478 DR GEOEGE WILSON ON THE ACTION OF THE DRY GASES 



philosophers and chemists have agreed to call the actinic influence of the sun- 

 beam, that the mixed gases contained in a graduated tube over water are found 

 to form a delicate actinometer, the intensity of the actinism being measured by 

 the rapidity with which the water rises in the tube, as it dissolves the hydrochlo- 

 ric acid produced by the union of the gases.* This actinic exaltation of afiQnity, 

 so striking when both gases are free, continues to manifest itself, though less 

 powerfully, when chlorine is in contact with substances containing hydrogen, 

 although the latter is in a state of combination. 



Chlorine water remains unchanged in the dark, but is rapidly converted by 

 sunlight into hydrochloric acid, and free oxygen. Dutch liquid, chloroform, and 

 chloric ether, besides various other bodies, are known to give up their hydrogen 

 to chlorine much more swiftly when exposed to the direct rays of the sun than 

 if shaded from them. It seemed in the highest degree probable that the hydro- 

 gen of organic colouring matters would, in like manner, resist the action of dry 

 chlorine for a much shorter period in sunlight than in diffuse daylight, or in 

 darkness. To determine this point, the following experiment was tried. A wide 

 glass-tube, open at both ends, was constricted in the middle so as to present a 

 narrow central canal, like that of an hoiu'-glass. Pieces of blue and of red 

 litmus-paper were then placed on either side of the constricted portion, and the 

 open ends of the tube drawn out at the blow-pipe, so as to admit of their being 

 put in communication, by means of caoutchouc connectors, with an arrangement 

 for drying the paper, and furnishing chlorine. After the paper had been ex- 

 posed to a current of dried air at the temperature of 220° Fahr. for three hours, 

 washed chlorine, transmitted through Nordhausen sulphuric acid, and a tube 

 three feet long containing fused chloride of calcium, was sent along the double 

 tube containing the papers, for five minutes. The ends of the tube were then 

 sealed whilst it remained full of gas, and the constricted middle portion closed 

 and divided at the blow-pipe, so that the double tube was converted into two 

 single hermetically sealed ones, each containing dried litnuis paper in an atmo- 

 sphere of chlorine. In this way two tubes were procured, each containing por- 

 tions of the same coloured paper, which had been dried in the same current of 

 air, and exposed in exactly similar circumstances to the same stream and the 

 same amount of dry chlorine. 



The one of these twin tubes was hung up inside a window, with a western 

 exposure, on or about the 31st of July 1847. The other was laid aside in a cup- 

 board, out of reach of the direct rays of the sun, but not protected from the influ- 

 ence of dull daylight. It was frequently brought out, moreover, to be examined, 

 and was at no time during the day in absolute darkness. I shew the Society 

 this tube after remaining in the circumstances described for more than eight 



* Lond. and Edin. Pliil. Mag., 1844, vol. xxw, pp. 2-3. 



