6 MITSCHERLICH ON CHEMICAL REACTIONS 
covered with the dry bladder be left for some time in boiling 
water, the gelatine dissolves, and it may be removed with ease. 
Although this attraction is very strong, it is however less con- 
siderable than that of the sulphate of barytes for the nitrate of 
soda. The attraction exercised by solids upon liquid and ga- 
seous bodies, is not only manifested by immediate contact, but 
also at a determinable distance. This may be proved by em- 
ploying thin plates of glass or quartz having completely smooth 
surfaces. The first is suspended, and the second furnished with 
an apparatus in which weights may be placed. From two plates 
thus arranged I removed all adherent moisture; the thinnest 
layer interposed would have been recognised by means of the 
coloured rings of Newton; on subsequently compressing these 
plates until the coloured rings begin to appear, it is easy to de- 
termine their distance. On the appearance of the second ring, 
the lower plate, the weight of which was fourteen grammes, re- 
mained attached to the first: the approached surfaces were only 
an inch square; when they were brought near so as to obtain 
the black of the first ring upon nearly the entire surface, several 
pounds might be suspended. On leaving this apparatus for 
some time under the receiver of an air pump, the plates did not 
detach themselves; atmospheric pressure does not therefore in- 
tervene to maintain them in contact. 
We know that this kind of attraction is chiefly exerted during 
the crystallization of bodies. A body dissolved is deposited upon 
threads or any solid bodies suspended in the liquid, much more 
speedily than if these were not there. Crystals are deposited 
much more speedily still around a crystal already formed than in 
any other portion of the liquid, when, for example, the power of 
the solvent is diminished by a lowering of the temperature. 
The solvent power of the water acts, then, with less energy 
near a formed crystal than at some distance from it. 
In some cases it is very easy to account for chemical combina- 
tions which the action of solid bodies on liquids and gases may 
induce, but in other cases the explanation may be more difficult. 
It is possible that condensation may be the sole cause with re- 
spect to gaseous bodies. In this manner the detonation which 
M. Thénard observed, on introducing charcoal into a mixture of 
hydrosulphuric acid and oxygen, may very well be exjlained ; 
when platinum black, on which oxygen has been condensed, is 
placed in contact with hydrochlosic acid, chlorides of platinum 
