132 ANNUAL OF SCIENTIFIC DISCOVERY. 



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been made to ascertain what substance allowed them to pass most freely. 

 The principal results attained to were as follows : 



Of the solids experimented with, rock-crystal, ice, rock-salt, Iceland 

 spar, and the diamond, in the order named, exhibited the greatest photo- 

 graphic transparency ; while thin glass, mica, iodide of potassium, and 

 nitrate of potash, considerably affected the transmission of the photo- 

 graphic rays. The photographic transparency of different liquids may 

 be indicated as follows : water, 74 ; alcohol, 63 ; chloroform, 26 ; oil of 

 turpentine, 8 ; of gases, hydrogen, nitrogen, oxygen, and carbonic acid, 

 have a photographic transparency indicated by the number 74 ; coal 

 gas, 37; sulphurous acid, and sulphuretted hydrogen, 14 each. This 

 remarkable fact was also noticed, namely, that solid bodies, when dis- 

 solved or melted, maintain exactly the same photographic transparency 

 as when in the solid state. The same was the case when they were 

 converted into vapor, which showed that this power was part of the 

 nature of the substance. 



The lecturer then described the phenomenon of fluorescence, and 

 showed that the chemical rays of the spectrum corresponded with the 

 rays of fluorescence, by taking a photograph in that part of the spec- 

 trum which, though otherwise invisible on the screen, lighted up a so- 

 lution of jesculine. He then showed that all metals give characteristic 

 photographic spectra, some of them bearing a strong family resemblance 

 to each other, as in the cases of iron, cobalt, and nickel ; the last metal 

 giving one of the longest spectra observed, and which extended to 190 

 of the scale. Arsenic, antimony, and tin showed as great differences 

 in the invisible as visible part of the spectrum. The most interesting 

 of the metals to study, in this respect, was magnesium, which opened 

 a wide field for investigation. There were certain points of resemblance 

 between the spectrum of magnesium and that of the sun, which led to 

 the supposition that this metal existed in the solar atmosphere. The 

 comparison of the spectrum of magnesium with that of the sun led also 

 to some important considerations as to the temperature of the sun. It 

 was known that the higher the temperature the more refrangible were 

 the rays of light emitted by a body. We have no conception of the 

 temperature of the electric spark. The heat of the strongest wind-fur- 

 nace was estimated at 4500 F., and that of the oxy-hydrogen jet was 

 supposed not to exceed 15,000 F. ; yet with neither of these could the 

 same effects be produced as with the electric spark. The lines of the 

 photographic spectrum of magnesium were not seen in photographs of 

 the solar spectrum, and yet there was no doubt that this metal was 

 present in the solar atmosphere. Kirchoff, had discovered that solids, 

 when heated, give a continuous spectrum, but that bodies in the form 

 of gas give rays of definite and limited refrangibility, each substance 

 emitting light of a definite property. He had also noticed that light 

 from a luminous mass, by passing through ignited vapor, which, per se, 

 would give bright lines in the spectrum, became furrowed out in dark 

 bands, occupying exactly the same position in the spectrum as the 

 bright lines. Now, ignited magnesium vapor emitted green rays which 

 were absolutely identical with the group of fixed lines b in the solar 

 spectrum, and it was, therefore, certain that magnesium was a constitu- 

 ent of the sun. It was, moreover, probable that the heat of the sun 

 was inferior to that oi' the electric spark, inasmuch as it was insufficient 



