ON ORGANIC COLOURING MATTERS. 487 
even the slightly moist gas produces. I shew the Society a tube, containing lit- 
mus-paper, which withstood, without change, a current of hydrochloric acid 
passed over it for five minutes. The experiment was made on 23d February 
1848 ; and the paper is at this date not a bright red, but only a dark lilac. 
Three explanations suggest themselves as to the ultimate though imperfect 
reddening of the blue litmus. 1. The gas may not have been quite dry. 2. 
Hydrogen acids, though perfectly dry, may, unlike the so-called oxygen acids, be 
able to modify the tints of colours. 3. Hydrochloric acid, which has a great affi- 
nity for water, can compel its elements to unite to form it, so that the gas may 
combine with the liquid. Hydrogen and oxygen are present, both in the colour- 
ing matter and in the paper. In the paper, indeed, they are present in the pro- 
portion to form water. This solvent, therefore, may be slowly generated within 
the sealed tube, and be the cause of the gradual reddening of the blue paper. 
As to the first of these views, I can neither disprove nor confirm it. It is pro- 
bably as difficult to render a gas absolutely anhydrous, as it is to produce a per- 
fect vacuum. Moreover, as I have stated already, we have no test of absolute 
dryness, applicable to a gas. It would interrupt the argument, however, to con- 
sider this question at length here; I have devoted, accordingly, the section which 
succeeds this to its discussion. 
Of the second explanation, I would say much the same as I have said of the 
first. It is highly probable that a powerful hydracid like the hydrochloric should 
retain, more or less, as a gas, its characteristic action on organic colours. 
I would speak most positively of the third view. The power of bodies which 
have a great affinity for water, to compel its formation and separation, is so great, 
that I ventured to predict, that it would be impossible to preserve, for any length 
of time, blue litmus unchanged in tint in the driest hydrochloric acid. I do not 
dwell at length, however, on the cause of the ultimate reddening, as the fact that 
the paper was only slowly and imperfectly reddened, is sufficient for my present 
purpose. At lowest, the experiments I have detailed demonstrate that the re- 
moval of water from hydrochloric acid gas delays its action on colours. 
Ammonia. 
The last of the gases I tried was ammonia. According to KANnz,* when dry 
it has no action on organic colours, although no body, when moist, affects these 
substances more powerfully. I might content myself with adducing this distin- 
guished chemist’s statement as to the negative action of dry ammonia, and add it 
to the list of gases which have their action on colouring matter arrested by the 
removal of moisture. My own experiments, however, have been so much less deci- 
sive than KaNr’s statement led me to expect they would be, that I cannot, without 
comment, avail myself of his evidence. ‘The difference between his results and 
* Elements of Chemistry, p. 852. 
VOL. XVI. PART. IV. 61 
