1313,] Observations on Radiant Heat. 101 



surrounding atmosphere) exposed to the action of radiant heat 

 rises in a given time, caeteris paribus, a quantity proportional to 

 the rays which it receives. If a certain number of rays make it 

 rise 1 degree, double the number will make it rise 2 degrees, 

 triple the number 3 degrees, and so on. This being established, 

 he placed two parabolic metallic mirrors at the distance of rather 

 more than three feet from each other. In the focus of one mirror 

 he placed a thermometer ; and in the focus of the other a hot 

 body, gradually increasing in temperature. The thermometer 

 was allowed to rise to its maximum without any screen. Then 

 a screen of transparent glass was interposed, and the experiment 

 repeated. Lastly, a screen of blackened glass was interposed, 

 and the experiment repeated a third time. By the blackened 

 screen all the radiant heat would be intercepted, and the effect 

 on the thermometer would be owing to the rise in the tempera- 

 ture of the screen. Hence the rise of the thermometer when 

 the blackened screen was used, subtracted from the same rise 

 when the transparent screen was used, leaves the effect produced 

 by the radiant heat passing through the glass. The following 

 table shows the ratio between the rays passing through the clear 

 glass, and the rays acting on the thermometer, when no screen 

 was interposed, at different temperatures :—- 



Temperature of the Rays •transmitted 



Hot Body in the through the Total Rays. 



Focus. Glass Screen. 



357° 10 263 



.655 10 139 



800 ,. 10 75 



1760 .... 10 34 



Argand's lamp without its chimney . . 10 29 



Ditto, with glass chimney 10 18 



Third Proposition. — The Calorific Rays which have already 

 passed through a Screen of Glass, experience, in passing through 

 a second Glass Screen of a similar nature, a much smaller Dimi- 

 nution of their Intensity than they did in passing through the 

 first Screen. 



This proposition was proved by interposing, first one glass 

 screen, and then two, and observing the difference in the effect. 

 The experiments appear conclusive. 



Fourth Proposition. — The rays emitted by a hot Bcdy 

 differ from each otlier in their J acuity to pass through Glass. 



This proposition is an obvious consequence from the third, 

 which can hardly have a doubt that the calorific rays (like those 

 ')f light) are of different kinds. 



