449 
REFEACTIVE INDEX OF GASES ON TEMPERATURE. 
containing dry calcium chloride. The calcium chloride absorbs large quantities of 
the gas. One end of the tube was then sealed up and the other attached to the 
apparatus, and the whole exhausted with the oil-pump. On gently warming the 
calcium chloride tube, the ammonia gas was liberated. The former method, of 
exhausting and filling several times while the tubes were kept at about 90° C., was 
adopted; and after the gas had been in the apparatus for a few days, the process 
was repeated. 
More care must be taken in the case of ammonia, since the gas does not strictly 
follow tlie ordinary gaseous law. 
If 'p be tlie pressure and t the temperature, the refractive index may be written 
p = 1 + + ¥). 
( l +«0 
Hence if p^ be the initial pressure in the tidies, 
p>i ,, final ,, ,, one tube. 
Pi )) >) ,, ,, second tube, 
the number of bands displaced 
oc -p^yp + X {pi + p.2)] . 
• • (1 ai) 
According to Mascaet" X for ammonia = '000178 per centimetre of mercury. 
We must, therefore, take care that tlie value of 2^1 + j^i hoes not vary to any 
extent throughout the series of measurements. Tliis point was carefully attended 
to, and the value of {jp + 2 ^ 1 ) was equal to 120 centims. throughout, the variation 
not exceeding 2 centims. 
My mam object being the temperature coefficient, and not so much the absolute 
value of /i, I did not make any measurements with another value of 2P + 2^- This 
omission I now regret; but Mascart’s value may be used, as he made experiments 
specially on this point, and had much greater ranges of pressure than my apparatus 
was an-anged for. In his papers I cannot find that he measured the temperature 
coefficient. 
* ‘ Comptes Rendus,’ vol. 86, 1878, p. 321. 
3 M 
VOL. CCI. — A. 
