56 KEPORT— 1902. 



thermometer, then, according to the best of the existing determinations, 

 the amount of heat required is -l:-2 joules. 



It will therefore be convenient to fix upon this number of joules as 

 a secondary unit of heat. 



This secondary thermal unit may be called a ' calorie.' 



For the present a second proposition is — 



Proposition II. — The amount of heat required to raise the tempera- 

 ture of one gramme of water 1° C. of the scale of the hydrogen thermometer 

 at a mean temperature which may be taken as 10° C. of that thermo- 

 meter is 4-2 joules. 



If further research should show that the statement in II. is not 

 exact, the definition could be adjusted by a small alteration in the mean 

 temperature at which the rise oi' 1° takes place. The definition in I. and 

 the number (1-2) of joules in a calorie would remain unaltered. 



These propositions, it will be observed, while reaffirming the names 

 'joule' as the equivalent of 10' ergs, and calorie, the equivalent of 4-2 joules, 

 as the amount of heat required to raise the temperature of one gramme of 

 water one degree centigrade on the hydrogen scale, leave undetermined 

 the mean temperature of the water so raised. Proposition II. states that 

 this may be taken as 10° C, but it is pointed out that if the heat required 

 to raise one gramme of water from 9°-5 to 10°-5C. should prove not to. 

 be 4-2 joules, a readjustment in the mean temperature employed in the 

 definition could easily be made. 



Accordingly in the Report, 1897, made at Toronto the Committee 

 ■wrote : — 



' At the Liverpool Meeting the committee agreed that the " calorie," 

 defined as the heat equivalent of 4-2 x 10^ ergs, should be adopted as the 

 unit for the measurement of quantities of heat, but the question as to 

 the exact part of the absolute thermodynamic scale of temperature at 

 v/hich this quantity of heat could be taken as equal to one water-gramme- 

 degree was for the time being left open. 



' This resolution has made it incumbent on the Committee to consider 

 carefully — 



' 1. The relation between the results of measurements of intervals of 

 temperature by accepted methods and the absolute scale. 



' 2. The specific heat of water in terms of the erg and its variation with 

 temperature. 



' With regard to the first point there appears to be no reason to doubt 

 that the scale of a constant-volume hydrogen thermometer is very nearly 

 identical with the absolute scale. The Committee have therefore decided 

 to recognise the standard hydrogen thermometer of the Bureau Inter- 

 national des Poids et Mesures as representing, nearly enough for present 

 purposes, the absolute scale. This convention has at least the advantage 

 of giving a definite meaning to statements of the numerical value of inter- 

 vals of temperature within any range for which comparison with the 

 hydrogen thermometer is practicable. If future investigation should 

 show that it is inaccurate to any appreciable extent, corresponding cor- 

 rections can be applied when necessary.' 



As regards the second point further research has shown that an 

 alteration in the temperature of measurement is required. The present 



