Conduction of Heat in Liquids. 39 



prevented. With, the weaker solutions a slight coating was sufficient, 

 but with, the stronger it had to be frequently renewed. 



In the cooling experiments, if we suppose the mass of water in 

 the dish to remain constant, the rate at which the thermometer falls 

 at any instant is directly proportional to the rate at which heat is 

 leaving the dish. Of this heat some passes into the material below 

 the dish, and some is lost by radiation from the water and the sides 

 of the dish. The material on which the cooling was slowest was 

 packing, and the heat given up to it seemed very small. Of this by 

 far the greater part occurred in the first minute, and this is precisely 

 the time when a small error in calculating the heat given to the liquid 

 in the experiments on conductivity is of least importance. Thus in 

 default of more accurate knowledge, the loss of heat by the dish when 

 on packing was taken as representing the loss by radiation when on 

 a liquid. Since the absolute amount, of heat given to the liquid is 

 not required, but only the ratio of the quantities given up for each 

 minute or half minute of the experiment, absolutely no error would 

 be introduced by neglecting the conductivity of the packing, pro- 

 vided the heat passing into the packing followed the same law as that 

 passing into the liquid. 



The liquids whose conductivities were determined are water, sul- 

 phuric acid solutions of various strengths, bisulphide of carbon, one 

 solution of methylated spirit, paraffin, and turpentine oils. For water, 

 methylated spirit, and paraffin two series of observations were made, 

 the water being siphoned from the dish in one case, and left in the 

 other. In the case of turpentine no observations were made with the 

 water siphoned. For the sulphuric acid solutions and the bisulphide 

 of carbon the siphon was always used. It was found that the con- 

 ductivity and the rate of cooling of the dish were nearly independent 

 of the strength of the sulphuric acid solution, and differed little from 

 the corresponding quantities for water. The law of cooling on bisul- 

 phide of carbon also closely resembled that on water. The relative 

 conductivities of these liquids would thus in all probability be most 

 correctly obtained by referring them to the value obtained for water 

 by the method employing the siphon. The following table gives the 

 quantity of heat given up to the liquids in consecutive minutes or 

 half minutes of the experiment, so far as is required in calculating 

 the conductivity. The unit employed is arbitrary, but is the same 

 throughout: — 



