( 20) 
If we want to compare them with those made by H. Brequeret, 
who has worked at a pressure of 1 atm. we must make use of a 
supposition on the relation of the rotatory constants at 100° and 
at 1 atm. The simplest supposition already made in the treatise men- 
tioned is, that the rotation is proportional to the density of the gas !). 
But then differences will be found between BecQuEREL’s results and 
mine, which in the case of oxygen amount to over 10°, and even 
more than that in the case of other gases. These differences might 
make us doubt the validity of our supposition *), and this caused 
me to make some measurements on the rotatory constants in oxygen 
at different pressures, in order thus to put to the test the propor- 
tionality of rotation and density. 
Till now the pressures were read on a metal-manometer, the 
corrections of which had been previously determined *). In order to 
obtain also a sufficient precision especially for lower pressures, the 
pressure was measured with the hydrogen manometer, used by 
VERSCHAFFELT in his investigations on the isothermals of mixtures 
of CO and H,*). I used this manometer in the same way as he 
did when he compared its indications with those of the standard 
open manometer °). The copper connecting-tube mentioned there was 
now fixed to my apparatus. Mr. HARTMAN kindly took upon him to 
take the readings of the manometer. 
The D'ARSONVAL galvanometer®) has now been clamped in a stand 
suspended as invented by JuLius, to protect it against vibrations’), 
so that the readings could be taken with greater accuracy. For the 
rest, everything was used in the same way as before. 
The observations are made with commercial oxygen at four pres- 
sures, at each of which three or two sets of adjustments have been 
made, in the same order as before®). The calculation®) then yields 
: : @ 3 
for each a number proportional to Ts where @ is the rotatory 
constant and d the density, borrowed from AMAGAT’s '°) observations. 
These numbers are: 
4) ‘Lhesis for the doctorate, Leyden 1899, p. 15. Comm. Phys. Lab. No, 45 and 47. 
5) Thesis for the doctorate, p. 17. 
9) Arch, Néerl. p. 305. 
7) Wied. Ann. 56, p. 151 (1895). 
8) l. c p.320. 
9) Le. p. 325—380. 
0) Ann. de Ch. et de Ph. (5) 19 p. 375 (1880). 
