( 677 ) 
already, and the lower rim cracked off. Later we made the connec- 
tion tube and the stopper more elastic (cf. H Pl. I fig. 2) by inserting 
between them a collar formed of several layers of paper glued together 
at the borders. In this way a closure was obtained which was 
perfectly tight, a quite necessary item, for otherwise the escaping 
liquid streams past the reading points as a cold vapour, which 
disturbs the uniform distribution of temperature supposed to exist 
in the ends of the rods and obtained by continually blowing air 
on to the points which is necessary also for keeping them dry. 
At the top, the rods are supported sideways so that no strain 
is caused in them. They are protected from the cold vapours which 
arise from the bath. From the front and side elevation of the 
upper end, Fig. 1, the arrangement of paper used for this protection 
can be clearly understood, and the course of the vapour can be 
followed as it streams over the wall of the bath through channels 
of cardboard. This arrangement has moreover the advantage, that 
the outer surface of the vacuum vessel is also cooled. This is of 
great importance in the beginning. The cold gas and cooled air are 
so conveyed away by various paper screens, that they do not come 
into the neighbourhood of the cathetometer or the standard scale, and 
also that air at the ordinary temperature remains between these and 
the points. At the commencement the liquefied gas is introduced in drops 
through an opening in the cork at the upper end, and afterwards 
carefully in small quantities. When the bath is once full, fresh liquid 
is continually added in small quantities to keep the level at the 
same height. The liquids used were nitrous oxide and oxygen obtained 
in the manner described in Comm. No. 14 (Dec. °94) and No. 51 
(Sept. 99). In both cases considerable purity was aimed at, in consequence 
the temperature of the bath did not change during the measurements. 
There is no doubt that the temperatures at the top and the bottom 
of the bath were not the same but this introduced no difficulty 
since in the calculation only the mean temperature as determined 
by the platinum resistance was required. 
3, The comparator (cathetometer and scale). We used the 
instruments which are described in Comm. No. 60 (Sept. ’00). The 
scale was very carefully enveloped in wool and paper to protect it 
from changes of temperature. Its temperature was read by two 
thermometers divided into */,, and symmetrically placed above and 
below, while the room temperature was maintained as constant as 
possible. The telescopes were provided with the microscope objec- 
tives which had been used for the measurements on the viscosity 
of liquid methyl chloride (Comm. No. 2, Febr. ’91) and which 
