185 



wLich affect colours, and teaching that elastic fluids, when anhydrous, 

 lose ill whole, or in part, the power to destroy or change the tints 

 of organic bodies, which they possess when associated with water. 



Section fifth reviews the methods employed for drying gases, and 

 the tests of gaseous dryness. It enters at length into the question, 

 how far it is possible to confer absolute dryness on an elastic fluid, 

 and suggests some modifications of the processes at present in use, 

 which tlie author thinks will prove serviceable. Non-action on 

 colouring matter is likewise pointed out as a negative test of dryness 

 of some value, in relation to the gases which act on colours. 



Section sixth is devoted to the question. Does water accelerate the 

 action of gases on colours, in virtue simply of its conferring mediate 

 liquidity on the gas ? The author thinks not, and refers to the slow 

 action which he has observed of liquid anhydrous bromine and sul- 

 phurous acid on blue litmus, as contrasted with their rapid produc- 

 tion of destruction or modificatidn of colour when dissolved in water, 

 as shewing that the liquefaction of the gas is not the only cause of 

 its rapid action when moist. He contends that this is only to be 

 fully accounted for by taking into consideration the power of water 

 to liquefy both the colouring matter and the gas, and thus to bring 

 them into a closeness of physical contact in the highest degree 

 favourable to energetic chemical action. The author was led, in 

 connection with this view, to infer that dry gases would act on dry 

 colours in other liquids besides water, provided only these could 

 dissolve both the gas and the colouring matter. He finds, however, 

 that this cannot be laid down as a general proposition, at least so 

 far as chlorine is concerned, the only gas on which he has had 

 opportunity to make researches in reference to this point. The volatile 

 oils of the type of spirit of turpentine (C 5, H 4) and chloroform, 

 which contain no oxygen, and sulphuret of carbon, which contains 

 neither of the elements of water, dissolve the colouring principle of 

 alkanet root, and also chlorine, but the gas does not destroy the 

 colour. Solutions of dry litmus, on the other hand, in chloroform 

 and sulphuret of carbon, are instantly bleached by dry chlorine. 



The author's final conclusion is, that the function of water in 

 bleaching, speaking generally, is to dissolve the colour and the gas, 

 and so to bring them within the sphere of chemical aflSnity ; and 

 that water is more efficacious in accelerating bleaching than other 

 liquids, simply because it excels most of them in solvent power. 



