224 



ing, without receiving or giving out energy in any form ; so that the 

 equation taken to represent their condition was 



A T= 0. 



This condition was realized in the early experiments of Mr Joule, 

 where, by the sudden opening of a stopcock, air previously confined 

 in one vessel was allowed to fill another also ; but it is not exactly 

 realized in the experiments now in progress by Messrs Joule and 

 Thomson, for which the correct equation is 



A (T + P V) = 0. 



Hence the approximate positions of the point of absolute cold cal- 

 culated by means of the former equation, require a small correction. 

 The author computes the values of this correction for two series of 

 experiments, made at a high and a low temperature respectively ; 

 and finds them to be — 



+ 0°*05 Centigrade for the high temperature, 

 — 0°'002 Centigrade for the low temperature ; 



so that for the experiments now in question, the correction is prac- 

 tically inapplicable. As it may, however, have a sensible amount 

 for greater ranges of temperature and pressure than those which 

 occur in the particular experiments referred to, and for gases denser 

 than atmospheric air, the author explains how it is to be calculated. 



The following Donations to the Library were announced : — 



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