34 
DR. ANDREWS ON THE THERMAE CHANGES 
Supplementary Note on the Determination of the Specific Heats of Fluids. 
The accurate determination of the specific heats of fluids is of so much importance 
in all inquiries connected with the heat of combination, that I have taken some pains 
to introduce greater simplicity and accuracy into the methods hitherto employed for 
that purpose. The process I am about to describe is a modification of that adopted 
by M. Regnault in his valuable researches on the specific heats of simple and com- 
pound bodies, and I am also indebted to the same accurate philosopher for a know- 
ledge of the most important precautions to be taken in inquiries of this kind. 
The general principle of the following method is to compare the increments of 
temperature produced by the cooling of a hot body in water and in the fluid under 
examination. But instead of taking, as is usually done, a ball of heated metal, whose 
temperature at the moment of immersion cannot be known with absolute precision, 
I employed a thermometer with a very large reservoir, and so adjusted that the mer- 
cury does not appear in the stem till it is heated to nearly the boiling point of water. 
The cylindrical reservoir is about two inches long, and half an inch in diameter. A 
mark is placed on the stem corresponding to 201° Fahr., which point is situated 
about an inch and a half from the reservoir. This instrument is easily heated by 
means of a very simple apparatus till the mercury rises a little above the mark. 
The first step of the process is to determine accurately the thermal value of the re- 
servoir with a small portion of the stem adjacent to it in terms of water. For this 
purpose, a certain weight of water is placed in a cylindrical vessel of thin brass, 
which is suspended within a larger vessel of tin plate. A very delicate thermometer, 
with a long cylindrical bulb (capable of being read with ease to -^jth part of a degree), 
is suspended in the water, and the whole is so arranged that the initial temperature 
of the fluid is about 5° below that of the surrounding air. 
The observer, previously agitating with a very light glass stirrer the water in the 
brass vessel, reads the temperature to an assistant, who notes it down and also marks 
the time. The former then removes the large thermometer from the heating appa- 
ratus (the disturbing influence of which is carefully prevented by a wooden screen) 
and, holding it at a suitable distance from the water, watches the descent of the mer- 
cury till it reaches the mark, when he instantly immerses it. The time of immersion 
is again noted, and the whole is gently agitated for 3^ minutes in the fluid. The tem- 
perature of the latter having now always attained a maximum, the new position of 
the thermometer is observed. The final temperature is never allowed to be more than 
2° higher than the air. 
The corrections for the heating and cooling influence of the air are very small 
when all the above conditions are fulfilled. They must not, however, be neglected. 
The rate of heating for each degree of depression per minute was found to be 0°01, and 
as from 12 to 20 seconds usually elapsed between the observation of the initial tempe- 
rature and the immersion of the heated instrument, the correction for that period of 
the observation was easily made. It was assumed that during the minute subsequent 
