TBANSACTIONS OF SECTION B. 5H9 



compiled) with a treatise of chemistry fifty years old. Let him compare a pub- 

 lished record of one year's progress of the science fifty years ago with one of 

 modern date. Let him compare, as far as may be possible, the number of men 

 who formerly devoted their whole time and energy to the advancement of 

 chemistry, or who were engaged in industrial pursuits involving a knowledge of 

 the science, with the corresponding number now-a-days. Let him count up the 

 services which chemistry had rendered to common life at the commencement of 

 the epoch with those which it has now to show. 



Everywhere he will see marvellous evidences of increasing growth. But if he 

 be a reflecting man, he will not be satisfied with wondering at results : he will 

 endeavour to trace them to their causes, and to discover the guiding principles, 

 which have brought them about : he will try to derive, from a knowledge of 

 those guiding principles, a perception of the means by which such progress can 

 best be continued and extended — how it can be most effectively directed to the- 

 benefit of his fellow-men. 



It is on this aspect of the question that I propose to address you to-day. 



The process of scientific investigation includes a great variety of operations, 

 which may be considered UT\der three headings, mental, sensual, and physical. We 

 think, we observe, and we work with our hands. lu planning a new "experiment 

 we call to mind what is known of the phenomena in question, and form an opinion' 

 as to what is likely to happen under conditions somewhat different from those 

 which existed in previous experiments. We regulate by careful observations the 

 necessary manual operations, so as to obtain with accuracy the desired conditions 

 for the new experiment, and we observe attentively the changes which take place 

 in the course of that experiment. The result of such observations is sometimes 

 in accordance with our anticipation, but very frequently at variance with it. If 

 it accords with our anticipation, we put on record the extension which it has given 

 to the application of the general theory on which that anticipation was founded. 

 But if the result is not what we expected, we carefully and critically revise the 

 reasoning which had led us to expect a particular result, and often repeat the same 

 experiment with greater care, or some modification of it. 



Materials for a new theory are gained when logically fiiultless reasoning, 

 checked by accurate observations, have led to results which could not have been 

 foreseen by the aid of any previous theory'. When a theory has thus gained a 

 footing in science, it serves as a guide in further work. It guides us in arranging 

 known facts. It guides us to the discovery of new facts. Sometimes it does 

 these things for a short time only, and is then superseded by some more general 

 theory derived from a wider and more comprehensive view of the faits. 



There is, perhaps, nowhere so severe and rigorous a test of the truth of an 

 idea as that which is afforded by its use in any accurate department of experi- 

 mental science ; and it is worth while, on philosophical grounds, to consider briefly 

 the conditions of growth of the chief chemical theories which have withstood this 

 ordeal and proved themselves to be trustworthy guides in experimental science. 



Now as far as I know them, the general theories wliich have played the chief 

 part in the development of chemistry are mere condensed statements "of fact. 



Every thoughtful man of science has doubtless indulged in speculations to find 

 the cause of facts which are as yet unexplained ; has imagined some fundamental 

 condition or property of matter which might cause it to produce effects such as 

 are witnessed. It is to be hoped that the time may be far distant when men of 

 science will confine their thoughts within the range of ideas which are proved to 

 be true._ But it is most important that they should not confuse such hypothetical 

 speculations with theories which have received experimental verification, and that 

 while employing any theory, they should not lose sight of the limits within which 

 it has been proved to be correct, beyond which it can only be used as an hypothesis. 



The foundation of the science of chemistry was laid by the discovery of chemical 

 elements; those distinct varieties of matter which we can neither produce nor 

 destroy. Chemical science treats of those changes of property in matter which can 

 be represented as due to changes of combination of elementary atoms. It knows 

 nothing of the production or destruction of those elementary atoms. Speculations 



