PRESIDENTIAL ADDRESS. MACKAY Ixi 



far beyond the powers of the most delicate balance to weigh or of 

 the most powerful microscope to reveal and we have a fair con- 

 ception of Dalton's atom and of the atomic hypothesis. 



A scientific hypothesis to be of value must serve two purposes: 

 it must satisfactorily explain the facts already known and it must 

 Doint the way to fresh discoveries. Judged by this criterion, the 

 atomic hypothesis is among the most valuable in the " history of 

 science. Its effect in stimulating research was immediate and 

 permanent. Under its influence, in the second decade of the last 

 century, the great Berzelius carried out the gigantic work necessary 

 to establish the laws of chemical proportions, which henceforth 

 became the foundation of all chemical research. And from that 

 time until the present the atomic theory has dominated chemical 

 thought. 



Dalton assumed the atom to be indivisible and of constant 

 mass or weight, but made no assumption regarding its other 

 properties, for example, its size or shape or colour or any of its 

 physical characteristics. But as investigation proceeded this con- 

 ception was modified in two directions. On the one hand the idea 

 was advanced that the atom might itself be composite ; on the other 

 hand it was endowed with certain new properties. The first of 

 these modifications was proposed within a decade of Dal ton's 

 publication of his theory. Prout, an English physician, observing 

 that atomic weights as then determined were all either whole 

 numbers or very nearly whole numbers if the weight for hydrogen 

 were made unity, put forward the hypothesis that hydrogen was 

 the one primordial substance of which all other elements were 

 composed, their atoms being simply groups of hydrogen atoms. 

 This hypothesis has proved itself one of the most seductive in 

 the history of science. It appealed to the imagination of the 

 chemical philosopher since it revived the ancient idea of the one- 

 ness of matter and provided a soul-satisfying unity underlying, the 

 infinite multiplicity of chemical changes. It was soon found, 

 however, that several atomic weights could not be expressed by 

 whole numbers. That of chlorine, for example, was certainly 

 nearer 35.5 than 35. To meet these facts it was assumed that the 



