42 M. Grotthuss on the [Jan. 
potash. The oxide of iron falls down, the red colour disappears, 
and the liquid when filtered becomes quite colourless. [t con- 
tains pure anthraxothionate of potash, which by gentle evapora~ 
tion is obtained in long channelled prisms terminated by four- 
sided pyramids. It frequently crystallizes likewise in long, white, 
brilliant needles ; it is much more soluble in boiling than in cold 
alcohol. Hence it frequently happens when a glass is half filled 
with a boiling hot saturated solution of this alcohol, that the salt 
during the cooling shoots up from the bottom of the glass in 
brilliant needles, the ends of which, in consequence of the adhe- 
sion of the liquid and its universal contraction, are some lines 
higher than the surface of the solution. 
Sect. 4,—Anthrazothionate of potash has at first a hot taste, 
similar to that of radishes, but leaves in the mouth a cooling 
salt impression. In summer, when placed in dry air, it retains 
its crystalline form unaltered; but as soon as the air becomes in 
the least moist, for example, a little before the dew begins to 
fall, at six in the evening, it becomes liquid, and remains in that 
state till ten o’clock the following morning. When it has not 
become solid by that time, or at the latest by 11 o’clock, it is a 
proof that the atmosphere is moist; and we may predict with 
some probability that it will rain either on that day or the follow- 
ing one. For such an experiment the salt must not be exposed 
to the sunshine, but placed in the open air in a watch glass, and 
in a place moderately shaded. The remarkable delicacy with 
which this salt indicates the kygrometrical changes of the atmo- 
sphere, the readiness with which it gives out the water which it 
has absorbed when put into a dry place, fit it, I think, particu- 
larly for hygroscopical investigations ; and if it were placed upon 
a scale properly balanced, it would form an excellent hygro- 
meter. When exposed to heat in a glass tube, it melts quietly 
into a colouiless and glass-looking mass, which on cooling con- 
geals again in crystals. It is capable of bearing a much higher 
temperature without decomposition than prussiate of potash. 
However, when it is exposed to a red heat in certain metallic 
vessels, for example in silver, sulphuret of silver is formed which 
blackens the metal, and at the same time ammonia is produced. 
Sect &.—Diluted sulphuric acid drives off the anthrazothionic 
acid undecomposed. We may, therefore, by its means easily 
obtain this acid in a free state in a proper receiver. The tedious 
and complicated method, therefore, which Porrett has given for 
obtaining it is quite unnecessary. Concentrated sulphuric acid 
drives off only a part of the acid undecomposed from the anthra- 
zothionate of potash. The remainder undergoesa decomposition: 
an effervescence takes place; the sulphur separates in flocks, 
while carbonic acid gas and sulphurous acid gas fly off. But 
the most remarkable circumstance (which Porrett seems to have 
entirely overlooked) is that during this decomposition, which 
goes on rapidly when assisted by heat, no trace of axotic gas 
6 
