Barrett — On a New Form of Calorimeter. 15 



supported by knife edges, E, somewhere near its centre of gravity. 

 From the end of the stem a pan, F, depends, and beyond this a 

 pointer, fixed to the stem, moves over a graduated arc. The pan 

 is made of the right weight to exactly equipoise the arrangement 

 at a given air temperature. The weighing of the liquid in the 

 cup, A, is taken at this temperature ; otherwise the Varying length 

 of the thread of mercury in the stem, B, would derange the balance. 

 Except as a thermometer- stopped funnel for the liquid, the burette 

 is, of course, not required in this arrangement. 



In making a determination of the specific heat of a liquid with 

 this instrument we require no cool liquid in A to mix with the 

 warm liquid from C, for the bulb of the thermometer, A, itself 

 forms both the containing vessel and the cool material to be 

 warmed. All that is necessary is a careful determination, made 

 once for all, of the heat capacity of the instrument. This constant 

 factor, K, may in fact be found and stamped upon the base of each 

 instrument before it leaves the maker's hands. As the constant is 

 determined in precisely the same way that subsequent measure- 

 ments are made, it includes, or rather it enables us to evade, all 

 corrections, such as those due to the heat capacity of the vessel and 

 of the thermometer, and also the loss of heat due to cooling if the 

 same range of temperature be employed ; and indeed, if otherwise, 

 the silvered jacket and cover (which latter is necessary to prevent 

 loss from evaporation) render the correction from this source negli- 

 gible in ordinary work. 



The constant, K, is found as follows : — 



Let W be the weight of water used, T its original temperature 

 (viz. that indicated by the thermometer-stopper) ; let t be the ori- 

 ginal temperature of the calorimeter (given on the stem, B), and 

 its highest reading after the water has entered. As the heat lost 

 on the one hand is equal to the heat gained on the other, and the 

 specific heat of water is unity, 



W(T-d)=K(6-t); 

 whence 



T-e 



K= W 



e-f 



When a determination of the specific heat, S, of a liquid has to be 

 made, warm the liquid, and pour it into the burette ; note as before 



