46 HEAT. 



the spectrum, while the most of the heat rays are 

 thrown into the region of the red, and below it. Nei- 

 ther the place of the heat rays nor the chemical rays 

 is visible to the eye, but a delicate thermometer proves 

 that there is most heat just below the red, and a piece 

 of paper covered with chloride of silver, (a substance 

 very sensitive to the chemical rays of light,) grows 

 black most rapidly in the region of the violet. The 

 place of the chemical and heat rays is thus shown, al- 

 though neither can be seen. It is not to be understood 

 that they are confined to the points indicated, but only 

 that they are accumulated there in largest proportion. 



94. BURNING GLASSES. The collection 



action th of of ra Y s of heat from the sun b y ordinary 

 burning glass- burning glasses, depends on the fact that 

 they are refracted, or bent out of their 

 course on passing from one medium to another, pre- 

 cisely as in the case of light. A lens made of two 

 watch-glasses, filled with water, answers for heat as 

 well as light, and may be used as a burning glass. 



95. DIFFERENT HEAT RAYS. There are 

 ray* of heat different kinds of heat rays, as there are of 



light rays ; some will pass through one 

 substance best, and some through another. Thus, a 

 piece of smoked rock salt allows the blue heat ray of 

 the spectrum to pass, while alum lets the lower or 

 red heat ray pass. 



96. ANALYSIS OF HEAT. The analysis 



How is the 



analysis of ot neat is enected. by the same means as 



keate/eeteJf are 



through a prism just as if light were to be analyzed, 



