118 
rierbad’”’, whose index of refraction lies as close as possible to that 
of the liquids to be investigated. Aqueous solutions with strong 
refractivity (e.g. of cane-sugar or cadmium salts), which are prefer- 
able to organic substances, because they do not attack the cement 
with which the windows are fastened in the reservoir W, proved 
unsuitable, on account of their high viscosity. After also a number 
of mixtures of little volatile organic liquids (paraffin-oil-methylsali- 
cylate and methylnonyl-ketone-methylsalicylate) had been tried, in 
which, however, difficulties also presented themselves, tetrachlor- 
ethane itself was taken for it. Previously the rims of the windows 
had, however, to be protected, for which purpose a layer of an 
aqueous glue appeared to be very effective, and the paint had to be 
removed from the reservoir. 
6. Ad B. The influence of the temperature on the index of refrac- 
ton of the solvent. 
The strong and always recurring curvatures of the bands are 
partially owing to the great value of the temperature coefficient of 
the index of refraction in organic liquids, accompanied with a very 
small specific heat. 
From KANONNIKOFF's determinations ') En = — 0.0005 may e.g. be 
calculated for tetrachlorethane at + 20°. 
Benzene has Wz — 0.00065 at 20°. 
A change of temperature of 0.01° gives, therefore, rise to a change 
in the index of refraction of 6 units in the 6 place of decimals, 
which when a 2 cm. vessel is used, corresponds to a displacement 
of 10 scalar divisions. For water these values are about eight 
times smaller 
Exceedingly slight temperature disturbances through addition of 
heat from outside, which occur especially on a prolonged stay of 
the observer in the neighbourhood of the apparatus, give much 
sooner rise to curvatures and displacements of the bands in such 
liquids than in water. 
7. To prevent this: 
1. the interferometer was surrounded by a large thermostat, filled 
with water. 
2. a stirrer was placed in the bath liquid. 
1) Journ. für prakt. Chemie, 32, 520 (1885). 
