INAUGURAL ADDRESS. 7 



truthfully from what we see to what we cannot see, but may none the 

 less know ? 



Our knowledge of the existence of molecules and the laws of 

 their movements in space came mainly from the study of gases. 

 Our knowledge of the atoms which compose them and of their 

 ■characteristic groupings or configurations came from the study 

 of those so-called chemical changes which occur when different 

 sorts of molecules collide together under favourable conditions — 

 ■changes which result in the re-arrangement of the atoms to foim 

 new sorts of molecules, and which therefore convert old forms of 

 matter into new. Luckily for science, all these chemical changes, 

 from which we can learn so much, are subject to our own control 

 and can be brought about at will as soon as we have learned the 

 requisite conditions ; and thus chemistr}^ is pre-eminently an 

 •experimental science, unlike astronomy ; for astrononi)-, after all, 

 •can only observe nature and can never force her hand. 



For chemical action to occur between molecules, they must be 

 free to collide with one another, so as to bring their constituent 

 atoms within the range of new attractions. The solid state of 

 aggregation hardly permits of this ; but the conditions may be 

 favourable when gas is mixed with gas, or liquid with liquid, or 

 when gases, liquids or solids are dissolved in a suitable liquid. So 

 important, indeed, is the process of solution as a condition ante- 

 ■cedent to chemical change that chemists have long felt impelled 

 to make the nature of that process itself a special study. I need 

 hardly remind you that most of the chemical changes that interest 

 the geologist and most of those in the domain of the physiologist 

 are changes that occur between substances in solution ; nor that, 

 though there are many other solvents, water is, as a fact, by far 

 the most frequent one in nature. Hence it was natural that 

 chemists should pay great attention to the study of aqueous 

 solutions. 



At the Christchurch meeting in 1891, in a presidential address 

 to Section B, I directed members' attention to the then recent 

 work on solution by van't Hoff and Arrhenius and to the new 

 light which it had thrown on our views about molecules and their 

 ■chemical changes. Briefly, van't Hoff ascertained from an exami- 

 nation of the experimental work of Pfeffer on osmotic pressure, 

 that of Raoult and others on the vapour pressures and freezing 

 points of solutions, and from other evidence, that the molecules 

 of dissolved substance follow the same quantitative mathematical 

 laws connecting their concentration, pressure, and temperature 



