50 CHANGES OF CONDITION.
entrance of water, the cell rises to the surface of the solution, and
remains there until, after ten or fifteen minutes, the constant
thickening of its walls so increases its weight as to make it once
more sink, and this time permanently.- Membranes of Colloidal Silicates of the Heavy Metals. Dilute
some commercial water-glass solution until a specific gravity of 1.1
is obtained. Into about 100 c.c. of this solution in a narrow
beaker, drop small particles of various salts, such as copper sulphate,
aluminium sulphate, ferric chloride, nickel nitrate, cobalt nitrate,
manganous sulphate, lead nitrate, and uranyl nitrate. Within a
few minutes the particles begin to swell and to send out shoots
which branch and grow toward the surface of the liquid until the
whole beaker is filled with what appear to be bright colored algse
growths. The salt, on being thrown into the solution, begins at
once to dissolve, and at the surface of contact between this solu-
tion and the silicate solution, an insoluble semi-permeable film of
metal silicate is formed. The dissolved salt within exerts an
osmotic pressure against this film, and forces it to expand, while
the water which is thereby drawn in through the film dissolves
more of the salt. The osmotic pressure is thus maintained and
the film is continuously forced to expand until it bursts in places
and forms outgrowths and side-arms.