MOLECULES AND ATOMS 829 



of carbon bisulphide, CS 2 =90, of ether, C 4 H, 0, = 94, of benzene, 

 C 6 H 6 , = 109, of alcohol, C 2 H 6 O, = 200, of chloroform, CHC1 3 , = 67, 

 Ac., show the amount of heat expended in converting one part 

 by weight of the above substances into vapour. A great uniformity 

 is observed if the measure of this heat be referred to the weight of the 

 molecule. For carbon bisulphide the formula CS 2 expresses a weight 

 76, hence the latent heat of evaporation referred to the molecular 

 quantity CS 2 = 76 x 90=6,840, for ether=9,656,for benzene=8,502, for 

 alcohol =9, 200, for chloroform =8,007, for -water =9, 6 20, &c. That is, 

 for molecular quantities, the latent heat varies comparatively little, 

 from 7,000 to 10,000 heat units, whilst for equal parts by weight it 

 is ten times greater for water than for chloroform and many other 

 substances. 27 



Generalising from the above, the weight of the molecule determines 

 the properties of a substance independently of its composition i.e. of 

 the number and quality of the atoms entering into the molecule 

 whenever the substance is in a gaseous state (for instance, the density 

 of gases and vapours, the velocity of sound in them, their specific 

 heat, <kc.), or passes into that state, as we see in the latent heat of 

 evaporation. This is intelligible from the point of view of the atomic 

 theory in its present form, for, besides a rapid motion proper to the 

 molecules of gaseous bodies, it is further necessary to postulate that 

 these molecules are dispersed in space (filled throughout with the 

 luminiferous ether) like the heavenly bodies distributed throughout 

 the universe. Here, as there, it is only the degree of removal (the dis- 

 tance) and the masses of substances which take effect, while those 

 peculiarities of a substance which are expressed in chemical trans- 

 formations, and only come into action on near approach or on contact, 

 are in abeyance by reason of the dispersal. Hence it is at once obvious, 



87 If the conception of the molecular weights of substances does not give an exact 

 law when, applied to the latent heat of evaporation, at all events it brings to light a 

 certain uniformity in figures, which otherwise only represent the simple result of obser-. 

 ration. Molecular quantities of liquids appear to expend almost equal amounts of heat 

 in their evaporation. It may be said that the latent heat of evaporation of molecular 

 quantities is approximately constant, because the vis viva of the motion of the molecules 

 is, as we saw above, a constant quantity. According to thermo-dynamics the latent heat 



of evaporation is equal to . (n' n) -P 13*59, where t is the boiling point, n' the 

 E dT 



specific volume (i.e. the volume of a unit of weight) of the vapour, and n the specific 

 volume of the liquid, dp/dT the variation of the tension with a rise of temperature per 1, 

 and 18*89 the density of the mercury according to which the pressure is measured. Thus 

 the latent heat of evaporation increases not only with a decrease in the vapour density 

 (i.e. the molecular weight), but also with an increase in the boiling point, and therefore 

 depends on different factors. 



