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it rise easily) till the coloured liquid is drawn up to zero on 
the scale, then the number of 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 thermo- 
meter, and so whenever a calorimetric determination is 
about to be made the thermometer is pushed down that 
amount beyond its standard level, and just before a deter- 
mination is made the index is introduced and sucked in (by 
means of the thermometer) 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 
the index being adjusted by this simple contrivance involves 
no special parts for the purpose. As this calorimeter 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 would 
have an instrument in which the external or accidental 
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 would have a differential calorimeter. The appa- 
ratus drawn as Fig. II. shows the construction of such an 
THE DIFFERENTIAL CALORIMETER —Fig. II. 
