THK HALorjKNS 459 



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

 of the resultant product as in detonating gas (Chap. III.). Diffused 

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

 an explosion. 12 The hydrochloric acid gas produced by the reaction of 

 chlorine on hydrogen occupies (at the same temperature and pressure) 

 a volume equal to the sum of the original volumes ; that is, a reaction 

 of substitution here takes place: H., + C1 2 = HC1 + HC1, the atoms of 

 chlorine and hydrogen change places there were two molecules and 

 two molecules are also obtained, although different molecules were first 

 taken and like molecules are formed. In this reaction twenty-two 

 thousand heat units are evolved for one part by weight of hydrogen. 13 



These relations show that the affinity of chlorine for hydrogen is 

 very great and analogous to the affinity between hydrogen and oxygen. 



- The quantity of chlorine and hydrogen \vliicheombineispropovtional 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 a means 

 for estimating the intensity of the chemical rays (as an actinometer), the influence of the 

 heat 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 given 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 enclosed 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. As 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 of 

 a foreign gas decreases the action of light; for example, 3^ of hydrogen by 38 p.c., 

 cfaj "f oxygen by 10 p.c.. , ( \ n of chlorine by 60 p.c., <tc. According to the researches 

 of Klimenko and IVkatoros ils.siM, 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 i> evolved in the reaction of chlorine on hydrogen, and as this reaction, 

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

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

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

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

 gas-s should be understood in this sense, as Pringsheim (1877) pointed out. 



15 In the formation of steam (from one part by weight of hydrogen) 29000 heat units 

 are evolved. The following are the quantities of heat (thousands of units) evolved in 



