Chemical Constitution of Saline Solutions. 265 
Composition and Formula. 
Calculated for Found. Erdmann’s 
Nile-7H20. Per cent. Analysis. * 
Wi, 5 «o o LOSs 13°29 2g 
“ . ea) Nil, . . . 72°82 
I, OESO 58:02 
IO, 5 o oo Berae 28°80 (mean) GLO), 5 o Bariles 
99:99 100-11 100:00 
These numbers agree so very closely with the composition of a salt of the 
formula Nil,7H,O that I have no hesitation in assigning it that formula, though it 
is usually stated to be a hexahydrate. 
It is completely dehydrated by standing over sulphuric acid and by heating to 
100° C. When dried without the action of heat, it is a black amorphous powder. 
These salts were prepared in my laboratory, and analysed by Mr. J. A. 
Cunningham, B.A., A.R.C.Se.1. 
PART II. 
Spectra of the Second Kind. 
The salts of cobalt for the most part present spectra characterised by broad 
absorption bands. The bands are readily weakened in intensity by diluting 
saturated solutions. In this respect the spectra differ widely from those of uranium 
salts, salts of (didymium) praseo- and neo-dymium, all of which exhibit absorption 
bands of the third kind and have been closely studied. 
The differences arise from differences in the constitution of the spectra, and 
may best be exemplified by stating that not only are they greatly modified by 
dilution, but very slight differences in the thickness of the layer of liquid present 
very different spectra. While in the case of uranium compounds and didymium 
salts, a smail difference in thickness of liquid or even great dilution does not alter 
the intensity or the width of the absorption bands in a proportionate degree. 
This is best shown by integrating the spectra, taking for ordinates the proportional 
thickness of liquid, and for the abscisse the wave-lengths or oscillation-frequencies 
of the rays absorbed. The curves so obtained differ so very widely in the two or 
three classes of salts that they cannot be reproduced as suitable illustrations. 
There can be no doubt, when all the chemical evidence available is taken into 
consideration, that the spectra belong to substances quite differently constituted. 
H. W. Vogelf examined a number of inorganic coloured substances, such as 
* Jour. fiir prakt. Chem., vol. 7, p. 254. 
+ Ueber die Verschiedenheit der Absorptionspectra eines und desselbeu Stoffs. Ber., vol. xi., p. 918, 
1878. 
