PRESIDENTIAL ADDRESS MACKAY Ixv 



making the compound and may point to new and unexpected 

 properties, which in turn can be verified by further experimenta- 

 tion; and thus knowledge grows from more to nlore. 



A striking illustration of this process is furnished by the 

 benzene theory. This is simply a mental j^ictnre of tlie relation 

 of the atoms in a molecule of the hydrocarbon benzene. Its publi- 

 cation in 1865 led to an unexampled advance in the knowledge 

 of that great class of organic compounds known as benzene 

 derivatives, which include, among many other substances, the 

 aniline dyes ; and it was this advance which made possible the 

 great German colour industry of to-day with its millions of capital, 

 its army of workmen and, last but not least, its alluring dividends. 

 No more practical proof than this of the utility of a theory can 

 reasonably be demanded. 



But this very utility lias had a train of evil consequences. So 

 universally useful has the atomic theory been in explaining the 

 properties of matter that in some quarters the existence of atoms 

 is tacitly assumed as a fact. Careless writers of elementary text- 

 books are especial sinners in this respect, and in consequence many 

 beginners in chemistry acquire as firm a belief in the reality of 

 atoms as in the existence of footballs or chocolates, to the complete 

 subversion of all clear thinking in chemical subjects. I have in 

 mind a text-book of elementary chemistry which I keep by me 

 as a constant reminder of how the subject should not be presented. 

 On one of the first pages of this book, the author, having defined 

 "mass" and "molecule" in the same breath, directs the attention 

 of the student to a piece of sugar and inquiries : 



"Cannot the smallest particle of sugar, the molecule, be 

 separated into still smaller particles of something else? May it 

 not be a compound bodv, and will rot somp force separate it into 

 fwo or n<nre substances? The next experiment will answer this 

 rjuestioii." 



The pupil is then instructed to ])our some sulphuric acid on 

 =ugar. The sugar is charred, and the author, after pointing out 

 *^hat this action is an example of chemical change continues as 

 follows: 



