482 Prof. Tyndall on the Action of Free Molecules on 



parent plates of rock-salt, the plates being protected from 

 crushing by washers of indiarubber. a is a cock leading to 

 an air-pump and a barometer-tube; b is a cock to which flasks 

 can be attached; while c is a cock connected with a purifying- 

 apparatus (not shown). This consists of a U tube filled with 

 fragments of clean glass moistened with sulphuric acid, and 

 of a second tube filled with fragments of Carrara marble 

 wetted with caustic potash. A plug of cotton-wool intercepts 

 the floating matter of the air. The source of heat L is a 

 cylinder of carefully prepared lime, against which a flame of 

 coal-gas and oxygen impinges. The incandescent lime faces 

 the concave mirror R, which receives the rays and sends them 

 back in a parallel beam through the tube TT'. P is the 

 thermopile, from which wires proceed to a distant galvano- 

 meter, the reflected dial of which is observed through a tele- 

 scope. S is an adjusting-screen. At C is what, in my former 

 researches, I have called the " compensating cube," used to 

 neutralize the radiation from the source L and to bring the 

 galvanometer-needle to zero. The chief object of this arrange- 

 ment is to enable the experimenter to send a calorific beam 

 along the axis of the tube T T' which shall never touch its in- 

 ternal surface. The great width of the tube, aided when 

 necessary by diaphragms, renders this easy of accomplish- 

 ment. 



The mode of operation is readily understood. Suppose the 

 heat impinging on the pile to produce a galvanometric deflec- 

 tion of 50°. This, the "total heat," is neutralized by the 

 radiation from the compensating cube. To render the com- 

 pensation accurate, the double screen S is shifted by an ex- 

 tremely fine screw-motion. Even when the total heat is very 

 large it is possible, by means of this screen, to neutralize 

 accurately the radiation of the source and bring the needle to 

 zero. Suppose it to be there when the tube T T' is exhausted; 

 on permitting a gas or vapour capable of absorbing radiant 

 heat to enter T l v , the preexisting equilibrium is destroyed, 

 the needle moves promptly away from zero, and from the 

 observed deflection the absorption may be calculated. 



Instead of the concave reflector, a rock-salt lens of great 

 purity is sometimes used to render the beam parallel. When 

 the lens is used, the incandescent portion of the cylinder of 

 lime is caused to face the tube T T', between which and the 

 lime the lens is introduced. Instead of the incandescent lime, 

 a spiral of platinum-wire heated to redness by a voltaic 

 current is frequently employed as the source of heat. The 

 spiral is placed in the centre of a brass globe G, fig. 4, and is 

 connected with the screws 8 S' fixed in the back of the globe, 



