350 THE PRINCIPLES OF SCIENCE. [chap. 



difficvilt as to measure liquids in porous vessels. To 

 determine the latent heat of steam we must condense a 

 certain amount of the steam in a known weight of water, 

 and then observe the rise of temperature of the water. 

 But while we are carrying out the experiment, part of 

 the heat will escape by radiation and conduction from 

 the condensing vessel or calorimeter. Yle may indeed 

 reduce the loss of heat by using vessels with double sides 

 and bright surfaces, surrounded with swans-down wool or 

 other non-conducting materials ; and we may also avoid 

 raising the temperature of the water much above that of 

 the surrounding air. Yet we cannot by any such means 

 render the loss of heat inconsiderable. Eumford ingeni- 

 ously proposed to reduce the loss to zero by commencing 

 the experiment when the temperature of tlie calorimeter 

 is as much below that of the air as it is at the end of the 

 experiment above it. Thus the vessel will first gain and 

 then lose by radiation and conduction, and these opposite 

 errors will approximately balance each other. But Reg- 

 nault has shown that the loss and gain do not proceed by 

 exactly the same laws, so that in very accurate inves- 

 tigations Rumford's method is not sufficient. There 

 remains the method of correction which was beautifully 

 carried out by Eegnault in his determination of the latent 

 heat of steam. He employed two calorimeters, made in 

 exactly the same way and alternately used to condense a 

 certain amount of steam, so that while one was measuring 

 the latent heat, the other calorimeter was engaged in 

 determining the corrections to be applied, whether on 

 account of radiation and conduction from the vessel or 

 on account of heat reaching the vessel by means of tiie 

 connecting pipes.^ 



4. MeiJiod of Compensation. 



There are many cases in which a cause of error cannot 

 conveniently be rendered null, and is yet beyond the 

 reach of the third method, that of calculating the requisite 

 correction from independent observations. The magnitude 



' Giali.'un's Chemical Reports and Memoirs, Cavendish Society, 

 l)p. 247, 268, &c. 



