity. 
may be either electrical, thermal or mechanical. We have by 
this theorem a mode of measuring affinity quantitatively; but- 
electrical theories do not establish the nature of this force 
beyond dispute, and this is shown by the fact that the English 
school of electricians, under the leadership of Sir William 
Thomson, have for some years been divided in opinion as to 
the sources of the current developed by the galvanic battery, 
between the old contact theory of Volta and the chemical 
theories of Davy and Faraday. While the electro-chemical 
theory was being developed the founders of the atomic theory 
were not idle, and in that wonderfully fruitful decade which 
witnessed the enunciation of Dalton’s views we find also the 
nucleus of an idea which has an important bearing on the 
nature of affinity. 
In 1811 Avogadro” formulated that law which is now 
regarded as the strongest bolt in the framework of the atomic 
eory; but in stating his conclusions in regard to the number 
of “elementary molecules” in equal volumes of gases he nec- 
essarily introduced the conception of what is now the funda- 
mental distinction between atoms and molecules, and its later 
development into our belief in the two orders of combination, 
atomic and molecular. 
This theorem, of the utmost importance to the atomists, can~ 
not, however, be considered as a material contribution to the 
theory of affinity, because the actions of this force are experi- 
mentally evident and are, therefore, not dependent for their 
verification on the molecular theory; but the form which this 
idea subsequently took in the mind of Brodie was, on the con- 
trary, of the highest importance for it is essentially a new 
conception. 
In his paper entitled, “On the state of the elements at the 
moment of chemical change,” this chemist regards the mole- 
cules of elementary bodies as composed of atoms. The 
en combined is due chiefly to the fact that when free the 
element is combined with itseif in the form of an integral mole- 
cule. He says, “The general object of the paper may be con- 
