Mr. M. M. Pattison Muir on Chemical Classification. 191 



recognition has closed their eyes to other truths. Do not the 

 upholders of what may be called the ^'energy theory" some- 

 times overlook the paramount importance of '^composition "? 

 Granting that " similarity of action is correlated with similarity 

 of composition " *, how are we to measure the '' action " of 

 chemical compounds ? 



What I have to say of general chemical analogies as aiding 

 us in answering this question I shall defer until I come to 

 speak of MendelejefF's periodic law. The physical properties 

 of compounds present us with measurable quantities ; and it 

 seems to me that by measuring certain of these we have a 

 means of arriving at a more definite knowledge concerning 

 the relations existing between structure and chemical function. 



21. In attempting to show that there is a connexion between 

 the physical properties of compounds and their composition, 

 as part of the wider problem of the connexion between che- 

 mical character and the valency of the combining atoms, I 

 must content myself with the merest outline of the field of 

 investigation. 



In a paper on Isomerism, published in this Magazine 

 (September 1876), I drew attention to the probable connexion 

 existing between the boiling-points and the composition of 

 groups of chemical compounds. Isomeric substances exhibit 

 different boiling-points or different degrees of fluidity : we can 

 scarcely do otherwise than attempt to correlate these with dif- 

 ferences in structure ; and in the paper cited I adduced ex- 

 amples to show that we have some evidence for such a corre- 

 lation!. The melting-points of solid substances are also, in 

 all probability, functions of the structure of these bodies. Few 

 attempts have as yet been made towards systematizing the 

 results of experiments upon melting-points; accurate and 

 numerous data must be first obtained. 



Most intimately connected with the boiling-points of liquids 

 is the question of the vapour-tensions of these liquids. Every 

 vapour exerts a certain pressure or tension, which is mainly 

 dependent upon the temperature of the liquid. The upholders 

 of the molecular theory of matter regard this tension as due 

 to the motion of the molecules, to the striving of these mole- 

 cules to escape into the less closely packed atmosphere sur- 

 rounding them. The higher the temperature of the liquid the 

 more rapid will be the movements of the molecules ; the greater 

 therefore will be the tension of the vapour. The tension of a 

 vapour is a measurable quantity; it may be regarded as a 

 variant, temperature being the variable. In order to trace the 



* Die modernen Thewien, 2nd edit. p. 173. 



t See H. Kopp, Ann. Chem, Pharm, xcvi. p. 2 ; Kekule, Lehr. der org. 

 Chem, i. p. 281 et seq. 



