PRESIDENT'S ADDRESS. xi 



synthesized urea. According to this hypthesis when a compound 

 of, say, carbon, hydrogen and oxygen is formed, the smallest particle 

 of the compound capable of existing is some sort of little group 

 or association of definite numbers of atoms of carbon, hydrogen 

 and oxygen ; and any given portion of the compound, say a pound 

 of it, is simply an exceedingly large number of such little groups, 

 massed together. These little groups of atoms are now called 

 molecules. This picture of what we may call the invisible 

 mechanism of matter may or may not be a true one; but the true 

 mechanism, whatever it is, produces exactly the same visible effects 

 as would result from the atomic hypthesis. This hypthesis, there- 

 fore, as far as it goes, serves the same practical purposes as a 

 knowledge of the actual constitution of matter. It will be noticed, 

 however, that it only provides us with a skeleton mechanism, leaving 

 details to be filled in; and chemists soon felt the need of supple- 

 menting it with additional hyptheses. Wohler's discovery, already 

 cited, furnishes an illustration of facts which made this need 

 apparent. Ammonium cyanate and urea have exactly the same 

 composition, which is expressed in terms of the atomic hypothesis 

 by the formula CH 4 N 2 O ; that is, in every smallest particle or 

 molecule we may suppose that one atom of carbon is associated 

 with four of hydrogen, two of nitrogen and one of oxygen. It 

 cannot be that these different atoms are associated in haphazard 

 fashion, like so many different coloured marbles thrown into a bag. 

 On the contrary, there must be one definite arrangement of them 

 that gives ammonium cyanate and another that gives urea. So 

 much is evident; but how is the arrangement in each case to be 

 determined ? And until the arrangement of atoms in the molecule 

 of a given compound is known, or whatever it is that corresponds 

 to this in the true mechanism of matter, how is the synthesis of 

 the compound to be anything more than a lucky chance? 



It is now clear why progress in the synthesis of organic com- 

 pounds had been slow. The problem of constitution had first to 

 be solved or at least some working hypothesis had to be formulated 

 which would be a sufficient approximation to the truth to serve 

 practical purposes. The history of Chemistry from 1820 to 1860 

 is characterized by successive attempts to attain this end. Berzelius' 

 electro-chemical theory, the radical theory, the substitution theory, 



