ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 669 
temperature, enables the salt to go into solution, and thus a dense lower layer is formed 
from which the modified salt is produced. 
The ammonium phosphate is another example of this action. A solution saturated at 
212° was filtered into a clean flask wetted with liquor ammoniee, and left some weeks as 
a supersaturated solution. It was then placed in a freezing-mixture, when it threw 
down a white powder, probably the anhydrous salt : the flask was now set aside, when 
after a few days the powder had partially disappeared, and a crop of transparent crystals 
had formed. But this result is not so readily produced if special means be taken to 
secure clean vessels. Five tubes were filled with sulphuric acid, rinsed with water, 
filled up with liquor ammonise, emptied, and a boiling saturated solution of the ammonium 
phosphate filtered into them. When cold, these tubes were put into a freezing-mixture 
composed of sodic sulphate and hydrochloric acid, and after half an hour transferred to a 
similar mixture, without producing any immediate effect. On examining the tubes next 
day a small quantity of white powder was found at the bottom of three of the tubes, 
with transparent crystals in the form of flat quadrangular prisms. 
Strontic nitrate would appear to be favourable to the exhibition of this class of phe- 
nomena, since a strong solution on cooling down to about 62° deposits anhydrous octa- 
hedral crystals. But these are bright, hard, and transparent, and apparently not affected 
when the tube is put into water at 212°, or into a freezing-mixture at 10°, or when the 
crystals are touched with a wire. On removing the cotton-wool and scratching the side 
of the tube, the scratches became immediately filled up with minute crystals, and a 
copious shower of the hydrated salt took place with the liberation of heat-currents. 
After a time these formed a considerable mass, and on putting the tube into water at 7 0° 
the salt melted down, but more rapidly at 90°; at 200° the remaining part of the mass 
fell down into an opaque white powder, from which the original octahedra emerged as 
sharp and transparent as when first formed. 
Many salts behave like the sodic carbonate in maintaining a state of supersaturation 
down to a certain temperature, and then suddenly becoming solid throughout. The 
rhombic sodic phosphate does this, as also the plumbic acetate. A boiling solution of 
plumbic acetate (2^ salt to 1 water) was left to cool slowly down to about 40° (the tempe- 
rature of the room), when it suddenly crystallized with a considerable rise in temperature. 
The whole formed a solid mass, not amorphous, but consisting of needles of the normal 
salt, as is nearly always the case when crystallization commences at the surface. On gently 
warming the mass, these needles fell apart, and revealed the fibrous structure. This is a 
common case in salts that have considerable solubility. A boiling saturated solution of 
zincic acetate, on the contrary, not only cooled down to 40° without crystallizing, but con- 
tinued liquid during some hours in a mixture of snow and water at 32°. On removing the 
thermometer crystallization did not set in until the inner side of the flask at the level of the 
solution had been scratched, when the marks became chalky white as usual, crystallization 
of the normal salt set in, and small well-shaped crystals continued to fall during some hours 
with but slight rise in temperature. The excess of salt beyond the point of saturation was 
4 t 2 
