47 8 PHYSIOLOGICAL PHYSICS. [Chap, xxxvm. 



1. One pound of water requiring 1 unit of heat, 50 

 pounds will require 50 units to raise their temperature 

 1 degree. The quantity may also be measured by 

 the number of degrees of temperature through which 

 the body has raised the unit mass of water. Thus, if 

 a hot iron ball be plunged into 1 pound of water 

 at an ordinary temperature, and if, when the ball and 

 the water have become of equal temperature, it is 

 found that the water is 10 warmer than before, then 

 the ball has given out 10 units of heat. The trouble 

 in such an experiment is to ensure that the pound of 

 water gets all the heat that is given out from the ball, 

 and that none of it is given off in other ways, which 

 would make the calculation erroneous. It is ne- 

 cessary, therefore, to plunge the ball into water 

 contained in a vessel surrounded by non-conducting 

 material, and arranged also to prevent radiation of 

 heat. The vessel would also be heated, and the heat 

 imparted to it would require to be taken into account. 

 Thus, suppose the water to weigh 10 pounds, and its 

 temperature to be raised 1, it has obviously gained 

 10 heat units. If the vessel weighed 5 pounds and it 

 is heated 1, it has not gained 5 heat units, since its 

 capacity for heat is not so great as water. Suppose 

 it to be made of iron, the capacity for heat of iron as 

 compared with that of water (its specific heat) is 

 about '114 for each unit of mass, and we take the 

 unit as 1 pound, so that for the 5 pounds the amount 

 is '570. So that if its temperature is raised 1, the 

 units of heat gained amount to '570. The total lost by 

 the ball is, therefore, 5*570 units. One form of 

 calorimeter consists, then, of a vessel filled with 

 water and surrounded by non-conducting material. 

 Into the water the hot body is plunged, and the dif- 

 ference in temperature of the water before and after 

 gives, by a simple calculation, the units of heat 

 gained by the water, that is, given off by the body. 



