50 PROFESSOR J. W. MALLET ON THE STRUCTURE OF GOLD-LEAF, 



sensitiveness of the film. The sixth (lowermost) strip shows the result of trans- 

 mission through the "flashed" ruby-red glass, with very long exposure 1 hour and 

 10 minutes. 



In these photographs there is no indication of well defined absorption bands. The 

 general absorption belongs mainly to the middle portion of the spectrum, and is, on 

 the whole, more marked at the less refrangible end, with notable increase of 

 absorption in this region as the amount of gold present is increased. The position of 

 maximum absorption is nearer to the long-wave end for the glass than for the 

 colloidal gold in water. It is interesting to note that, while no photographed results 

 could be obtained from the saphirine glass, the absorption being too far in the red for 

 the sensitiveness of the film, eye observation of this glass, using sunlight and a glass- 

 prism spectroscope, showed a distinct belt of absorption extending from about 5700 

 to 6250, beside the general absorption of rays of shorter wave-length. Allowance 

 has to be made in the photographs for insensitiveness of the film at the red end of 

 the spectrum. 



Ultra-violet Spectrum. 



Tins was examined with a quartz prism, and for the liquids a tube closed at the 

 ends by plates of quartz. The source of light was electric sparks between cadmium 

 poles placed pretty near each other. The results are shown in figs. 13, 14 and 15, a 

 few of the positions being indicated by the wave-lengths of the cadmium lines, as 

 before. No results could be obtained for the saphirine or the ruby glass, the glass 

 alone absorbing all rays in the ultra violet. Fig. 13 represents the water with 

 colloidal gold in suspension, 75 milligs. to the litre, in a column of 2 '25 centims. long. 

 Fig. 14 represents a like liquid, with 50 milligs. per litre, and in a column 9'25 

 centims. long. Fig. 15 is the same, with 20 milligs. per litre, and in a column 

 also ( J'25 centims. long. 



In each of these three figures the three uppermost strips represent exposures for 

 3, 5 and 10 minute* respectively (counting from above downwards), the light passing 

 through the colloidal gold liquid, while the four lower strips exhibit the results from 

 sparks through air (no gold liquid interposed) for 1, 2, 5 and 10 seconds respectively. 

 The general absorption, without indication of dark bands, begins to be well marked 

 at about 3500, and increases toward the more refrangible portion of the spectrum, the 

 effect increasing also with the amount of gold present. 



Infra-red Spectrum. 



This was examined, by the obliging permission of Professor S. P. LANGLEY, 

 Secretary of the Smithsonian Institution, Washington, D.C., in the astrophysical 

 laboratory of that institution, using sunlight, a rock-salt prism, and Professor 



