NATURAL PHILOSOPHY. 10*7 



be analyzed by the prism. A commutator is also used to invert the 

 direction of the current, for the purpose of better exhibiting that 

 part of the action due to the positive and negative poles. This 

 study, the end of which cannot yet be seen, has already given me 

 the following results. 



" The arc from charcoal points furnishes by the prismatic analysis 

 the most curious and brilliant appearance. Its spectrum is crossed 

 along its whole extent by a multitude of irregularly-grouped lumi- 

 nous lines ; but among these a double line is remarked situated on 

 the boundary between the yellow and the orange. As this double 

 ray recalled by its form and position the line D of the solar spectrum, 

 I was desirous of examining whether it corresponded to it, and for 

 want of instruments to measure the angles I had recourse to a pecu- 

 liar process. I projected upon the arc itself an image of the sun 

 formed by a converging lens, which permitted me to observe at once 

 the solar and the electric spectra superposed, and in this way I ascer- 

 tained that the double bright line of the arc coincides exactly with the 

 double black line of the solar light. This process of investigation 

 furnished me with the means of several unexpected observations. In 

 the first place, it proved to me the extreme transparence of the arc, 

 which but faintly shadowed the light of the sun. It showed me that 

 this arc placed in the path of a beam of solar light absorbed the ray 

 D, so that this line of the solar spectrum is considerably strengthened 

 when the two spectra are laid exactly over each other. When, how- 

 ever, they overlap, the line D appears blacker than usual in the solar 

 light, and comes out more brilliant in the electric spectrum, so that we 

 can easily judge of their perfect coincidence. Thus the arc offers to 

 us a medium, which itself emits the rays D, but at the same time 

 absorbs them when they come from another source. To make the 

 experiment still more decisive, I threw upon the arc the reflected 

 image of one of the incandescent points of charcoal, which, like all 

 bodies in ignition, gives no lines, and under these circumstances the 

 line D appeared as in the solar light. Passing, then, to the examination 

 of the arcs furnished by other matters, I have almost always found 

 the line D positive and at its place, and I have ascertained that it 

 coincides exactly with the ^brilliant line from the flame of a candle. 

 When the poles employed are of metals which give the ray D but 

 feebly, such as iron and copper, it can always be revived with ex- 

 traordinary intensity by touching them with potassa, soda, or by one o? 

 the salts formed by lime, or by one of these bases. Before conclud- 

 ing any thing from'the almost constant presence of the line D, it will 

 without doubt be necessary to ascertain whether its appearance does 

 not indicate the existence of the same substance mixed with all arc- 

 conductors. Nevertheless, this phenomenon already appears to us a 

 pressing invitation to the study of the spectra of the stars, for if 

 this same line should be discovered, stellar astronomy will be able to 

 make the discovery available. 



" I also endeavoured to make these different arcs coincide, and I 

 was again astonished by the appearance of unexpected phenomena. 

 During the coincidences of these various spectra I saw the electric 



