1 835.] different Solid and Liquid Bodies. 47 



The colours introduced into a diaphanous medium always 

 diminish more or less its diathermanity ; but they do not 

 communicate the property of detaining certain kinds of rays 

 of heat. They act upon the transmission of radiating heat, 

 as brown substances on the transmission of light. There is 

 an exception in opaque green and black glass, but these two 

 colouring matters appear to act by modifying the diather- 

 manity, a quality which is independent of colouration. The 

 quantity of radiating heat which traverses two polarizing 

 plates of tourmaline does not change, when the angle of 

 the axes of crystallization is increased ; the rays of heat 

 cannot be polarized by this mode of transmission, and in this 

 respect they differ totally from the rays of light. They 

 resemble light however, in the property of refracting, which 

 is proved by rock salt, the only one of the diathermous 

 bodies capable of transmitting calorific rays from any source. 



In ordinary prisms they cannot produce refraction except 

 upon a certain portion of radiating heat, for the glass inter- 

 cepts several kinds of rays of heat proceeding from very hot 

 sources, and absorbs almost the whole heat which is emitted 

 by bodies below incandescence. Hence, the doubt which 

 has been hitherto entertained upon the refrangibility of 

 obscure heat. 



Article VII. 



Products of the Distillation of Pit Coal. By F. F. Runge. 

 (Poggendorff, Annalen xxxi. 65.^ 



From the oil of pit coal rectified over oxide of copper, three 

 bases and three acids are partly separated, or are partly 



