SECT. II.] PRELIMINARY DEFINITIONS. 27 



the dilatation of a fluid or of air. We assume the construction, 

 use and properties of this instrument to be accurately known. 

 The temperature of a body equally heated in every part, and 

 which keeps its heat, is that which the thermometer indicates 

 when it is and remains in perfect contact with the body in 

 question. 



Perfect contact is when the thermometer is completely im 

 mersed in a fluid mass, and, in general, when there is no point of 

 the external surface of the instrument which is not touched by one 

 of the points of the solid or liquid mass whose temperature is to be 

 measured. In experiments it is not always necessary that this con 

 dition should be rigorously observed ; but it ought to be assumed 

 in order to make the definition exact. 



24. Two fixed temperatures are determined on, namely : the 

 temperature of melting ice which is denoted by 0, and the tern- 

 perature of boiling water which we will denote by 1 : the water is 

 supposed to be boiling under an atmospheric pressure represented 

 by a certain height of the barometer (76 centimetres), the mercury 

 of the barometer being at the temperature 0. 



25. Different quantities of heat are measured by determining 

 how many times they contain a fixed quantity which is taken as 

 the unit. Suppose a mass of ice having a definite weight (a kilo 

 gramme) to be at temperature 0, and to be converted into water at 

 the same temperature by the addition of a certain quantity of 

 heat : the quantity of heat thus added is taken as the unit of 

 measure. Hence the quantity of heat expressed by a number C 

 contains C times the quantity required to diaoolvo a kilogramme 

 of ice at the temperature zero into a mass of water at the same 

 zero temperature. 



26. To raise a metallic mass having a certain weight, a kilo 

 gramme of iron for example, from the temperature to the 

 temperature 1, a new quantity of heat must be added to that 

 which is already contained in the mass. The number C which 

 denotes this additional quantity of heat, is the specific capacity of 

 iron for heat; the number C has very different values for different 

 substances. 



