the Thermal Unit. 411 



We thus see that the determinations of this ratio vary 

 from 1*013 to '9957, a difference of one part in 60, and an 

 additional uncertainty is introduced by the differences of 1 in 

 250 above referred to in the determinations by Bunsen, 

 Schiiller and Wartha, and Velten, as to the weight of mercury 

 corresponding to the " mean calorie." 



I think the considerations adduced in the preceding pages 

 are sufficient to justify the statement that our endeavours to 

 establish the capacity for heat of water as the standard of 

 calorimetric measurements have, so far, not met with success. 

 Nevertheless progress is essential. Whatever may be the 

 calorimetric standard adopted, it will still be necessary to 

 obtain more exact information concerning the changes in the 

 capacity for heat of water, otherwise the reduction of the 

 results of different observers to some common standard will 

 remain an impossibility. I believe that the obstacles are now 

 not insurmountable. Recent advances in the methods of 

 calorimetry and thermometry have placed more efficient in- 

 struments at our disposal. For example, the use of differential 

 platinum thermometers does away with many of the difficulties 

 hitherto connected with the measurement of small changes of 

 temperature, and, I would venture to add, the method which 

 I found efficient when tracing the changes in the specific 

 heat of aniline is mutatis mutandis applicable in the case of 

 water. 



It is now chiefly a financial difficulty which blocks the way, 

 for the apparatus would have to be on a large scale and 

 necessarily expensive. 



If this Association could see its way to provide that apparatus 

 and capable hands to work it, I believe our present doubts 

 would soon be dispelled and be replaced by a reasonable 

 certaintv. 



Part II. 



Consideration of certain Thermal Units other than those 

 dependent on the Capacity for Heat of Water. 



Tn the paper by Mr. Macfarlane Gray, previously referred 

 to, he defines his unit of heat as follows : — " That quantity of 

 constitutional molecular energy which is mechanically equiva- 

 lent to the variation of the pv product for the unit mass of 

 hydrogen gas per degree, at the temperature of melting ice 

 and atmospheric pressure." 



Mr. Gray gives the mechanical equivalent of this unit 

 of heat as 422*45 kilogramme-metres, but I differ from 

 him considerably in the numerical value of this constant*. 



* My own calculations of this quantity give 429-17 kilogramme-metres. 



