480 EEPORX— 1882. 



theory of heat, and of the molecular theories which have grown up in consequence 

 of it, have done much to set our minds free from preconceived notions, and to 

 induce us to huild chemical theories on something more than luiverified con- 

 jectures. 



But how far can we say that mechanical principles are actually recognised as 

 the true basis of rational chemistry ? So far as I know no chemist denies that it is 

 so, and yet how little do our text-books, even the most recent and the most highly 

 reputed, show the predominance of this idea ! How very small a portion of such 

 books is taken up with it, how much seems utterly to ignore it or to be couched in 

 language antagonistic to it ! We still find chemical combinations described as if 

 they were statical phenomena, and expressions used which imply that two perfectly 

 elastic bodies can, by their mutual action alone, bring each other into fixed relative 

 positions. We still find change of valency described as a suppression of ' bonds 

 of affinity,' as if a suppression of forces were the usual course of nature, or as if it 

 were possible that the same two forces, acting at the same place and in the same 

 direction, should at one time neutralise one another, and at another time not 

 neutralise one another. We still find saturated compounds spoken of as if the 

 stability of a compoimd were independent of circumstances, and chemical combina- 

 tion no function of temperature and pressure. Beginners are sometimes helped by 

 the invention of intermediate reactions in explanation of final residts, without any 

 reference to the dynamical conditions of the problem, without any consideration 

 whether the fancied intermediate reactions imply a winding-up or a runniug-dowu 

 of energy. In fact our long familiar chemical equations represent only the conser- 

 vation of matter, and to keep always in mind the mechanical conditions of a 

 reaction is as difficult to some of us as it is to think in a foreign language. More- 

 over, we still find in many of our text-books the old statical notion of chemical 

 combination stereotyped in pictures of molecules. I do not, of course, mean to 

 accuse the distinguished inventors of graphic formidee of meaning to depict mole- 

 cules, for I believe that they would agree with me in thinking that these diagrams 

 do not any more nearly represent actual molecides than they represent the solar 

 system ; but unfortunately we cannot prevent beginners from regarding them as 

 pictures, and moulding their ideas upon them. They present something easily 

 grasped by the infant mind, and schoolmasters are fond of them ; but only those 

 who have each year to combat a fresh crop of misconceptions, and false mechanical 

 notions engendered by them, can be aware how much they hinder, I won't say the 

 advance, but the spread of real chemical science. If it be true that the illustrations 

 of an artist like the late Hablot Browne give to our conceptions of the characters 

 of a story a more definite and permanent, though perhaps a much modified form of 

 what the author of the story intended to portray, it is equally true that the illus- 

 trations by which some, even great names amongst us, have tried to make us fancy 

 that we had a true conception of some natural process, have become so fixed in our 

 minds as to prevent our realising the true meaning of nature. 



What, then, is the progress which I think has been made in physical chemistry ? 

 In the first place, notwithstanding the slowness with which new ideas replace old 

 familiar images, the molecular theories developed by Clausius, Clerk Maxwell, and 

 Boltzmann, and by Sir W. Thomson, have been long enough before the world to 

 have greatly loosened the hold upon our minds of many old notions. The rigid, 

 unbreakable, impenetrable atoms of the Epicurean philosophy, made familiar to us 

 by Lucretius, always presented difficulties whicli were only perhaps exceeded by 

 those of the elastic atmospheres with which modern philosophers fimcied them to 

 be surrounded ; but now the vortex theory, whether we think it probable or not, 

 at least gives us a standing ground for the assertion that the supposed impenetra- 

 bility of matter, and the curious compound of nucleus and atmosphere which had 

 been invented to account for elasticity, are not necessary assumptions. 



The kinetic theory of gases has analysed for us the different motions of the 

 molecules in a mass of matter, and has facilitated the conception of the part which 

 beat plays in chemical action. Hence we have had of late several attempts to 

 reduce to a form susceptible of mathematical calculation the problems of chemistry. 

 Most of these attempts have proceeded on the well-known mechanical principle 



