INTROD UCTOR Y. 3 



weight and the Centigrade scale. Hence to pass from the 

 Centigrade scale to the Fahrenheit scale multiply by the 

 factor 1.8, that being the ratio of the two scales. 



In this work calories referred to the kilogram (kilo- 

 calories) will be used, and the calorie will be the quantity of 

 heat necessary to raise the temperature of that amount of pure 

 water one degree Centigrade. We will omit consideration of 

 the variations in specific heat of water; to consider these it 

 would be necessary to state that the initial temperature was 

 o C. But, as remarked by Berthelot, " the calorie varies 

 only to a very slight degree if we take the water at a slightly 

 increased temperature at 1 5 or 20, for example ; so that we 

 are accustomed to regard as constant the specific heat absorbed 

 by the water for each degree comprised in this interval of 

 temperature, thus simplifying the calculations." We may 

 lessen this little -error by referring the calorie to a litre of 

 water instead of a kilogram, that is, by measuring the water 

 instead of weighing it; the weight of a litre of water diminish- 

 ing from its maximum density at 4 C., while its specific heat 

 gradually increases. The error of calculation is thus made 

 less than the error of experiment. 



HEAT OF COMBUSTION. 



When the fuel contains hydrogen, its heat of combustion 

 may be expressed in two ways. Hydrogen in burning pro- 

 duces water, and this water may be either condensed or in the 

 state of vapor. The same number does not apply to both 

 cases, since the vaporization of the water formed consumes 

 heat, which is not given up to the calorimetric bath. We 

 usually consider the heat of combustion, the result of the 

 experiment made under ordinary conditions, or when the 

 water is in the liquid state; this is the general acceptance of 

 the term heat of combustion. Some authors, however, prefer 

 to consider the water as vapor. 



It is easy, however, to change from one system to the 



