62 . MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
saturated solution of the so die sulphate. As this double salt contains only four propor- 
tionals of water of crystallization, its supersaturated solutions are not very stable ; but 
by mingling the salts in atomic proportions in a small quantity of water (such as 287 
grains of zinc sulphate, 322 of the sodic sulphate, and half an ounce of water) so as to 
ensure supersaturation, raising the temperature to about 100° F. until the solution be- 
comes clear, then boiling and filtering into clean tubes and plugging with cotton-wool, 
such a solution may he kept and even reduced to 10° F. without any separation of the 
salt, although on removing the plug for an instant crystallization sets in from the surface 
in tufts. 
But by repose, even in clean tubes and in the absence of nuclei, long crystals of the 
separated salts occupy the length of the tube ; but they are invisible on account of 
having the same refractive index as that of the solution in which they are immersed. 
By inverting the tube so as to drain off the liquor, the crystals become visible, air 
enters the interstices, and on restoring the tube to its original position the bubbles of 
air have a very solid appearance, and may he taken for amorphous masses of crystals. 
These air-bubbles may be driven away by gently heating the tube. By further repose 
of some weeks a portion of the water of the solution evaporates through the cotton- 
wool, and as the solution becomes more dense, the crystals become more and more 
visible. These crystals are easily fusible at about 90°. 
It may happen that the anhydrous powder thrown down by the zincic sulphate during 
the cooling is not taken up again. If in this state the tube be suddenly heated, the 
sodic sulphate will throw down an anhydrous salt, and on attempting to boil the salt there 
will be much kicking of the tube. But as the temperature rises this kicking ceases, and 
the mixed anhydrous powders pass rapidly into solution, the one apparently assisting 
the solubility of the other ; for in the case of strong sodic-sulphate solutions alone, when 
once the anhydrous powder is deposited by heat, the continued application of heat 
increases the quantity of the deposit and aggravates the soubresauts. 
If a saturated solution of the double salt be reduced to a low temperature, such as 
that of 0° F., the anhydrous powder of Glaubek’s salt is thrown down ; but in very clean 
tubes the solution passes into a transparent jelly-like mass that has, the appearance of a 
liquid. A platinum spatula plunged into it while the tube is inverted does not produce 
any flow of liquid. By repose the pulpy mass becomes crystalline, the mother liquor 
separates, and the crystals become invisible in it. If the tube be put into warm water 
at 75° or 80°, the crystals liquefy in a few minutes, but they form again in a freezing- 
mixture at 14° or 16° F. 
The double salt fuses at about 140° into an opaque mass ; if raised to about 180° there 
is much kicking of the vessel, the anhydrous powder separates, and the clear liquor col- 
lects above. The powder consists chiefly of anhydrous sodic sulphate. If a little water 
be added the solution may be boiled, but only a portion of the powder is taken up. By 
reduction of temperature, this powder, influenced by the solidification of the zinc salt, 
forms cauliflower masses which creep a little way up the tube. On opening the tube 
