( 102) 
The Method. 
In their investigation Ramsay and TRAVERS found that the 
refractivity of a mixture of gases may be found, not accurately 
indeed, but vet with a high degree of approximation, from the 
molecular proportion of mixing and the refractivity of the compo- 
nents. In order to determine whether this deviation occurs always, 
or must be attributed to accidental circumstances, but at the same 
time in order to try and find the cause, I have first examined a 
series of mixtures of gases; viz. carbonic acid and hydrogen in 
different proportions. These gases were chosen, because their refrac- 
tivity differs considerably and because they may be prepared suffi- 
ciently pure in a simple way. 
The carbonic acid was prepared according to the method described 
by Mr. Kuenen ©), by dripping a solution of Na HCO; in Hy SO, 
and by drying the gas by means of H,SO, and Ps O;. 
Hydrogen was obtained by electrolysis of diluted HCl with an 
apparatus similar to that used by Prof. KAMERLINGH ONNES for 
filling the hydrogen-thermometer. *) ; 
The determination of the refractivity was made according to the 
method of Lord Rayrreran®), which was also followed by RAMSAY 
and Travers. A nearly parallel pencil of light is split into two 
parts, which pass through tubes of equal length closed by plate 
glass and which are made to converge by an achromatic lens. The 
interference phenomenon obtained in this way was observed with 
an eye-piece, consisting of two cylindric lenses. These tubes (A and 
B fig. 1) are respectively connected with the open manometer Z 
and £ and with the reservoirs C and G; the latter are partly filled 
with mercury and the pressure in them may be changed by moving 
the bulbs D and Z up and down. 
The right half of this apparatus is filled with dry air freed from 
CO,. The left side was filled with the gases and gaseous mixtures which 
were to be examined. To this end G was connected with another 
reservoir H with a bulb K filled with mercury, by means of the 
three-way cock ¢, and it was also connected with the tube MN, which 
may be connected by means of the taps g and A with two gas- 
reservoirs L and M; by means of # with the air pump; and by means 
of f with the apparatus for the preparing of gas. G and / serve 
for preparing the mixtures. 
1) Phil. Mag. (5) Vol. 44, p. 179. 
*) Versl. Kon. Akad. 50 Mei 1896. 
3) Proc. Roy. Inst, Vol. XV Jan. 1896, 
