Modification of 'Bunsen's Calorimeter. 173 



greatly to the large bore of the capillary tube, which neces- 

 sitated the use of a large quantity of substance, the conse- 

 quence being that the mercury in the bulb was so much heated 

 that the loss of heat by radiation was not inappreciable during 

 the time of performing the experiment. 



Since then a new instrument has been obtained from Mr. 

 Casella, having a much finer capillary stem with an empirical 

 scale engraved on it. By means of it very good results have 

 been obtained with very small quantities of substances ; in 

 fact, it has been found (within certain limits) that the smaller 

 the mass of the substance the better. 



At first we endeavoured to make allowance for the loss of 

 heat during the experiment by constructing a curve of coolings 

 and adding to the rise obtained by dropping in the substance 

 the amount that had been lost while the experiment was 

 being performed, according to the curve of cooling. This 

 method, however, did not yield good results. 



Our final method of operatiug is comparative only ; that is 

 to say, one metal, whose specific heat is accurately known, is 

 chosen as the standard to which all other substances are re- 

 ferred, and, in order that the conditions may be the same in 

 all cases, we first ascertain the rise obtained by introducing 

 a given weight of mercury at a given temperature (that of 

 the air), and when the instrument has cooled throw in the 

 substance whose specific heat is desired. During the two 

 experiments the temperature of the air is usually nearly con- 

 stant ; and in that case no correction for temperature is 

 required. 



A second point to be noted, is to put in such a weight of 

 substance as will produce about the same rise as was obtained 

 with the mercury. It is thus possible to make the determi- 

 nation of the relative specific heats under almost precisely the 

 same circumstances, a condition which is eminently conducive 

 to good results. 



In practical working with the instrument the following 

 hints may prove useful : — 



1. The test-tube is always filled with water to the same 

 height before being cooled, this height being that of the level 

 of the copper or slightly above it. 



2. About *6 grm. of mercury is placed at the bottom of 

 the tube, unless the substance to be experimented on is 

 attacked by mercury. The object of this is to allow the solids 

 that are introduced to part with their heat more freely. The 

 substance comes thus into much more intimate heat-con tact 

 with the mercury of the thermometer than it would if it rested 

 upon the glass alone. If, for instance, some pieces of sulphur 



Phil. Mag. S. 5. Vol. 12. No. 74. Sept, 1881. 



