Effect of Bodies on Light and Heat. 405 



about the heating apparatus to volunteer to be vaporized and thus make 

 its mark. A millionth part of a milligram of Lithium is big enough to 

 write its name in legible characters. This metal was thought to be 

 very scarce till the spectrum proved it to exist almost everywhere in 

 many kinds of rocks, in plants and in animal tissues, in milk and in hu- 

 man blood. Five elements, not known before to exist at all, were dis- 

 covered by their unexpected writing across the spectrum. They are 

 Caesium, Rubidium, Thallium, Indium and Gallium. Newton, who first 

 philosophized on the solar spectrum, was ignorant of the fact that.it 

 Was crossed by many dark lines. This fact was discovered in the be- 

 ginning of this century, by simply allowing the light to fall on the prism 

 through a narrow slit placed parallel with the refracting edge of the 

 prism, instead of through a round hole, as Newton had done. The nar- 

 rower the slit the more distinct are the dark lines. In 1814, Fraunhofer, 

 a German optician, counted and mapped these lines to the number of 

 576, and lettered some of them as we now have them. From this cir- 

 cumstance they are called the Fraunhofer lines. These lines were 

 found to be fixed and constant in all sunlight, whether obtained direct 

 from the sun, or by reflection from the moon, or Venus, or any other 

 planet. It was not, however, till 1849 that it was proved what these 

 lines were due to. Kirchoif, a German, formed an artificial continuous 

 spectrum by means of a Drummond or lime light, and allowed this light 

 to pass through a flame colored by common salt. The result on the 

 spectrum was the dark line D in the very position in which the same 

 line is a bright } T ellow one in the spectrum of salt alone. The same 

 thing was done with potassium, lithium, calcium, strontium, barium 

 and copper. Repeated experiments from that day to this have proved 

 beyond all doubt that the solar spectrum is made up, first, of the contin- 

 uous colored spectrum which every incandescent solid or liquid body 

 gives, on top of which are piled, as it were, the discontinuous spectra of 

 a great number of substances in a state of incandescent vapor or gas. 

 It is shown that a vapor has the power of absorbing or quenching such 

 of the rays that pass into it as have the same wave length as those rays 

 which it ordinarily emits, while it is transparent to all the other rays. 

 Consequently, when the light of an incandescent solid or li'quid passes 

 through the light of an incandescent gas, the spectrum of the gas is 

 turned from a spectrum of colored lines to a spectrum of black lines, 

 the position of the lines, however, being precisely the same in both 

 cases. Since Fraunhofer's time many observers have watched and ex- 

 amined the solar spectrum. Over 2, 000 lines have been mapped on 

 the spectrum, and more than 800 have been identified as belonging to 

 elements on earth. The following is a table of the elements and num- 

 ber of lines of each, determined by Prof. A. J. Angstrom, of Upsala : 



