588 PRINCIPLES OF CHEMISTRY 



must remark, however, that as the specific heat or the amount of heat 

 required to raise the temperature of a unit of weight one degree 6 is a 



extent be seen in the specific heat of the oxides. Thus for alumina, A1 2 O S (Q= 0-217), 

 MQ = 22-8, and therefore the quotient MQ/n = 4'5, which is nearly that given by 

 magnesium oxide, MgO. But if we ascribe the same composition to alumina, as to 

 magnesia that is, if aluminium were counted as divalent we should obtain the figure 

 8*7, which is much less. In general, in compounds of identical atomic composition 

 and of analogous chemical properties the molecular heats MQ are nearly equal, as 

 many investigators have long remarked. For example, ZnS = 11-7 and HgS = 11-8 ; 

 MgS0 4 = 27'0 and ZnSO 4 =28'0, &c. 



6 If W be the amount of heat contained in a mass m of a substance at a temperature 

 1, and dW the amount expended in heating it from t to t + dt, then the specific heat 

 Q dWt(m x dt). The specific heat not only varies with the composition and complexity 

 of the molecules of a substance, but also with the temperature, pressure, and physical 

 state of a substance. Even for gases the variation of Q with t is to be observed. Thna 

 it is seen from the experiments of Regnault and Wiedemann that the specific heat of 

 carbonic anhydride at = 0'19, at 100 = 0'22, and at 200=0-24. But the variation of 

 the specific heat of permanent gases with the temperature is, as far as we know, very in- 

 considerable. According to Mallard and Le Chatelier it is = O'^jp 6 per 1, where M is 



the molecular weight (for instance, for O 2) M = 82). Therefore the specific heat of those 

 permanent gases which contain two atoms in the molecule (H 2 , O 2 , N 2 , CO, and NO) 

 may be, as is shown by experiment, taken as not varying with the temperature. The 

 constancy of the specific heat of perfect gases forms one of the fundamental propositions 

 of the whole theory of heat and on it depends the determination of temperatures by means 

 of gas-thermometers containing hydrogen, nitrogen, or air. Le Chatelier (1887), on the 

 basis of existing determinations, concludes that the molecular heat that is, the 

 product MQ of all gases varies in proportion to the temperature, and tends to become 

 equal ( = 6-8) at the temperature of absolute zero (that is, at -273); and therefore 

 MQ = 6'8-fa(273 + ), where a is a constant quantity which increases with the complexity 

 of the gaseous molecule and Q is the specific heat of the gas under a constant pressure. 

 For permanent gases a almost Q, and therefore MQ = 6*8 that is, the atomic heat (if the 

 molecule contains two atoms)= 8'4, as it is in fact (Chapter IX., Note 17 bu ). As regards 

 liquids' (as we as the vapours formed by them), the specific heat always rises with the 

 temperature. Thus for benzene it equals 0-88 + 0-0014*. R. Sohiff (1887) showed that the 

 variation of -the specific heat of many organic liquids is proportional to the change of 

 temperature (as in the case of gases, according to Le Chatelier), and reduced these 

 variations into dependence with their composition and absolute boiling point. It is very 

 probable that the theory of liquids will make use of these simple relations which recall 

 the simplicity Jot the variation of the specific gravity (Chapter II., Note 84), cohesion, 

 and other properties of liquids with the temperature. They are all expressed by the 

 linear function of the temperature, a + bt, with the same degree of proximity as the property 

 of gases is expressed by the equation pv=Bt. 



As regards the relation between the specific heats of liquids (or of solids) and of their 

 vapours, the specific heat of the vapour (and also of the solid) is always less than that 

 of the liquid. For example, benzene vapour 0-22, liquid 0-38 ; chloroform vapour 0*18, 

 liquid 0-28 ; steam 0*475, liquid water 1-0. But the complexity of the relations exist- 

 ing in specific heat is seen from the fact that the specific heat of ice = 0-502 is less 

 than that of liquid water. According to Regnault, in the case of bromine the specific 

 heat of the vapour =0-055 at (150), of the liquid = 0-107 (at 80), and of solid bromine 

 =0-084 (at -15). The specific heat of solid benzoic acid (according to experiment and 

 calculation, Hess, 1888) between and 100 is 0-81, and of liquid benzoio acid 0*50. 

 One of the problems of the present day is the explanation of those complex relations 

 which exist between the composition and such properties as specific heat, latent heat, 

 Kpaasion by heat, compression, internal friction, cohesion, and so forth. They can 



