THE HALOGENS 465 



with spongy platinum, or a strongly heated substance, or when subjected 

 to the action of an electric spark. The explosion in this case takes place 

 for exactly the same reasons i.e. the evolution of heat and expansion of 

 the resultant product as in the case of detonating gas (Chapter III.) 

 Diffused light acts in the same way, but slowly, whilst direct sunlight 

 causes an explosion. 12 The hydrochloric acid gas produced by the 



by Draper, Bunsen, and Roscoe. Electric or magnesium light, or the light emitted by 

 the combustion of carbon bisulphide in nitric oxide, and actinic light in general, acts in 

 the same manner as sunlight, in proportion to its intensity* At temperatures below- 12? 

 light no longer brings about reaction, or at all events does not give an explosion. It was 

 long supposed that chlorine that had been subjected to the action of light was afterwards 

 able to act on hydrogen in the dark, but it was shown that this only takes place witH 

 moist chlorine, and depends on the formation of oxides of chlorine. The presence of 

 'foreign gases, and even of excess of chlorine or of hydrogen, very much enfeebles the 

 explosion, and therefore the experiment is conducted with a detonating mixture 

 prepared by the action of an electric current on a strong solution (sp. gr. T15) of hydro- 

 chloric acid, in which case the water is not decomposed that is, no oxygen becomes 

 mixed with the chlorine. 



12 The quantity of chlorine and hydrogen which combine is proportional to the intensity 

 of the light not of all the rays, but only those so-termed chemical (actinic) rays which 

 produce chemical action. Hence a mixture of chlorine and hydrogen, when exposed to 

 the action of light in vessels. of known capacity and surface, may be employed as anactino- 

 meter that is, as a means for estimating the intensity of the chemical rays, the influence of 

 iheheat rays being previously destroyed, which may be done by passing the rays through 

 water. Investigations of this kind (photo-chemical) showed that chemical action is 

 chiefly limited to the violet end of the spectrum, and that even the invisible ultra-violet 

 rays produce this action. A colourless gas flame contains no chemically active rays ; the 

 flame coloured green by a salt of copper evinces more chemical action than the colourless 

 flame, but the flame brightly coloured yellow by salts of sodium has no more chemical 

 Action than that of the colourless flame. 



As the chemical action of light becomes evident in plants, photography, the bleaching 

 of tissues, and the fading of colours in the sunlight, and as a means for studying the 

 phenomenon is given in the reaction of chlorine on hydrogen, this subject has been the 

 most fully investigated in photo-chemistry. The researches of Bunsen and Roscoe in 

 the fifties and sixties are the most complete in this respect. Their actinometer contains 

 hydrogen and chlorine, and is surrounded by a solution of chlorine in water. The hydro- 

 chloric acid is absorbed as it forms, and therefore the variation in volume indicates the 

 progress of the combination. A.S was to be expected, the action of light proved to be 

 proportional to the time of exposure and intensity of the light, so that it was possible to 

 conduct detailed photometrical investigations respecting the time of day and season of 

 the year, various sources of light, its absorption, &c. This subject is considered in detail 

 in special works, and we only stop to mention one circumstance, that a small quantity ot 

 a foreign gas decreases the action of light ; for example, s&j of hydrogen by 88 p.c., 

 f$9 of oxygen by 10 p.c., r&s of chlorine by 60 p.c., &c. According to the researches of 

 Klimenko and Pekatoros (1889), the photo-chemical alteration of chlorine water is 

 retarded by the presence of traces of metallic chlorides, and this influence varies with 

 different metals. 



As much heat is evolved in the reaction of chlorine on hydrogen, and as this 

 reaction, being exothermal, may proceed by itself, the action of light is essentially the 

 same as that of heatthat is, it brings the chlorine and hydrogen into the condition 

 necessary for the reaction it, as we may say, disturbs the original equilibrium ; this is 

 the work done by the luminous energy. It seems to me that the action of light on the 

 mixed gases should be understood in this sense, as Pringsheim (1877) pointed out. 



