HEAT OF EVAPORATION OF WATER. 283 
buta small effect on the temperature) ; (b) its electric conductivity, which would have 
necessitated the covering of the platinum-silver coil with some insulator. 
I therefore gave my first attention to the selection of some more suitable liquid. 
The ideal one ought to have small “ volume heat,”* and should be a perfect insulator, 
and therefore I at first selected aniline. I determined its specific heat over a range 
of from 15° to 52° C., and found that in some respects it suited my purposes admir- 
ably. A full description of this work will be found in paper A. 
Early in September I had, in consequence of an accident, to take the calorimeter to 
pieces and withdraw the aniline. I was then alarmed by the darkening in colour 
which it had undergone. In the discussion on the paper above referred to, Dr. 
ARMSTRONG expressed his opinion that the change in constitution indicated by this 
change in colour was not of a nature to render it likely that it would produce any 
appreciable effect on the specific heat, as it was probably due to the formation of a 
body whose properties were similar to those of aniline. I may add that Professor 
Ramsay independently expressed an opinion to the same effect. My observations 
show that no alteration in the specific heat of aniline had been indicated by the 
change in colour referred to, and I am still of opinion that it may be regarded as a 
liquid admirably adapted for calorimetric purposes, as it is but rarely that it would 
be required for experiments extending over a period of months or years. 
If I am able to carry out my investigations into the latent heat of evaporation of 
water and other liquids according to the plan I have designed, it is probable that the 
enquiry will occupy my leisure time for some years, and as the exact determination of 
the capacity for heat of the calorimeter and contents throughout a large range of 
temperature is a most laborious one, I was anxious to employ some liquid about the 
constancy of whose composition I should have no anxiety. 
At this time, Mr. THomas suggested to me that I should try a particular kind of 
petroleum oil supposed to consist of hydrocarbons only. ‘This is a singularly limpid 
oil, without colour, smell, or taste. I tested its insulating powers very severely over 
a range of temperature 10° to 150° C., and although I placed two large electrodes 
within a quarter of an inch of each other, and used a potential difference of 10 volts, 
I could cause no permanent deflection in a high resistance galvanometer throughout 
this range of temperature. Its specific gravity at 15° is °865, and, as I shall show 
hereafter, its “volume heat” is smaller than that of aniline.t After several 
experiments of different kinds, I came to the conclusion that this oil was a most 
suitable liquid for my purpose.{ 
The replacement of aniline by oil necessitated a re-determination of the capacity 
* IT propose to use the above term to denote the capacity for heat of any volume of a substance as 
compared with the capacity for heat of an equal volume of water. The phrase has already been used in 
a similar sense by Derxuy, ‘Chem. Soc. Journ.,’ 1893, p. 854. 
+ This oil appears to me to be well adapted to many physical purposes. 
t See note at end of this Section. 
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