74 THE PROGRESS OF SCIENCE n 



practice of our epoch have passed into this de- 

 ductive and synthetic stage, that they are entitled 

 to the name of the " New Chemistry " which they 

 commonly receive. But this new chemistry has 

 grown up by the help of hypotheses, such as those 

 of Dalton and of Avogadro, and that singular 

 conception of " bonds " invented to colligate the 

 facts of " valency " or " atomicity," the first of which 

 took some time to make its way ; while the second 

 fell into oblivion, for many years after it was pro- 

 pounded, for lack of empirical justification. As 

 for the third, it may be doubted if any one regards 

 it as more than a temporary contrivance. 



But some of these hypotheses have done yet 

 further service. Combining them with the mechani- 

 cal theory of heat and the doctrine of the conserva- 

 tion of energy, which are also products of our time, 

 physicists have arrived at an entirely new con- 

 ception of the nature of gaseous bodies and of the 

 relation of the physico-chemical units of matter to 

 the different forms of energy. The conduct of 

 gases under varying pressure and temperature, 

 tin 'ir diffusibility, their relation to radiant lie; it 

 and to light, the evolution of heat when bodies 

 combine, the absorption of heat when they are 

 dissociated, and a host of other molecular pheno- 

 mena, have been shown to be deducible from the 

 dynamical and statical principles which apply to 

 molar motion and rest; and the tendency of 

 physico-chemical science is clearly towards the 



