QUANTITY OF HEAT. SPECIFIC HEAT. 77 



It has long been recognised that, at any rate, there is no evidence 

 for its constancy, but determinations of the nature and magnitude of the 

 variation have been so conflicting till recently that experimenters have 

 been too often content to leave it out of account. But measurements 

 of temperature and quantity of heat have so greatly advanced in accuracy, 

 that now the results of different workers begin to show agreement, and 

 there is no longer any doubt as to the existence of a variation, and even 

 its magnitude at ordinary temperatures is probably fairly determined. 



Before giving an account of more trustworthy work, we may illustrate 

 the difficulties of exact calorimetry by briefly describing two earlier 

 researches. 



In 1847 Regnault published an account of experiments in which he 

 sought to determine the specific heat of water at different temperatures 

 by the method of mixtures. A large boiler was so arranged that the 

 water in it could be boiled under different pressures, and therefore at 

 different tempera tures, these ranging from 107 to 190. When the 

 water was at the boiling temperature a quantity of about 10 kgm. was 

 rapidly forced into a large gauged calorimeter containing a known 

 quantity about 100 kgm. of cold water, at an observed temperature 

 ranging from 8 to 14. From the temperature of the mixture the ratio 

 of the mean specific heat over the rise of temperature in the one case 

 to that over the fall in the other could be determined. A series of 

 experiments led Kegnault to express the specific heat at t, taking the 

 specific heat between and 1 as 1, by 



Specific heat at t = 1 + '00004* + -0000009* 2 . 



From this the specific heat at 15 is 1-0008, while the mean specific heat 

 from to 100 is 1-005. 



In 1870 Jamin and Amaury described an electrical method. The 

 calorimeter was of thin copper and contained 350 grammes of water. 

 It was surrounded by a spiral of insulated German-silver wire of known 

 resistance, through which a known current could be passed, to supply a 

 known quantity of heat. Outside the spiral was a layer of swansdown 

 of such low conductivity that nearly all the heat generated was con- 

 ducted inwards to the water. Outside the swansdown packing was a 

 thin polished copper vessel. This was placed in the middle of a double- 

 walled enclosure, containing water between the walls. In this water 

 was another spiral through which a current could be passed, so as to 

 make the temperature of the enclosure rise at the same rate as that 

 of the outside of the calorimeter, and thus eliminate any radiation 

 correction. The results obtained were expressed by 



Specific heatat e = l + -0011< + -0000012< 2 . 



We shall realise how widely this differs from Regnault's value by 

 noting that it gives the specific heat at 15 as 1-0168, and the mean 

 specific heat from to 100 as T06. 



The first results to which any value can now be attached were pub- 

 lished by Rowland in 1879 * in an account of experiments on the 

 mechanical equivalent of heat, to be described later. Here it is sufficient 



* Physical Papers, p. 343. A recalculation of the results in terms of the scale of 

 the Paris hydrogen thermometer is given by Dr. W. S. Day, Phil. Mag., July 1898. 



