ADDRESS. 17 



thinking that the next great advance, of which we ah-eady have some 

 foreshadowing, will come on this side.' 



There is, perhaps, no branch of our science in which the doctrine of 

 the Daltonian atom plays a more conspicuous part than in organic chemis- 

 try or the chemistry of the carbon compounds, as there is certainly none 

 in which such wonderful progress has been made during the last fift}' 

 years. One of the most striking and perplexing discoveries made rather 

 more than half a century ago was that chemical compounds could exist 

 which, whilst possessing an identical chemical composition, that is con- 

 taining the same percentage quantity of their constituents, are essentially 

 distinct chemical substances exhibiting different properties. Dalton was 

 the first to point out the existence of such substances, and to suggest that 

 the difference was to be ascribed to a different or to a multiple arrange- 

 ment of the constituent atoms. Faraday soon afterwards proved that 

 this supposition was correct, and the research of Liebig and Wohler on 

 the identity of composition of the salts of fulminic and cyanic acid gave 

 further confirmation to the conclusion, leading Faraday to remark that 

 ' now we are taught to look for bodies composed of the same elements in 

 the same proportion but differing in their qualities, they may probably 

 multiply upon us.' How true this prophecy has become we may gather 

 from the fact that we now know of thousands of cases of this kind, and 

 that we are able not only to explain the reason of their difference by 

 virtue of the varying position of the atoms within the molecule, but even 

 to predict the number of distinct variations in which any given chemical 

 compound can possibly exist. How large this number may become will 

 be understood from the fact that, for example, one chemical compound, 

 a hydrocarbon containing thirteen atoms of carbon combined with twenty- 

 eight atoms of hydrogen, can be shown to be capable of existing in no less 

 than 802 distinct forms. 



Experiment in every case in which it has been applied has proved the 

 truth of such a prediction, so that the chemist has no need to apply the 

 cogent argument sometimes said to be used by experimentalists enamoured 

 of pet theories, ' When facts do not agree with theory, so much the worse 

 for the facts ! ' This power of successful prediction constitutes a high- 

 water mark in science, for it indicates that the theory upon which such a 

 power is based is a true one. 



But if the Daltonian atom forms the foundation of this theory, it is 

 upon a knowledge of the mode of arrangement of these atoms and on a 

 recognition of their distinctive properties that the superstructure of 

 modern organic chemistry rests. Certainly it does appear almost to 

 verge on the miraculous that chemists should now be able to ascertain 

 with certainty the relative position of atoms in a molecule so minute 

 that millions upon millions, like the angels in the schoolmen's dis- 

 cussion, can stand on a needle's point. And yet this process of orientation 

 is one which is accomplished every day in our laboratories, and one which 

 more than any other has led to results of a starthng character. Stdl this 

 1887. J 



