1903.] Specific Heats, especially at Low Temperatures. 



187 



Ice Method* 



When an object cooled by means of liquid air is plunged in water,, 

 a firm coating of ice is rapidly formed. If the water is not stirred, 

 the ice remains unmelted for a considerable time. This suggested the 

 weighing of the ice as a possibly practical method of determining the 

 specific heat of the material. After a number of preliminary experi- 

 /ments, it was found that the method was capable of yielding very con- 

 sistent results. Thus five experiments were made with a ball of 

 commercial zinc weighing 106J grammes, giving the following 

 numbers : 



The calorimeter contained water kept at very nearly C. by means 

 of an ice jacket, and the time of immersion in the calorimeter in each 

 of the above experiments was five minutes. The initial temperature 

 was deduced from the reading of a platinum thermometer observed 

 J or | minute before extracting the object from the bath of liquid air. 

 After removal from the calorimeter, the object was transferred to 

 a vessel in which the ice was dried by contact with filter-paper cooled 

 by means of melting ice, and was finally transferred to another vessel in 

 which the ice was weighed. The number assumed for the latent heat 

 of water is 80 '0, this appearing to the author the best value he could 

 take for the purpose of reducing his results to the unit used in the 

 preceding part of the investigation. 



The method appeared to offer advantages in its comparative simplicity 

 and in its avoidance of thermometrical difficulties, and the author felt 

 encouraged to attempt an extended examination of the method, with 

 a view to the estimation of the relative importance of the sources of 

 error. He here selects the more important considerations. 



(i) An object which carries a quantity of liquid air on its upper 

 surface, for instance, a cylinder with axis vertical, may by the method 



* [Note added 2?th May, 1903. The author has learned that the method of 

 weighing an ice- jacket is not new. It was briefly described by Joly in ' Nature,' 

 1895, vol. 52, p. 80, and was subsequently independently invented and used by 

 Dewar. See also a paper by Bedford and Green, * B. A. Report,' 1901, p. 544. ii* 

 which is described a calorimeter resembling in some of its features that used by/ 

 the author. 



