34 ON THE DETERMINATION OF VERDET’S CONSTANT IN ABSOLUTE UNITS. 
during which it has been in progress ; and secondly, to Mr. Whipple, Director of the 
Kew Magnetic Observatory, who sent me all the data for calculating the value of H, and 
had several series of experiments and calculations made for me at the Observatory. 
Appendix. 
Analysis of the Bisulphide of Carhon employed. — Mr. J. M. Thomson, Demonstrator 
of Chemistry at King’s College, London, had the kindness to test the purity of the 
bisulphide for me. He writes : — “ As far as I can see, the CS 2 you sent me is perfectly 
good. Its sp. gr. (the mean of three determinations) is T275 at 15°, the sp. gr. of pure 
CS 2 being L271 at 15°. Its boiling point is rather above 47°, so that the sample may 
be considered as very nearly pure. It leaves a trace of sulphur on evaporation, but not 
a quantity that can be weighed.” 
During the year which elapsed between the experiments and the testing of the 
bisulphide the latter was kept in a stoppered bottle of thick blue glass. 
The temperature of the bisulphide , at the time of the optical experiments, was unfor- 
tunately not taken; but I believe it was about 55° F., as it was a frosty night, while a 
good fire kept the laboratory pleasantly warm. 
The logarithms used in the calculations are from Chambers’s Mathematical Tables, 
Ed. 1873. 
