48 FRAGMENTS OF SCIENCE 



for the face of the thermo-electric pile a combustible body, 

 the invisible rays are competent to set that body on fire. 



6. Visible and Invisible Rays of the Electric Light 



We have next to examine what proportion the non- 

 luminous rays of the electric light bear to the luminous 

 ones. This the opaque solution of iodine enables us to 

 do with an extremely close approximation to the truth. 

 The pure bisulphide of carbon, which is the solvent of 

 the iodine, is perfectly transparent to the luminous, and 

 almost perfectly transparent to the dark, rays of the elec- 

 tric lamp. Supposing the total radiation of the lamp to 

 pass through the transparent bisulphide, while through the 

 solution of iodine only the dark rays are transmitted. If 

 we determine, by means of a thermo-electric pile, the total 

 radiation, and deduct from it the purely obscure, we ob- 

 tain the value of the purely luminous emission. Experi- 

 ments, performed in this way, prove that if all the visible 

 rays of the electric light were converged to a focus of daz- 

 zling brilliancy, its heat would only be one- eighth of that 

 produced at the unseen focus of the invisible rays. 



Exposing his thermometers to the successive colors of 

 the solar spectrum, Sir William Herschel determined the 

 heating power of each, and also that of the region beyond 

 the extreme red. Then drawing a straight line to repre- 

 sent the length of the spectrum, he erected, at various 

 points, perpendiculars to represent the calorific intensity 

 existing at those points. Uniting the ends of all his per- 

 pendiculars, he obtained a curve which showed at a glance 

 the manner in which the heat was distributed in the solar 

 spectrum. Professor Muller of Freiburg, with improved 

 instruments, afterward made similar experiments, and con- 



