Determinations of the Absorption of Heat by Liquids. 207 



The general outline of the apparatus employed is shown in 

 fig. 1. The instruments were Professor Tyndall's, who kindly 



Fig. 1. 



1 



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i 



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allowed me their use, and also opportunity for making the expe- 

 riments. The source of heat, S, was a platinum spiral raised to 

 bright incandescence by an electric current, and surrounded by 

 a glass globe having an aperture in front. Before this was a 

 cell, C, with moveable rock-salt sides to contain the liquid under 

 examination. Before this, again, and precisely opposite, was a 

 thermo-electric pile, P, fitted with its conical reflector and 

 attached to a very delicate galvanometer. The rock-salt cell 

 was supported on a little shelf behind a perforated metal screen, 

 D, from which the pile was distant some 18 inches. The rock- 

 salt walls of the cell, in the experiments first to be described, 

 Were separated by a partition formed of an annular plate of mica 

 0*02 inch (-J millimetre) thick : this thickness determined that of 

 the liquid layer under examination. The heat from the spiral was 

 sometimes concentrated by the use of a rock-salt lens (not shown 

 in the figure) which was placed between the source and the cell. 

 To obtain greater sensitiveness, in most of the experiments the 

 " compensating " arrangement was adopted. A blackened 

 canister, H, filled with boiling water, was placed before the oppo- 

 site face of the pile. By adjusting a screen, T, between the 

 canister and the pile, the deflection of the galvanometer, caused 

 by the radiation from S, could be exactly neutralized. The 

 needle was thus brought to zero and maintained in its most sen- 

 sitive position. 



The empty cell having been mounted on its support in 

 the path of the rays, the screen was moved until the needle 

 stood precisely at zero. A liquid (bisulphide of carbon) was 

 now poured into the cell through a fine funnel, and the needle 

 of the galvanometer immediately observed through a telescope. 

 Instead of moving in the direction of absorption, a marked de- 

 flection took place on the opposite side of zero, the needle finally 

 coming to rest at 15° on the side of increased heat from the 



