AND THE ABSORPTION SPECTRUM OF GOLD. 49 



2. Gold chemically reduced in a dilute aqueous solution of its chloride so-called 



colloidal gold the metal being in sufficient quantity and state of aggrega- 

 tion to transmit greenish-blue light. 



3. Ditto, a less amount of more finely distributed gold transmitting ruby-red 



light. 



4. Glass coloured by gold so as to transmit greenish-blue light the so-called 



saphirine glass. 



5. Glass coloured ruby-red by very finely divided gold. 



It was found to be impracticable to secure any result for the gold-leaf, on account 

 of the very small amount of light transmitted. For the colloidal gold in water, and 

 the gold-coloured glass, the following results were obtained. 



Visible Spectrum. 



The source of light was a strong electric arc between closely placed carbon poles, 

 with a slit of about ^ millim. in width. Dispersion was obtained by a Rowland 

 concave grating of 21'5 feet focal length, railed with about 15,000 lines to the inch, 

 using the spectrum of the first order. The photographs were taken on M. A. Seed 

 dry plates (" orthochrornatic, L"), specially sensitized for the region from green to 

 red inclusive. The original photographs were laid side by side, so that the positions 

 of like wave-lengths were the same for all, and then re -photographed together on a 

 reduced scale. The results are shown in tig. 12, with a few positions indicated in 

 Angstrom units. 



Taking the strips in order from the top downwards, the first (uppermost) strip 

 represents the light transmitted simply through a sheet, about 2 rnillims. thick, of 

 colourless glass of the same kind as that on which the gold ruby-red is " flashed," and 

 which also formed the end plates of cells containing the colloidal gold in aqueous 

 suspension time of exposure about 2 minutes the darkness at the less refrangible 

 end is due, not to absorption, but to the insensitiveness of the photographic film for 

 rays in this region. The second strip shows the effect of transmission through a 

 column of water, 2 '2 5 centims. long, containing 75 milligs. of metallic gold to the 

 litre, reduced from the chloride by potassium acid carbonate and formic aldehyde, 

 and exhibiting dark greenish-blue colour time of exposure 30 minutes. The third 

 represents also colloidal gold in watery suspension, but in a column of 9 '2 5 centims. 

 long, with 50 milligs. of gold per litre, and showing a blue or slightly violet-blue 

 colour time of exposure 20 minutes. The fourth represents the same, in a column 

 of same length as the last, but with only 20 milligs. of gold per litre, and showing a 

 clear ruby-red colour time of exposure 10 minutes. The fifth, shifted over to the 

 right to secure correspondence of position for equal wave-lengths, is the same as the 

 first, but with shorter exposure; the right-hand end is in the region of slight 



VOL. ccm. A. H 



