290 MR. E. H. GRIFFITHS ON THE LATENT 
probable error was certainly less than 2, and, I believe, less than 1). Assume the 
probable error to be as great as 2, it follows that the differences of temperature 
could be determined to :0004°, and I am confident that differences of :0001° could 
be detected. 
There can be little doubt therefore that the quantity =q¢=C,, {()— 6) + (d’—d’)} 
could be determined to a far higher degree of accuracy than subsequent experience 
proved to be necessary. 
Norr.—December 4, 1894. At Mr. Heycocr’s suggestion, I have to-day tested 
as follows the oil supplied to me by Mr. Tomas, and referred to in the preceding 
section. We placed a lump of sodium in a test-tube filled with the oil, and gradually 
raised the temperature to above 160°C. No action whatever was visible, and the 
surface of the melted sodium remained as bright as that of pure mercury. We may 
assume, therefore, that the manufacturers are justified in their statement that it 
consists of hydrocarbons only, and the probability that different samples would 
possess approximately the same composition is sufficient to make the determination 
of its specific heat of some value. 
Section VII.—TuEe Heat DUE TO THE STIRRING (Q,). 
I have already pointed out the necessity for rapid stirring. Throughout these 
experiments the stirrer revolved at a rate of about 310 to 320 revolutions per minute, 
which is a slow rate as compared with that adopted during my enquiry into the value 
of J. The conditions, however, are different. In my former work I had at times to 
make observations with the calorimeter only partially filled with liquid, and I found 
that unless the water was thrown over every part of the calorimeter, its capacity for 
heat varied with the depth of the liquid. Throughout my present work the calori- 
meter was full, the liquid almost touching the lid, and, in consequence, I considered 
the slower rate sufficient to ensure an even distribution of temperature. 
In my aniline experiments previously referred to, I eliminated the effect of the 
stirring-heat by finding the temperature (Ax) at which the stirring supply exactly 
balanced the loss by radiation, &c., and thus the rate of rise at @, enabled me to 
determine the effect of the electrical supply. As throughout my observations on 
latent heat I had to maintain the temperature of the calorimeter at an equality with 
the surrounding walls, this mode of elimination was not applicable. 
Two methods of finding the value of the mechanical supply suggested themselves. 
I. That of finding the mass of water which it was necessary to evaporate per hour 
in order to maintain the calorimeter at a constant temperature, when the supply of 
heat was that due to the stirring only. 
II. That of determining the rate of rise across the null-point (a) when the heat 
supply was that due to stirring only ; (b) when the heat was due to the stirring and 
a given potential difference. 
