870 
PEOFESSOE W. A. MILLEE ON THE PHOTOGEAPHIC 
power ; but coloured bases, like oxide of iron, nickel, cobalt, or copper, are very inferior ; 
and when the salts which they form are green or yellow, they are nearly opaque. 
It is remarkable that, notwithstanding the high diactinic quality of silica, none of the 
different varieties of glass transmit rays extending beyond one-fifth or one-sixth of the 
range afforded by quartz. This absorptive action is produced by a lamina of glass less 
than the one-hundredth of an inch in thickness, which cuts off the more refrangible rays 
nearly as completely as a piece of glass of twenty times the thickness. All glass appa- 
ratus must therefoi’e be abandoned in these experiments, and apparatus of quartz substi- 
tuted for them. 
15. I had no encouragement in my attempts to construct prisms of other materials 
than rock-crystal. Eock-salt ofiers no advantage, and it is too soft and deliquescent to 
yield prisms or lenses comparable with those of quartz. I did indeed make a consider- 
able number of experiments with a hollow prism furnished with thin quartz sides, and 
filled with water. But the refractive power of water is less than that of rock-crystal, 
and its dispersive power is not higher. The addition of pure chloride of sodium till the 
water is saturated does not materially increase the refractive or dispersive power, whilst 
it appeared (in a very slight degree it is true) to diminish the amount of the more 
refrangible rays, so that, on the whole^ I found it more convenient to work with a quartz 
prism, the double refraction of which in the position in which I used it, was so shght 
that it was not a source of any inaccuracy of importance. 
b. Absorption by transmission through lAguids. 
16. In the experiments with liquids, the same plate-glass trough with quartz sides 
was used as when solutions were employed, and the apparatus was arranged in exactly 
the same manner. Great care was taken in the purification of each specimen. The 
ivood-spirit was prepared from oxalate of methyl by Wohlee’s method, and it, as well as 
the alcohol oxi^f ousel oil employed, was in the anhydrous state. The glycerin was per- 
fectly colourless, and was prepared by Mr. Geo. Wilson by distillation Avith superheated 
steam ; it retained about 4 per cent, of water. The specimen of glycol was the only one 
about which I had any doubt ; it had a barely perceptible yellowish tinge, and a very 
slight empyreumatic odour. The carbolic acid, given me by Mr. Ceace-Calveet, was a 
beautiful colourless specimen, which, by a slight reduction of temperature, solidified to 
a mass of delicate Avhite needles. The benzol was a specimen prepared from benzoate of 
lime, purified by congelation at 32°, and carefully rectified. The paraffin oil was a perfectly 
limpid colourless specimen obtained from Rangoon petroleum ; it had a specific gravity 
of 0'831, and boiled steadily at 360° F. The bisulphide of carbon, and indeed most of 
the other liquids, Avere rectified immediately before proceeding to experiment upon them. 
The folloAving Table contains a list of the liquids operated on, and the lengths of the 
different spectra, in terms of the scale already explained (par. 7). The compounds 
included in the folloAving Table, with the exception of nitric and hydrochloric acids, aie 
not simply solutions, but liquids to which a definite chemical formula may be assigned. 
