( 245 j 
inside the jacket. This arrangement proved very satisfactory, the 
liquid in the testing tube is perfectly quiet and free from bubbles 
of vapour. 
At first the space round the nicols formed one continuous space 
with that round the testing tube, and so the nicols were also sur- 
rounded with an atmosphere of methyl chloride vapour. In conse- 
quence of this, vapour still condensed on the nicols and accurate 
adjustments could not be made. In order to avoid this the spaces 
round the nicols have been separated from that round the testing 
tube by the brass rings £, fastened on the testing tube by means of 
sealing wax and closed by means of india rubber rings on the jacket. 
As at the low temperature a considerable decrease of pressure is to 
be expected in the imperfectly closed nicol spaces, the caps V have 
been pierced by copper tubes MZ, connected to U-tubes with narrow 
openings and filled with pieces of sodium hydroxide. 
Before closing the apparatus, the nicols have been adjusted at a 
given angle by fastening to one of them a long bent copper-wire, 
the end of which could be moved over a divided scale. This adjust- 
ment is not accurate and no use has been made of this angle in the 
ealeulation of the results, the rotations having been compared with 
those in water. 
The rotation at different wave-lengths is again obtained by varying 
the strength of the current, by doing which, the dark band can be 
caused to move over the whole spectrum. 
The optical and magnetical part is almost the same as that de- 
seribed in the previous communication, except that for the measure- 
ment of the current we have again used a pb’ ARsonvAL-galvanometer 
with shunt (comp. Comm. Suppl. 1 p. 25, Arch. Néerl, (2) 2 p. 305). 
3y comparing it with a Weston-millivolimeter it is found that the 
sensibility of the galvanometer is constant within the limits we had 
to fix for the accuracy. 
Fig. 2 gives a general survey of the apparatus used. C represents 
an are lamp, ZB Arons-LumMMER’s mereury electric lamp, A a colli- 
mator, ) a water reservoir, P? a prism and Q a telescope on a 
circle from MryersrriN. Further G represents the coil with the 
methyl chloride apparatus, 47 the cock, by which the supply of liquid 
methyl chloride for the jacket is regulated. 
The arrangements for regulating the current and measurement, 
with resistances and switches, are similar to those for the experi- 
ment on the magnetic rotation in gases *). 
') Comm. Leiden Suppl. 1 p. 25; Arch. Néerl. (2) 2 p. 315. 
