242 



Mr. J. Joly. 



in which the surface extent of the same substance is varied con- 

 siderably more especially point to this conclusion. If these are to be 

 taken as conclusive, the error must be most generally less than the 

 proportion one in a thousand. The variations in successive experi- 

 ments are of about this magnitude, but are not in any special direction, 

 and so point to no source of error in particular. These experiments 

 are on conductors. (See "Method of Condensation," p. 362.) 



The change of medium around the substance from one of greater 

 to one possessing less buoyancy affects the balance and has to be 

 allowed for by calculation based on experiment. It is improbable that 

 more than a very small error arises from this source. Special cases 

 may, however, arise in which this may not be so ; and it was chiefly 

 to avoid error from this last source that, in dealing with the large 

 spheres of thin metal, used in the determination of the specific heat 

 of air at constant volume, I resorted to the use of the differential 

 calorimeter.* This has the further advantage of eliminating the 

 radiation error affecting observations in the simple calorimeter. Nor 

 need the thermal expansion of the substance any longer be con- 

 sidered. I describe the apparatus briefly here, as it seems in fact to 

 eliminate to a great extent, if not entirely, what small risk of error 

 obtains in the previously described apparatus. Its advantages, how- 

 ever, are most conspicuous in the use for which it was designed, the 

 only use to which, up to the present I have applied it — the calori- 

 metry of gases. Its application to this branch of physics must ever be 

 its most important use. Indeed in the conditions obtaining in the 

 calorimetry of solids or liquids it is hardly called for. However, even 

 in these latter cases, where it is advisable to enclose the substance from 

 contact with the steam, the differential calorimeter would enable us 

 to effect the experiment somewhat more accurately than would be 

 possible with the use of the single calorimeter. 



Plate 7 shows, in side sectional elevation, fig. 2, and front 

 sectional elevation, fig. 1, to a scale of one-tenth, the differential 

 calorimeter which I have at present in use in dealing with gases. 

 The spheres, one of which is used to hold the gas, are shown 

 hanging in the calorimeter. The drawing needs little explanation 

 after what has been said about the single calorimeter. The principle 

 is obvious. Apart from its special application to gases, it may be 

 said that the calorimeter is so constructed that carriers depending^ 

 from both arms of the balance are hung within it, side by side 

 and only a few centimetres removed from one another, the balance 

 used being a short-beam balance. The substance may be enclosed 

 in a receptacle of thin platinum or copper, with a screwed, air- 

 tight lid, and placed upon one carrier; a similar receptacle, also- 



* "On the Specific Heats of G-ases at constant Yolume" (Preliminary ]STote) r 

 ' Eoy. Soc. Proc.,' vol. 45, p. 33. 



