REFRACTIVE INDEX OF GASES ON TEMPERATURE. 43<) 
As a source of light, I used an ordinary Bunsen burner, placed about 150 centims. 
from the first glass block, and a small piece of bicarbonate of soda was held in tlie 
flame. This gave a brilliant yellow flame for a long time. The position of the 
interference bands was observed in a telescope with a micrometer scale in the 
eye-piece. 
Tlteory of the Measiirempnt. 
Mascart and others have established that for pressures in the vicinity of 
atmospheric pressure the refractive power is proportional to the pressure, or 
p =: I + Kp, 
where /c is a function of the temperature. 
This is only true in cases where Boyle’s law practically bolds. In the case of 
such gases as ammonia, where the deviation from Boyle’s law is aj^preciahle, a 
correction is required. 
Let d he the length of either tube, 
X the wave-length of Na light, 
Pq the initial pressure in the tubes, 
'Pi the final pressure in first tube, 
■p ,2 the final pressure in second tube, and 
n the number of bands displaced, then 
_ 71 X X 
{Pi- P^) 
The measurements were made as follows:—Steam or water was allowed to run 
through the jacket for over an hour until the temperature was steady, and no drift 
of the hands was observed when the tap K was open. The Imnds were then adjusted 
so that a band was on the cross wire in the telescope. The two limbs of the 
manometer were read, and also the two thermometers in the jacket, and the 
thermometer hung beside the manometer. The tap K was then shut, the reservoir 
B lowered, and the tap P opened. When about 100 hands had passed, the tap P 
was shut and the position of the band observed, tlie manometer read, and also the 
three thermometers. B was then raised, the tap P opened, and the mercury allowed 
to flow back to A. The tap P was shut when the original position was attained and 
the readings again made. This method provides a test of any possible drift of 
the hands in one direction due to creeping changes of temperature. The proper 
temperature to take is readily seen to he the temperature as recorded when the 
pressure has been reduced, and not the mean of the initial and final temperatures. 
With regard to the accuracy I consider that I was able to estimate millim. on 
the manometer scale. An interference band being a fuzzy thing and not sharp, I 
found it impossible to estimate more than wotli of a band. The hands appeared about 
