RADIATION. 299 



endless screw, tLis linear thermo-electric jnle may bo moved tlironirh tlie entire 

 spectrum, each of its rays being selected in succession, the amount of heat fall- 

 ing upon the pile at every point of its march being declared Ity an associated 

 magnetic needle. 



When this instrument is brought up to the violet end of the spectrum of the 

 electric light, the heat is found to be insensible. As the pile gradually moves 

 from the violet end towards the red, heat soon manifests itself, augmenting as 

 we approach the red. Of all the colors of the visible spectrum the red possesses 

 the highest heating power. On pushing the pile into the dark region beyond 

 the red, the heat, instead of vanishing, rises sudd(!nly and enormously in inten- 

 sity, until at some distance l)eyond the red it attains a maximum. Jiloving the 

 pile still forward, the thermal power falls, somewhat more suddenly than it rose. 

 It then gradually shades away, but for a distance beyond the red greater tlian 

 the length of the whole visible spectrum, signs of heat may bo detected. Draw- 

 ing, as Sir William Herschel did, a datum line, and erecting along it perpendic- 

 ulars proportional in length to the thermal intensity at the respective points, we 

 obtain the extraordinary curve which exhibits the distribution of heat in the 

 spectrum of the electric light. In the region of dark rays beyond the red, the 

 curve shoots np in a steep and massive peak — a kind of Matterhorn of heat, 

 which dwarfs by its magnitude the portion of the diagram representing the 

 luminous radiation. Indeed, the idea forced upon the mind by the inspection of 

 this diagram is that the light rays are a mere insigniticant appendage to the 

 dark ones, thrown in as it were by nature for the purposes of vision. (See figure, 

 where the space ABCD represeuta the non-luminous, and CBE the luminous 

 radiation.) 



The diagram drawn by Professor Miiller to represent the distribution of heat 

 in the solar spectrum is not l)y any means so striking as that just d(!scribed, and 

 the reason, doul)tless, is that pnor to reaching the earth the solar rays liave to 

 traverse our atmosphere. The aqueous vapor there diffused acts very eneigeti- 

 cally upon the ultra red rays, and by it the suunnit of the peak representing the 

 sun's invisi]>le radiation is cut off. A similar lowering of tlie mountain of invisi- 

 ble heat is ol)served when the rays from the electric light are permitted to pass 

 through a film of water, which acts upon them as the atmospheric vapor acts 

 up(jn the rays of the sun. 



VII. — COMBUSTION BY INVISIBLE RATS. 



The sun's invisible rays transcend the visible ones in heating power, so that 

 if the alleged performances of Archimedes during the siege of Syracuse had any 



