SECT. IX.] GENERAL REMARKS. 127 



in number, namely, the unit of length, the unit of time, that of 

 temperature, that of weight, and finally the unit which serves to 

 measure quantities of heat. For the last unit, we might have 

 chosen the quantity of heat which raises a given volume of a 

 certain substance from the temperature to the temperature 1. 

 The choice of this unit would have been preferable in many 

 respects to that of the quantity of heat required to convert a mass 

 of ice of a given weight, into an equal mass of water at 0, without 

 raising its temperature. We have adopted the last unit only 

 because it had been in a manner fixed beforehand in several works 

 on physics ; besides, this supposition would introduce no change 

 into the results of analysis. 



158. The specific elements which in every body determine 

 the measurable effects of heat are three in number, namely, the 

 conducibility proper to the body, the conducibility relative to the 

 atmospheric air, and the capacity for heat. The numbers which 

 express these quantities are, like the specific gravity, so many 

 natural characters proper to different substances. 



We have already remarked, Art. 36, that the conducibility of 

 the surface would be measured in a more exact manner, if we had 

 sufficient observations on the effects of radiant heat in spaces 

 deprived of air. 



It may be seen, as has been mentioned in the first section of 

 Chapter L, Art. 11, that only three specific coefficients, K, h, C, 

 enter into the investigation ; they must be determined by obser 

 vation ; and we shall point out in the sequel the experiments 

 adapted to make them known with precision. 



159. The number C which enters into the analysis, is always 

 multiplied by the density D, that is to say, by the number of 

 units of weight which are equivalent to the weight of unit of 

 volume ; thus the product CD may be replaced by the coeffi 

 cient c. In this case we must understand by the specific capacity 

 for heat, the quantity required to raise from temperature to 

 temperature 1 unit of volume of a given substance, and not unit of 

 weight of that substance. 



With the view of not departing from the common definition, 

 we have referred the capacity for heat to the weight and not to 



