ON ORGANIC COLOURING MATTERS. 493 



of the dry and moist gases. In our own day, however, the problem takes a 

 somewhat different shape, for we have learned to liquefy the gases without the 

 intervention of a solvent. Three of the fom* gases last referred to, which simply 

 dissolve in water, viz., chlorine, sulphurous acid, and sulphuretted hydrogen, 

 admit of liquefaction, although quite anhydrous. It has been held, accordingly, 

 that the liquefaction of a gas changes its properties, in the same way as dissolving 

 it in water would. 



With a view, so far at least, to examine this point, I exposed carefully dried 

 blue litmus-paper to the action of liquid bromine (which is equivalent to a lique- 

 fied gas), repeatedly rectified from chloride of calcium, and supposed to be an- 

 hydrous. Ultimately the paper was quite bleached ; but the decolorising action 

 was slow, certainly much slower than that of hydrated bromine. Specimens ac- 

 company this paper. The dark colour, however, of that element makes it an 

 unsatisfactory substance to work with, in relation to changes of tint in the bodies 

 upon which it acts. From experiments such as I have described, as well as fi-om 

 theoretical observations, it has been inferred that the function of water in rela- 

 tion to the gases I have been considering is simply to effect their mediate lique- 

 faction, and thereby to bring them into closer physical contact with the colom-ing 

 matters than their elastic condition permits. So general a conclusion, however, 

 as this, which would imply that a hquefied gas has the same properties as a dis- 

 solved one, is certainly in the meanwhile without proof, and is probably unten- 

 able. So far as they have been examined, the liquefied gases present properties 

 very different fi'om those exhibited by the same bodies when in aqueous solution, 

 although their action on colouring matters has been less inquired into than might 

 have been expected. 



There is, moreover, this manifest distinction between the action on a colouring 

 matter of a liquefied gas, and of an aqueously dissolved one, that in the former 

 case the gas only is in the liquid form, the colouring matter remaining solid, 

 whilst in the latter the water dissolves alike the colouring principle and the gas, 

 and brings both into a condition far more favourable to chemical action than 

 whei'e the one only is liquid . 



It is further certain that much must depend on the force of the adhesive 

 attraction of the liquidised gas for the dry colour. A liquid which cannot wet a 

 solid will exert little, perhaps no chemical action upon it, although it may pro- 

 duce a marked effect when both are dissolved in water. 



Again, if the liquidised gas can dissolve the colouring matter, we may be cer- 

 tain that, sooner or later, it wiU affect it ; but if it cannot dissolve it, the latter 

 may be totally unaltered by its presence. The slow action of dry bromine is 

 probably related, either to incapacity of quickly wetting, or of dissolving litmus ; 

 perhaps to both. 



In connection with this subject, I tried an experiment with liquefied anhydrous 



