34 LIGHT, 



19. LIGHT PRODUCES MAGNETISM. 



This was first observed more than twenty years since by Morrichini, 

 at Rome, and has been recently confirmed by Mrs. Somerville. (Ph. 

 Tr.) A sewing needle, an inch long, being half covered with paper, 

 had the other half exposed, during two hours, to the violet rays, which 

 imparted north polarity ; the indigo rays produced nearly the same 

 effect, and the blue and green in a still smaller degree. The yellow, 

 orange, red, and invisible rays were inert, having produced no effect 

 in three days. Similar effects were produced when the needles 

 were enclosed in green or blue glass, or ribands of the same color ; 

 one half being always covered with paper. The calorific rays pro- 

 duced no effect. In these experiments it was not necessary to dark- 

 en the room. 



Iron* ore not magnetic, becomes so by exposure to light, f 



REMARK. 



When we have considered radiant heat, certain discrimination* 

 may be made between it and light, properly so called. 



Some of the effects, above described, probably belong to one sort of 

 solar rays, and some to another, but the facts are stated with refer- 

 ence to the undecomposed rays, as they come to us from the sun. 



20. SOURCES OF LIGHT, most of ivhich are also sources of heat. 



1. The Sun and fixed stars. 



2. Combustion. 



3. Heat without combustion, as in an ignited stone. 



4. Percussion and friction. 



5. Chemical action without combustion. 



6. Electric and Voltaic action. 



7. Animal poiver, as in phosphorescent living animals. 



" Organization, sensation, spontaneous motion and all the opera- 

 tions of life, exist only at the surface of the earth, and in places ex- 

 posed to the influence of light. Without it nature would be lifeless 

 and inanimate. By means of light, the benevolence of the Deity 

 hath filled the surface of the earth with organization, sensation and 

 intelligence." Lavoisier. 



PHOTOMETER. 



Mr. Leslie, by having one ball of his differential thermometer J 

 made of black glass, adapts it, as he conceives, to the measurement 

 of light ; but it seems difficult to distinguish in this case, between the 

 effects of heat and of light, unless we adopt the opinion of the in- 

 genious inventor, that light, when absorbed, is converted into heat. 



* Fer oxidule of Hatty. t Am. Jour. I. 89 



t For the notice of this instrument, see thermometers. 



