ON THE SOURCES AND EFFECTS OF HEAT. ' 639 



which of the rays would furnish the greatest quantity of light, without sub- 

 jecting the eye to the inconvenience of unnecessary heat. He first observed 

 that the heat became more and more considerable as the thermometer ap- 

 proached the extreme red rays in the prismatic spectrum; and pursuing the 

 experiment, he found not only that the heat continued beyond the visible 

 spectrum, but that it was even more intense when the thermometer was at a 

 little distance without the limits of the spectrum, than in any point within 

 it. (Plate XXXIX. Fig. 546,547.) 



Sir Henry Englefield has repeated these experiments with many additional 

 precautions, and Mr. Davy was a witness of their perfect accuracy: the excess 

 of heat beyond the spectrum was even considerable enough to be ascertained 

 by the sense of warmth occasioned by throwing it on the hand. The skin 

 appears, when compared' with a thermometer, to have its sensibility more 

 adapted to the perception of radiant heat than to that of heat imparted by 

 contact, perhaps because a much smaller quantity of heat is sufficient to raise 

 the temperature of the thin cuticle very considerably, than would be required 

 in order to affect any thermometer in the same degree. 



It was first observed in Germany by Ritter, and soon afterwards in 

 England by Dr. Wollaston, that the muriate of silver is blackened by invisible 

 rays, which extend beyond the prismatic spectrum, on the violet side. It is 

 therefore probable that these black or invisible rays, the violet, blue, green, 

 perhaps the yellow, and the red rays of light, and the rays of invisible heat, 

 constitute seven different degrees of the same scale, distinguished from each 

 other into this limited number, not by natural divisions, but by their effects 

 on our senses: and we may also conclude that there is some similar relation 

 between heated and luminous bodies of different kinds. 



The effects of heat, thus originating, and thus communicated, may be 

 divided into those which are temporary only, and those which are permanent. 

 The permanent effects are principally confined to solids, but the temporary 

 eitects are different with respect to substances in different states of aggrega- 

 tion, and they also frequently comprehend a change from one of these states 

 to another. The effect of heat on an elastic fluid is the simplest of all these, 

 being merely an expansion of about one five hundredth of its bulk for each 

 degree of Fahrenheit that the temperature is raised ; or an equivalent aug- 



