President's Address for the year 1887. 



xix 



them is that doctrine concerning the constitution of matter, 

 which for want of a better name, I will call ' molecular ;' 

 the second is the doctrine of the conservation of energy ; 

 the third is the doctrine of evolution. Each of these was 

 pre-shadowed, more or less distinctly, in former periods in 

 the history of science ; but, so far is either from being the 

 outcome of purely inductive reasoning, that it would be 

 hard to overrate the influence of metaphysical, or even 

 theological considerations on the development of all three. 

 The peculiar merit of our epoch is, that it has shown how 

 these hypotheses connect a vast number of seemingly 

 independent partial generalisations, and it has given them 

 that precision of expression which is necessary for their 

 exact verification, and that it has practically proved their 

 value as guides to the discovery of new truth. All these 

 three doctrines are intimately connected, and each is 

 applicable to the whole physical cosmos. But as might 

 have been expected from the nature of the case, the first 

 two grow mainly out of the consideration of physico- 

 chemical phenomena, while the third in great measure owes 

 its rehabilitation, if not its origin, to the study of biological 

 phenomena." 



To dilate upon these three great doctrines would take far 

 more time than could be spared to-night, and would need 

 language intelligible but to few. I shall, therefore, content 

 myself with stating that as regards the first, atoms are no 

 longer matters of speculation as in the days of Democritus 

 and Lucretius, but are real recognisable units, the relative 

 weight of volume of which are well-known, while several 

 independent but converging lines of investigation enable us 

 to approximate to their actual size. That as regards the 

 second, the last half century has witnessed numerous careful 

 experiments, demonstrating that heat and mechanical energy 

 are interchangeable, so much heat disappearing for so much 

 mechanical work done, or so much mechanical work expended 

 m the production of a corresponding quantity of heat. 



In connection with these researches the name of Joule has 

 acquired an imperishable fame, and his labours in this 



