1882.] 



On Spectrum Photography. 



83 



oessively short lines or dots, which are totally absent from the photo- 

 graphs of solutions of zinc made from the same metal. Certain dis- 

 continuous lines in the spectrum of iridium become continuous when 

 moistened with calcic chloride solution. It has been remarked by me 

 elsewhere (Joe. cit.) that the more volatile, and I may now add, the 

 more unoxidisable a metal, the more continuous are its lines. The 

 compounds in solution are more volatile than the metals, and hence the 

 greater continuity in the lines. In the case of graphite it is doubtless 

 a volatile carbon compound, either carbon dioxide or a hydrocarbon, 

 which is formed by the heat of the spark when the points are 

 moistened with water. In the case of iridium it is difficult to suppose 

 that the calcic chloride solution forms a chloride by the simple action 

 of heat on such a refractory metal ; but this is the only explanation 

 that will account for the greater continuity of the lines. Insoluble 

 compounds give no spectra when mixed with water or glycerine and 

 exposed to the spark. The non-metallic constituents of salts do not 

 yield any marked series of lines, and therefore do not obscure the 

 metallic spectra. 



The spectrum of aluminium as obtained from perfectly pure solu- 

 tions is free from a group of short or discontinuous lines seen in my 

 published photographs of spectra. By prolonged exposure, as I have 

 elsewhere shown, these lines have been proved to be due to iron. 

 The spectrum of aluminium is thus proved to be a very simple one. 

 In all these spectra the rays lying between 4500 and 2000 on the 

 scale of wave-lengths are completely focussed on one plate, and the 

 relative intensities of the lines exhibit the relative intensities of the 

 rays. Any modification in the relative intensity of a line or in its 

 length is accurately registered on the sensitive plate. As many as 

 fifteen different spectra have been photographed on one plate, and 

 developed by one immersion in the developing solution. It has been 

 proved experimentally that accidental alterations in the period of normal 

 exposure, which are not very noticeable, do not affect the spectra. 

 Any irregularities such as may be unavoidable in the passage of the 

 spark do not alter the normal densities of the images of the various 

 rays. The development of the photographs is completed in about 

 thirty seconds. These points are of vital importance in placing this 

 method of working on such a basis that it may be employed in quanti- 

 tative methods of chemical analysis, for if the intensity of the rays 

 be so great that the period of exposure is rendered much shorter, 

 difficulties would arise in obtaining photographs with neither more 

 nor less than the requisite density. And, again, were the exposure 

 much prolonged the method would become somewhat tedious, or, at 

 least, it would be impaired in value. 



(4s.) Of all methods likely to yield results of practical importance 

 in estimating the relative proportions of the constituents of either an 



