DR. T. S. HUMPIDGE ON THE ATOMIC WEIGHT OF GLUCINUM. 
007 
IV. Determination of specific heat. 
The most accurate method for determining the specific heat of substances of which 
small quantities only are available is undoubtedly that devised by Bunsen, in which, 
as is well known, the quantity of ice at 0° which is melted by a given weight of the 
substance heated to a high temperature is measured by the diminished volume which 
the water produced occupies. Unless, however, the whole apparatus is kept exactly 
at the freezing-point, accurate results are not possible, and even under the most 
favourable conditions there is usually a mean error of nearly one per cent. Bunsen 
originally immersed his instrument in a large box of pure freshly-fallen snow to pre¬ 
serve it accurately at 0°—a proceeding which has been somewhat simplified by 
Schuller and Wartha, who coat the exterior vessel with a thick layer of ice, and work 
in ice-cold water. But even with this modification it is scarcely possible to use the 
apparatus in England, and especially on the west coast, where frost in the winter is 
the exception rather than the rule. 
Several kinds of rough calorimeters based on the same principle have been proposed 
in which the heated substance produces an expansion in a liquid at a definite tempera¬ 
ture. Among these are the atometer of Mr. Reynolds, which was used to determine 
the specific heat of his glucinum, and a similar form described by Professor Balfour 
Stewart. * No instrument of this description can, however, give accurate results 
unless most carefully shielded from external sources of heat and carefully calibrated, 
especially when a liquid expanding so irregularly as alcohol is used. As far as can be 
judged by the published results, no calibration has been attempted, and therefore the 
results obtained cannot be of much value. 
A very simple form of apparatus has been devised by Koppf who places the sub¬ 
stances in a glass tube with naphtha, and heats in a bath of mercury. Considerable 
dexterity must be requisite to remove the heated substance from the bath of mercury 
to the calorimeter in exactly equal times, and a further objection is the small range of 
temperature through which the substances can be heated. His results, too, do not 
agree so closely with one another as those obtained by Bunsen’s or Begnault’s methods, 
especially when only small quantities are used. 
To overcome these various objections a modification of Begnault’s method of 
mixtures has been adopted, which with small quantities of the substances (0'7 to 
7 grammes) gives results with a mean error of one per cent, or less, and which can be 
easily worked by one person without assistance. The errors in Begnault’s apparatus 
have been pointed out by Neumann and Pape.| They showed how a cooling effect 
must be produced by an upward current of cold air when the heating arrangement 
was opened at the top, and explained that, in many cases, the water equivalent of the 
* Proceed, of the Phvs. Soc., iv., 52, 342. 
t Phil. Trans., 1865, 71. 
% Pogg. Ann., cxx., 337, 579. 
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