RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
315 
circular apertures 3-J- inches in diameter. By means of screw caps these apertures 
may he closed air-tight by transparent 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 frag¬ 
ments 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 in¬ 
candescent lime faces the concave mirror B, which receives the rays and sends them 
back in a parallel beam through the tube T T\ P is the thermopile from which wires 
proceed to a distant galvanometer, the reflected dial of which is observed through a 
telescope. S is an adjusting screen. At C is what, in my former researches, I have 
called the “compensating cube,” used to neutralise the radiation from the source L, 
and to bring the galvanometer needle to zero. The chief object of this arrangement 
is to enable the experimenter to send a calorific beam along the axis of the tube T T', 
which shall never touch its internal surface. The great width of the tube, aided when 
necessary by diaphragms, renders this easy of accomplishment. 
Fig. 3. 
The mode of operation is readily understood. Suppose the heat impinging on the 
pile to produce a galvanometric deflection of 50°. This, the “total beat/' is neutralised 
by the radiation from the compensating cube. To render the compensation accurate, 
the double screen S is shifted by an extremely fine screw motion. Even when the 
total heat is very large it is possible, by means of this screen, to neutralise 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 
2 s 2 
