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 four 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 from 
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 colouring 
matters than their elastic condition permits. So general a conclusion, however, 
as this, which would imply that a liquefied 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 from 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 
where 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 will 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 
