370 Mr. J. P. Cooke 



on 



To those who consider that Avogadro's law, and therefore the 

 modern theory of chemistry, are direct deductions from the 

 mechanical theory of heat, this course seems inconsistent ; and 

 this inconsistency deprives the work of a very considerable 

 degree of simplicity which might otherwise have been secured. 

 The remarkable progress made in organic chemistry during 

 the last twenty years has resulted almost wholly from the cir- 

 cumstance that the investigators have worked back from the 

 elementary substances to the elementary atoms, and discussed 

 the various modes in which these atoms might be grouped in 

 the molecules. In the same way, in thermo-chemistry we 

 shall find no satisfactory basis until we go back likewise to 

 the atoms, and discuss the thermal effects which attend their 

 union or their separation. One generalization we can already 

 make in regard to atomic work with a great degree of cer- 

 tainty — that the union of atoms is attended with the evolution 

 of heat, and the parting of the same atoms in the same asso- 

 ciations with an equal absorption of heat: and it must be 

 remembered that, as defined by modern chemistry, atoms are 

 definite masses of matter ; so that in enunciating this general 

 principle we refer to a palpable effect as resulting from a well- 

 defined process, entirely independently of the theoretical views 

 we may have in regard to the nature of the chemical atoms or 

 of the modes by which they are united and grouped together. 

 The general principle just stated explains a great many facts 

 of thermo-chemistry which are otherwise anomalous and ob- 

 scure. It, moreover, gives us the basis for a clear theoretical 

 distinction between a chemical and a physical process — the 

 first consisting in the separation or union of atoms, the last in 

 the separation or drawing together of molecules. It is true 

 that the distinction here drawn is, as yet, theoretical; but the 

 theory involved gives us a basis from which to work, and this 

 is enough for the present. The problem of finding what we 

 have called the thermal potential of the atoms is not more 

 remote than many problems which have been successfully 

 solved in organic synthesis; and it is in this direction, as it 

 seems to us, that we can alone expect to reach a measure of 

 chemical affinity. It may, indeed, be found that the pro- 

 blem cannot be solved; and attempts to solve it may lead 

 to results which will modify or supplant our present theories. 

 It may appear that the difference between a chemical and 

 a physical process is one of degree, and not of kind ; but, 

 whatever the result may be, there can be no doubt that the 

 investigation will lead to larger knowledge and clearer con- 

 ceptions. 



As it seems to us, the principle of molecular work should 



