524 
Proceedings of the Royal Society of Edinburgh. [Sess. 
was used for obtaining all the results given in this paper. A single 
straight Nernst filament without a heater, of the kind known in Germany 
as Nernststiften an Brticken, was used as source. It took a current of one 
ampere, which was supplied by a battery of 113 storage cells. The current 
through the filament, and consequently its brightness, was thus very steady. 
The filament was shunted by a current balance and resistance for the purpose 
of checking the voltage on it. The light from the filament was made parallel 
by an achromatic lens ; it then passed through a prism set approximately at 
minimum deviation, and was focussed by a second lens on the slit. By 
rotating the prism the various colours could be passed across the slit. The 
arrangement thus practically consisted of two spectroscopes in series, the 
filament itself being the slit of the first spectroscope. The cells and wooden 
screen were placed in front of the slit of the second spectroscope as before. 
Another cause of error arose from the fact that the cells were usually 
not quite plane parallel but slightly prismatic, or perhaps they were not 
placed in position accurately. With the first arrangement this moved the 
image of the filament slightly off the slit and altered the deflection appreci- 
ably ; but with the final arrangement the error was not so serious, and 
with a little care it could be avoided. 
The salts experimented on were the fluoride, chloride, bromide, iodide, 
nitrate, and sulphate. They were obtained from Kahlbaum. 
The curves on the next page, which give the results of three independent 
determinations of A for the nitrate, give an idea of the accuracy obtained. 
The solutions were made up independently, and different cells were 
employed for each determination. 
The following table gives the results : — 
Values of A for Cobalt Salts in the Infra-Red. 
Scale 
Reading. 
A. 
d = 3 cm. 
d=roo cm. 
c = *077. 
Fluoride. 
c = *67. 
Chloride. 
c = -65. 
Bromide. 
c = *417. 
Iodide. 
c = -67. 
Nitrate. 
c — - 67. 
Sulphate. 
50 25 
•645,u 
•21 
•41 
•27 
•53 
26 
•25 
50 10 
•684 
•25 
•24 
•15 
•25 
•11 
•14 
49 58 
•720 
•14 
•31 
•15 
•12 
•10 
•05 
49 49 
•750 
•05 
•28 
•09 
11 
•10 
•03 
49 40 
•794 
•12 
•28 
•12 
•14 
•12 
•07 
49 31 
•850 
•12 
.•28 
11 
12 
•13 
•12 
49 22 
•910 
•15 
•28 
14 
•16 
•18 
•12 
49 13 
•980 
•16 
•40 
•23 
•30 
•33 
•22 
49 4 
1-070 
•46 
•70 
•60 
•61 
•66 
•59 
48 55 
1170 
•68 
1*14 
1-05 
1-06 
1-15 
104 
48 46 
1-270 
1-02 
1-40 
1-40 
143 
T42 
1-54 
