1911-12.] Absorption of Light by Inorganic Salts. No. V. 45 
relative intensity being occasionally met with.* The arc was watched 
through a piece of red glass and the exposure stopped if it appeared 
unsymmetrical ; also a telescope was mounted so as to receive the light 
reflected from the first surface of the Cornu prism, thus giving an image 
of the slit, and it was used to watch the relative intensities of the two 
beams. 
In the arc finally used the electrodes were horizontal. The kathode 
slid in a tube in which it could be rotated by hand and contact was made 
by hand. After every exposure the electrodes were filed so as to remove 
the iron oxide that had formed. This was found necessary in the interests 
of the symmetry of the light. 
It may be mentioned in passing that in a recent paper j* A. H. Pfund 
describes a method of producing an iron arc that burns steadily for hours. 
It depends on the formation of a bead of oxide in a cup on the anode. I 
found that the symmetry of the ultra-violet radiation from this arc varied 
considerably even when the arc was apparently steady, and hence it could 
not be used. 
All the salts investigated, with the exception of sodium chloride, were 
obtained from Kahlbaum. The curves on p. 46 give the results for cupric 
sulphate, cupric chloride, cupric nitrate, and cupric bromide. The absorp- 
tion is so much greater in the ultra-violet that a different scale is used 
there. The thicknesses employed varied from 1 mm. to 4 cm., and the 
concentrations in grm.-mols. per litre were as follows : — 
Infra-red. 
Ultra-violet. 
Sulphate .... 
c= ’050 
c = ’040 
Chloride .... 
c=- 0496 
c = ' 037 
Nitrate .... 
c = * 049 
c = '039 
Bromide .... 
c = ’0541 
c = -039 
The difference between the different curves in the infra-red is just about 
the error of observation. In the ultra-violet in the case of the nitrate 
there is just a trace of the nitrate band. The points marked with os in. 
the ultra-violet in the case of the sulphate are taken from a former paper.J 
* The results for the ultra-violet given in the paper are probably right to about 7 per 
cent. In the near ultra-violet, where a Nernst filament can be used instead of the iron arc,, 
an accuracy of 1 or 2 per cent, can be attained. 
+ “ Metallic Arcs for Spectroscopic Investigations,” A. H. Pfund, Astroph. Jr ., xxvii.,, 
1908, p. 296. 
X “On the Absolute Measurement of Light : A Proposal for an Ultimate Light 
Standard,” R. A. Houstoun, Proc. Roy. Soc., 85 A, p. 275 (1911). 
