Chemistry . 401 
perceived in each of the cavities, which gradually disappears as 
the crystal becomes less cold. The appearances are such as one 
might expect would arise from very highly condensed carbonic 
acid contained in the bubbles, assuming alternately the liquid 
and gaseous form, by very slight elevations and depressions of 
temperature.— En.” 
As the preceding description of Mr Allan’s specimen appears 
to us quite the reverse of that given by Dr Brewster, we shall 
here state how the case actually stands. When the crystal is 
very slightly warmed, the three oblong cavities, mentioned 
above, are actually filled with a liquid , as may be seen by its gra- 
dually rising till it fills the cavity ; but, upon cooling the crystal, 
the liquid contracts in dimensions, and a bubble, either of va- 
cuity or gas, occupies about one-third of the cavity. That the 
fluid is not a condensed gas, in the usual sense of the word, is 
certain, because it retains its fluidity when in the open air, and 
leaves a residuum behind it after evaporation. Mr Allan’s very 
fine specimen contains both the new fluids discovered by Dr 
Brewster, but it requires particular methods of observation to 
see the second one. 
26. Crystallisation of Acetic Acid by Pressure . — Mr Perkins, 
we understand, has succeeded in crystallising acetic acid by the 
pressure of 1000 atmospheres. The transparent crystals thus 
formed are pure acid ; and the residuum is acidulous water. 
Mr Perkins has crystallised several other acids by the same means. 
27. Conversion of Atmospheric Air into a Fluid by Pressure. 
— Mr Perkins has, we learn, compressed atmospheric air to such 
a degree, that a small portion of fluid appears at the end of the 
compressed column. This fluid does not wholly recover its ga- 
seous state when the pressure is removed. It was supposed to 
be water, but this is not yet certain ; several other gases have 
been converted into liquids by the same powerful agency. 
28. Persulphate of Iron and Ammonia . — This triple salt was 
obtained by Dr Eorchhammer of Copenhagen, in the following 
manner. Having prepared a solution of gold by means of nitric 
acid, and muriate of ammonia, and precipitated the gold by pro- 
tosulphate of iron, th,e clear solution was concentrated to a sy- 
rupy consistence. After remaining a month, fine wine-yellow 
VOL. IX. no. 18 . OCT. 1828 . 
c c 
