130 Mr. Ritchie on a new form of the 
thermometer, contains a coloured fluid, such as sulphuric 
acid tinged with carmine, alcohol, &c. The exterior surfaces 
of the cylindrical chambers are coated with lamp black, for 
the purpose of absorbing the radiant heat, which is rapidly 
conducted to the interior of the chamber, and thus causes an 
expansion of the inclosed air. As the results of this instru¬ 
ment are only comparable with each other, the scale has no 
reference to a fixed standard, and consequently may be divi¬ 
ded into any convenient number of equal parts. 
This instrument is well calculated for ascertaining the 
relative radiating powers of different surfaces ; but the only 
application which I shall now illustrate, is that of determining 
the law according to which radiant heat diminishes, as the in¬ 
strument is removed to different distances from the radiating 
source. 
Experiment i. 
Place a cylindrical vessel of tin plate filled with hot water, 
and having the same diameter as the chambers of the instru¬ 
ment, at different distances from the thermometer ; the results 
will deviate very considerably from the ratio of the squares 
of the distances. 
Experiment 2. 
Repeat the preceding experiment with a canister having a 
smaller diameter, and the results will approach more nearly 
to the squares of the distances. 
Experiment 3. 
Instead of the canister, I employed iron balls about two 
inches in diameter, and found that the effects were, (within 
