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



connexion probably existing between the tension and the che- 

 mical composition of vapours, it would be necessary to deter- 

 mine the tensions of many groups of compounds at the same 

 temperature, or, better still, to determine the connexion exist- 

 ing between tension and temperature for each compound 

 throughout a considerable range. Meyer* has well pointed 

 out the disadvantages of the usual practice, which is to deter- 

 mine the temperature at which each vapour exerts a tension 

 equal to 760 millims. of mercury. There is no doubt that 

 careful determinations of vapour-tensions of groups of com- 

 pounds would throw much light upon the composition, and so 

 upon the classification of these compounds!. 



Connected with determinations of boiling-points and of 

 vapour-tensions is the general question of specific and latent 

 heat. We say that bodies have different capacities for heat, 

 or that different quantities of heat must be communicated to 

 equal masses of different bodies in order to produce equal in- 

 crements of temperature in these bodies. Evidently part of 

 the heat communicated has disappeared j and different propor- 

 tions of the total heat have disappeared in each case. We have 

 adopted a practical unit for measuring the amount of heat 

 which has disappeared. We believe that this heat has gone 

 to do work of some kind. Different amounts of work must 

 be done in different elements and compounds before the heat 

 added begins to be apparent to our measuring-instruments. 

 There must surely be differences in the inner structure of these 

 elements and compounds. May we not reasonably hope that 

 careful determinations of those quantities which we call spe- 

 cific and latent heats, carried out for groups of compounds as 

 they have been partly carried out for groups of elements, 

 will throw considerable light upon the composition of these 

 compounds ? 



In connexion with this subject we must look with much 

 interest to the results of Weber's promised researches upon 

 the specific heats of carbon compounds. From his preliminary 

 researches, Weber thought himself justified in concluding that 

 " the specific heat of carbon when in combination is a function 

 of the temperature, and of all those circumstances which in- 

 fluence the quality of the non-carbonated part of the com- 

 pound molecule "J. 



* Die mod, Theor. 2nd edit. pp. 203, 204. 



t See H. Landolt, Ann. Chem. Pharm. Suppl. Bd. vi. p. 129. 



My frieud Mr. J. B. Hannay is now working at tliis question by an 

 original method ; lie has abeady obtained good results, 



\ See translation, by the author, of Weber's paper, " On Specific Heats 

 of Carbon, Boron, and Silicon," in this Magazine for April 1875, [IV.] 

 vol. xlix. pp. 300-302. 



