Radiation of Heat by Gaseous Matter. 279 



necting piece were attached to one end of the experimental tube, 

 and from them a length of india-rubber tubing led to the flue of 

 the laboratory stove. A gas-holder of air was put in connexion 

 with the other end of the experimental tube, a system of drying- 

 tubes intervening between the latter and the holder. By a slight 

 water-pressure a stream of dry air was carried gently through the 

 tube to the flue, and in this way the gases,- which if pumped out 

 would have injured the pistons, were speedily removed. As the 

 dry air replaced the gases, the needle gradually descended to 

 zero, its arrival there being indicative of the complete displace- 

 ment of the gas. The perfect dryness of the air thus made use 

 of was beautifully proved. Had the air contained moisture, it 

 would instantly on its mixture with hydrochloric acid have ren- 

 dered the medium within the tube turbid. However slight this 

 turbidity might be, and however invisible to the eye, the galva- 

 nometer would have revealed it. But there was no movement 

 in an upward direction; the needle gradually sunk from the 

 moment the air entered. 



As regards the influence of chemical union in the absorption 

 of radiant heat, a still more severe test than that furnished by 

 the substances last examined is presented by bromine and hydro- 

 bromic acid ; for the opacity of the former to light is far greater 

 than that of chlorine, while the two compounds are equally 

 transparent. The density of bromine, moreover, is 5*54, whereas 

 that of hydrobromic acid is only 2*75. The difficulty of opera- 

 ting with the acid compound of bromine is at least equal to that 

 attendant on hydrochloric acid; and several successive days were 

 spent in endeavouring to arrive at safe conclusions in connexion 

 with this subject. Bromine dried with phosphoric acid was in- 

 troduced into a flask furnished with a screw cap, which enabled 

 it to be attached to the experimental tube. By turning a stop- 

 cock, the pure vapour was allowed slowly to enter until the 

 mercury column was depressed two inches. From more than 

 twenty experiments made with this substance, I should infer 

 that the absorption of the quantity mentioned does not exceed 11, 

 while the absorption of hydrobromic acid of the same tension 

 amounts to 30. 



The hydrobromic acid was prepared by the action of glacial 

 phosphoric acid (for a pure specimen of which I have to thank 

 my friend Dr. Frankland) on bromide of potassium. If the 

 above figures represent the truth (and I have spared no pains to 

 arrive at aright conclusion), we have here a most striking instance 

 of transparency to light and opacity to obscure heat being pro- 

 moted by the self -same chemical act*. 



* A layer of liquid bromine, sufficiently opake to intercept the entire 

 luminous rays of a gas-flame, is highly diathermanous to its obscure 



