screens of extreme tenuity by radiant heat. 
141 
Experiment 2. 
Let two air thermometers be procured, with bulbs blown 
as thin as possible. Let the interior hemisphere of one of 
them be coated with a fine opaque coating of pounded char- 
coal. Place the bulbs of the thermometers at the same 
distance from a heated ball at the temperature of about 200 
degrees. Divide the space through which the fluid descends 
in each into the same number of equal parts. Raise the ball 
to the temperature at which it has just ceased to be visible in 
the dark, place it a greater distance from the two thermo- 
meters, and it will be found that the liquid will have sunk 
farther in the thermometer having the coated bulb, than in the 
other. 
This experiment evidently leads to the same conclusion as 
the preceding ; viz. when the temperature of the ball was 
low, the whole current of radiant heat was arrested by the 
external hemispheres of the bulbs of the thermometers ; but 
when the temperature of the ball was elevated, a portion of 
the radiant heat freely permeated the transparent bulb, which 
portion was arrested by the opaque coating in the other, and 
gave rise to an increase of temperature in the included air. 
Experiment 3. 
Procure a frame of a moderate size ; stretch across it a 
number of very fine threads of glass, or of fine wire, parallel 
to each other. At right angles to these, and at the same 
distance, stretch other threads of glass or wire, so as to divide 
the whole frame into a great number of small squares. Brush 
the whole over with a very broad camel-hair pencil, dipped 
