MR. J, B. HANNAY ON A NEW CALORIMETER. 245 



the indeXj I have found mercury not sufficiently mobile ; 

 so I have used instead a coloured aqueous solution pre- 

 ferably containing a little glycerine. The calorimeter is 

 firmly clamped and enclosed in wool, and a small drop of 

 coloured liquid is made to touch the end of the index-tube, 

 when it is at once drawn. The number on the scale of the 

 thermometer level with the stopper is read off, and the 

 thermometer, which has been slightly greased, is pulled up 

 (screwing it from side to side to make it rise easily) till 

 the coloured liquid is drawn up to zero on the scale ; then 

 the number on the thermometer-stem level with the stopper 

 is again read off. We have then the volume of the scale- 

 tube in terms of degrees on the thermometer ; and so, when- 

 ever a calorimetric determination is about to be made, the 

 thermometer is pushed down that amount beyond its 

 standard level, and just before a determination is made the 

 index is introduced and sucked in (by means of the ther- 

 mometer) to zero. It will thus be seen that the calorimeter 

 is open to the air and in a state of equilibrium until the 

 experiment is about to be made ; and as the index is ad- 

 justed by the above-mentioned simple contrivance, no 

 special parts are required for the purpose. As this calo- 

 rimeter can be made and used by any student, I have 

 found it extremely useful in inculcating the principles of 

 thermal chemistry. 



It naturally occurred to me that by connecting the end 

 of the index -tube with another air-vessel, which would be 

 kept in the same conditions as the calorimeter-vessel, we 

 should have an instrument in which the external or acci- 

 dental variations affecting both sides of the system would 

 in no way affect the equilibrium, and the index would only 

 be moved by any difference affecting only one vessel — or, in 

 other words, we should have a differential calorimeter. 

 Fig. 2 shows the construction of such an instrument. Two 



