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be provided with certain molecular interstices, which is not 
the case with the Silver compound, since the Silver Nitrate 
does not contain any water of erystallization. Ifa erystal of 
Copper Sulphate be placed in the solution of Potassium Sul- 
phide, the Copper Sulphide formed takes the place of the 
group SO.Cu whilst the water, having surrounded this group, 
in a solid condition, becomes liquid, and, escaping, leaves in 
the precipitated crust of Copper Sulphide interstices through 
which other particles can move. In the layer following next 
beneath, the normal decomposition can only proceed when 
the molecule of Potassium Sulphide can penetrate such 
interstices to reach the interior, or when the molecule of 
Copper Sulphate dissolved by the entering water, can pene- 
trate the crust to reach the exterior. But if these interstices 
are too small to permit such motion to take place, then a 
‘‘Chem-Osmotic” decomposition will set in, provided the 
other conditions are favorable. The larger molecules split 
and give this a partially modified decomposition ; the Sulphur 
of the Potassium Sulphide remains in the exterior liquid, 
forming Bisulphide, while the Potassium, uniting with the 
SO, group, precipitates metallic Copper. In the case of the 
Copper salt this movement is soon finished, while in the ease 
of the Silver salt this movement seems to be entirely ar- 
rested after a while; the interstices can, in this case be only 
formed by the difference of the molecular volumes of Silver 
Nitrate and Silver Sulphide, and are perhaps entirely 
covered over by several layers; or, at least, contracted. 
On the application of Caustic Potash instead of Potassium 
Sulphide to the Copper salt, metal is not separated, for 
several reasons, (1.) a body analogous to Potassium Bisul- 
phide cannot be formed under those conditions; (2.) the 
intermediate formation of voluminous Copper Oxy-Hydrate 
produces an entire change of the original positions of the 
molecules of Copper; making the formation of a crust im- 
possible. In the case of the Silver salt an intermediate 
formation of an Oxy-Hydrate cannot take place and the layer 
of Oxide protects the remainder of the crystal from further 
action. When caustic Soda is used it behaves in a different 
