by Radiant Heat. 349 



jirst experiment arrested by the screen ; whereas in the other ^ 

 a portion of it freely radiated thrcmgh the screen^ a/ndjbimd 

 its way directly to the bulb of the tJiermoineter r 



Exp. 2. Two air thermometers, with the thinnest possible 

 bulbs were taken, and the interior hemisphere of one of them 

 was coated with a fine opaque film of pounded charcoal. The 

 bulbs of the thermometers were then placed at the same dis- 

 tance from a heated ball at the temperature of about 200% 

 and the space through which the fluid descended in each was 

 divided into the same number of equal parts. The heated 

 ball, just visible in the dark, was placed at a greater distance 

 from the two thermometers, and the liquid sunk further in the 

 one with the coated bulb than in the other. 



Hence it follows, " that when the temperatui'e of the ball 

 was low, the whole current of radiant heat was arrested by the 

 external hemispheres of the ball's, but when it was high a por- 

 tion of the radiant heat freely permeated the transparent bulb, 

 which portion was arrested by the opaque coating in the other, 

 and raised the temperature of the included air. 



Exp. 3. A number of very fine threads of glass or wire 

 were stretched parallel and at right angles to each other across 

 a frame of moderate size. A broad camel-hair pencil dipped 

 in the white of an egg was brushed over the whole, so as to 

 cover the small square with a delicate transparent liquid film. 

 The screen was then placed between the differential thermo- 

 meter described in this Number, and the heated body, when 

 the following effects were produced. 



1. The temperature of the ball being low, and the screen 

 kept at almost the same temperature by constantly applying 

 the white of an egg mixed with cold water to the upper side 

 of the frame, no sensible effect on the thermometer was per- 

 ceived. 



S. The ball being just visible in the dark, was placed at a 

 greater distance, and a striking effect produced. 



Hence radiant heat freely permeates a very thin transparent 

 liquid screen. Mr Ritchie also found that a liquid screen is 

 more permeable by radiant heat than a solid one. 



Exp. 4. When the screen was placed at different distances 

 from the heated ball, a very little difference was observed in 



