690 Proceedings of the Royal Society 
The liquid, whose specific heat was to be determined, was placed 
in a thin glass beaker of about 350 cubic centimetres capacity, which 
in its turn was contained within a thick porcelain vessel of about 
two centimetres greater diameter. The two vessels were separated 
by a packing of cotton wool, which prevented in great measure loss 
of heat, and at the same time permitted the inside beaker to be 
removed and replaced with facility. To measure the temperature 
of the solution a very sensitive mercury-in-glass thermometer was 
employed. This thermometer, which had also been compared with 
the Kew standards, was graduated to tenths of a degree centigrade, 
and the distance between two successive divisions was about one 
millimetre. 
It is evident from what has been stated above that a rise of 
temperature of one degree could be determined within two per cent, 
of its true amount, and therefore a rise of temperature of four or five 
degrees could be measured with great accuracy. There are other 
causes of inaccuracy, however, than incorrect reading of the tempera- 
ture, of which the most important is perhaps the variation of tem- 
perature in the course of the experiment. All these causes of error 
were carefully allowed for, and in most cases three experiments made 
for each density of solution, the arithmetical mean of the results of 
which was taken as the true specific heat for the solution of the 
density in question. 
The heater was arranged to have about one-tenth of the thermal 
capacity of the liquid, so that the temperature of the liquid experi- 
mented on should not be raised much above that of the atmosphere, 
and consequently only a small amount of heat be lost by radiation. 
The method of experimenting was as follows : — 
In the first place the thermal capacity of the glass beaker was 
determined. This was done by filling it to about half the required 
height with water at the temperature of the atmosphere, and at the 
same time a similar vessel was filled with water about 10° above 
atmospheric temperature. The temperature and rate of cooling of 
this water were accurately determined, and then the temperature of 
the water in the beaker. The two quantities of water were then 
mixed, and the temperature read at the end of two minutes and 
again at the end of four minutes. The difference between these two 
readings added to the first reading gave the temperature of the 
