( 103 ) 
{f the substances used followed the laws of BoyLe and Gay- 
Lussac, the refractivity would be directly found from the pro- 
portion of the changes in pressure, which the gas and the air must 
undergo, when the interference phenomencn is twice brought into 
that position which it occupies when the two parts of the pencil 
of light pass through equal ways; so for instance when the two 
tubes are filled with air of one atm. So if p and p’ are the press- 
ures for air; pj and p’ those for the gas whose refractivity B is 
gar 
Pr Pi 
The gases, however, do not follow these laws, and therefore the 
refractivity is not to be found from the proportion of the change 
of the real pressure (p,p’,p; and p';), but from that of the pressures 
which would prevail, when the gases followed the laws of BOYLE 
and Gay-Lussac and had the same volumes and temperatures 
(P, P', P;, P’)). So that we may put: 
is to be determined, we get: B= 
bl Arae za 
EET EPN, 
P=p 
from which follows with some approximation : 
Eem Melt Lt (FE) (et) 
BAPE Bip’ ET EAS, 
met (=) (aha) 
Bs hi (FP) [oto (et gig) ete (eb) Ë 
This correction was also applied in calculating the composition 
of the mixtures. 
In order to find the values a and b for the mixtures CO, and 
Hs, I made use of the formula, deduced by Prof. vaN DER WAALS 
from the experiments of Dr. VERSCHAFFELT '). 
y = 0.999546 + 0.001618 (1 — z) + 0.00497 (1 — 2)? 
Re ieee | T 
this gives for 18°C. (7=291) at once the value of ax — bz agi 
) Proc. Roy. Acad. April 1899. 
Proceedings Royal Acad. Amsterdam. Vol. IL 
