668 ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
5, 4, 3, and 2 parts salt to 1 of water : the solutions have been left to themselves, or they 
have been placed in freezing-mixtures at various temperatures ; hut the general result 
is a sudden crystallization from some point near the surface of the solution, rapidly con- 
verting the whole into a solid, with a rise of temperature of 35° or 40°. 
M. Lowel also points out three varieties of the magnesic sulphate, the ordinary 7-atom 
salt, which he names 7HO a, a modified salt, 7HO b, and also a 6HO salt. 
In the case of this salt also I have placed highly charged supersaturated solutions 
(2 salt to 1 water), contained in chemically clean tubes, in freezing-mixtures without any 
effect in producing crystallization. In one experiment a solution made with two ounces 
of salt and one of water was boiled and filtered into five tubes, four of which were chemi- 
cally clean, while the fifth had been used for a solution of this salt and left lying on the 
table for two or three days. All five tubes on being filled were plugged with cotton- 
wool, and set aside to cool. When cold the four clean tubes were 1 put into a freezing- 
mixture at 10°, and when this had slowly risen to between 30° and 40°, another freezing- 
mixture was prepared, to which the tubes were transferred ; but still after another half 
hour the solutions remained perfectly bright and transparent. One tube was removed 
from the mixture and opened, when on touching the surface with a metal point, crystal- 
line tufts, like thistle down, were propagated downwards, and soon the whole solution 
had solidified into the usual 7-atom salt. The fifth tube, which was not chemically clean, 
did not retain its solution in the liquid state : on examining it some hours after it had 
been filled, the contents had solidified into the ordinary salt. The other three tubes, on 
being removed from the freezing-mixture, showed no disposition to crystallize. 
The magnesic sulphate parts readily with an atom of water and becomes milky 
w r hite. A group of these milky crystals is sometimes deposited at the bottom of a loug 
tube containing a strong and still warm solution of the salt, and partly immersed in cold 
water. A certain difference in density between the lower and upper portions of super- 
saturated solutions is favourable to this result. Indeed there is a tendency to stratifi- 
cation during the cooling of all these solutions, as may be seen on gently shaking the 
flask, when the effect is that of the mingling of layers of different densities, as when 
water is resting on syrup. I long thought that when a salt of a lower degree of hydra- 
tion is produced, it was due to the formation of denser and more highly charged saline 
substrata. Gay-Lussac’s experiment* teaches that saturated solutions contained in long 
tubes, and maintained at the .same temperature during many months, are of the same 
density at every part of the column-; and it seems probable that such is also the case with 
supersaturated solutions when cold and left undisturbed some time ; but there are vari- 
ous difficulties in the way of determining this point experimentally. When the denser 
lower stratum is formed at all, it is, I believe, by the process already explained in the 
case of zincic sulphate and sodic sulphate. A salt of a lower degree of hydration, or an 
anhydrous salt, is thrown down on a reduction of temperature of the supersaturated solu- 
tion, and the heat given out during a change of state, assisted by a rise of atmospheric 
* Annales de Chimie et de Physique, vol. vii. p. 79. 
