296 Professor Tyndall on the Actioii [Jan. 18, 



To prevent the transmission of heat by conduction from the cube 

 C to the tube a, the chamber /8 is partly embraced by an annular 

 space, in which cold water continually circulates. 



3. A thermo-electric pile furnished with two conical reflectors, and 

 connected with an excellent galvanometer. One of the faces of the 

 pile receives the rays which have passed through the tube a. 



4. A second copper cube C, also filled with boiling water, and 

 whose rays fall upon the second face of the thermo-electric pile. The 

 two cubes C and C, thus radiating upon the opposite faces of the pile, 

 tend, of course, to neutralise each other. 



Between the cube C and the adjacent face of the pile a screen S is 

 introduced, being attached to an apparatus of Ruhmkorffs, capable of 

 extremely fine motion ; by the partial advance or withdrawal of this 

 screen the two sources of heat can be caused to neutralise each other 

 perfectly. 



The tube a and the chamber y8, being both exhausted, the needle 

 of the galvanometer is brought exactly to zero by means of the screen 

 S. The gas or vapour to be experimented with is now admitted into 

 the tube a, and if it possess any sensible absorbing power, it will 

 destroy the previously existing equilibrium. The consequent deflec- 

 tion of the galvanometer, properly reduced, is the measure of the 

 absorption. In this way the action of eight gases and thirteen vapours 

 have been examined, and also the action of atmospheric air. 



Oxygen, hydrogen, nitrogen, and atmospheric air, respectively 

 absorb about * 3 per cent, of the qalorific rays ; this is the feeblest 

 action which has been observed. 



The most energetic action is that of olefiant gas, which at the ten- 

 sion of one atmosphere absorbs 81 per cent, of the calorific rays. 

 Between those extremes stand carbonic oxide, carbonic acid, nitrous 

 oxide, and sulphuretted hydrogen. 



Below a certain tension, which varies for different gases, the 

 amount of heat absorbed is exactly proportional to the density of the 

 gas. Above this tension, the rays on which the principal absorptive 

 energy is exerted, become gradually exhausted, so that every augment- 

 ation of density produces a diminished effect. 



In the case of olefiant gas, for example, where a unit measure 

 g?^th of a cubic inch in capacity was made use of; for a series of fifteen 

 such measures, the absorption was exactly proportional to the quantity 

 of gas ; subsequently, the ratios of the successive absorptions approached 

 gradually to equality. The absorption produced by a single measure 

 of olefiant gas of the above volume, moved the index of the galvano- 

 meter through an angle of 2 • 2 degrees ; the tension of the gas being 

 only TToo^o^h of an atmosphere. 



In the case of vapours, the most energetic is that of sulphuric ether ; 

 the least energetic is that of bisulphide of carbon. Comparing small 

 volumes and equal tensions, the absorptive energy of sulphuric ether 

 vapour is ten times that of olefiant gas, and ten thousand times that of 

 oxygen, hydrogen, nitrogen, or atmospheric air. 



