PHENOMENA OF HEAT. 87 



window, I feel the heat as great through the window as if I 

 were sitting outside. 



This singular and important difference has been the sub- 

 ject of many curious experiments, and it is found that terres- 

 trial heat (that is, heat radiating from fires or heated bodies) 

 is intercepted and detained by glass or other transparent sub- 

 stances, while solar heat is not; and that terrestrial heat 

 being so detained, heats the bodies through which it passes, 

 which solar heat is incapable of doing. More recent re- 

 searches, however, show that this detention is complete only 

 when the temperature of the source of heat is low, and that 

 as the temperature becomes higher, a portion of the heat ra- 

 diated, acquires the power of penetrating glass. 



ESTHER. 



Then it is only because the sun is so much hotter than any 

 terrestrial heat, that it is thus able to penetrate? 



MRS. c. 



Precisely so; and therefore this discovery is important, be- 

 cause it shows that solar and terrestrial heat are of the same 

 nature; and, at the same time, it leads us to regard the actual 

 temperature of the sun as far exceeding that of any earthly 



flame.* 



.' / 



ESTHER. 



I have read that in many parts of Russia large sheets of 

 talc are substituted for glass in windows. 



MRS. c. 



And in the Province of Yakutsk, in Siberia, the inhabi- 

 tants sometimes cut large blocks of ice, the size of the win- 

 dow frames, which they put in and let them freeze fast. 

 These serve them the winter through; and though they give 

 rather an opaque kind of light, they are perfectly tight and 

 warm, and remain unthawed until the spring. f 



* Herschel's Preliminary Discourse, 

 t DobelPs Travels in Kamschatka. 



