Dr. Gladstone on Refract ion- Equivalents. 311 



of the gas ; and this is in fact the case, for electric sparks passed as 

 nearly as possible under similar conditions through hydrogen, oxygen, 

 chlorine, and sulphurous anhydride emit light the intensity of which 

 is very slight in the case of hydrogen, considerable in that of oxygen, 

 and very great in the case of chlorine and sulphurous anhydride. 

 When liquefied sulphurous anhydride is sealed up in a strong tube 

 furnished with platinum wires, and the temperature then allowed to 

 rise until the internal pressure amounts to three or four atmospheres, 

 the passage of induction-sparks through the enclosed gas is attended 

 with very brilliant flashes of light. Further, if a stream of induc- 

 tion-sparks be passed through air confined in a glass tube connected 

 with a condensing syringe, and the pressure of the air be gradually 

 augmented to two or three atmospheres, a very marked increase in 

 the luminosity of the sparks is observed, whilst on allowing the con- 

 densed air to escape, the same phenomena are observed in the reverse 

 order. 



The electric arc from fifty cells of Grove's battery is incomparably 

 more brilliant when mercury vapour, instead of atmospheric air, is 

 interposed in the path of the discharge between the carbon points. 

 The gases and vapours just mentioned have the following relative 

 densities : — 



Hydrogen . 1*0 



Air 14-5 



Oxygen .... , 16*0 



Sulphurous anhydride 32*0 



Chlorine „ 35 '5 



Mercury 100-0 



It is obvious that the above results have a very direct bearing upon 

 the views now generally held regarding the constitution of the sun, 

 stars, and nebulee ; but I refrain from making any such application 

 of them until I have the honour of laying before the Royal Society 

 a complete account of these experiments. 



June 18. — Lieut. -General Sabine, President, in the Chair. 



The following communications were read : — 



"Researches on Refraction-equivalents." By J. H. Gladstone, 

 Ph.D., F.R.S. 



Since the paper of the Rev. T. Pelham Dale and myself " On the 

 Refraction, Dispersion, and Sensitiveness of Liquids "*, our re- 

 searches have been continued from time to time, and a good deal of 

 attention has been paid to the subject in Germany. The permanence 

 of the specific refractive energy of a body, notwithstanding change of 

 temperature, aggregate condition, solution, or even chemical combi- 

 nation, has been confirmed, and upon this has been built the doc- 

 trine of Refraction -equivalents. 



Our " specific refractive energy " is the refractive index of any sub- 

 stance minus unity, divided by the density — in symbolic language 



* Philosophical Transactions, 1863, p. 317. 



