MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 671 
The coldest part is the bottom, and the coldest part of the bottom is that nearest the 
window. If the flask be suspended in the air crystallization will first begin on the window 
side. If the flask be placed on metal or other good conductor of heat, a ring of crystals 
will be formed at the bottom, while the thermometer is 6° or 8° above the point at which 
crystallization ought to take place ; the bottom of the flask and the ring of solution in 
contact with it are prematurely chilled, while the rest of the solution is above the point 
of crystallization. If the flask be left on a thick block of wood or other bad heat-con- 
ductor, the crystals at the bottom will be formed later. A solution containing 125 parts 
of nitre in 100 of water ought, according to the Table, to begin to deposit salt at 150°. 
The flask containing such a solution at boiling heat was placed on a block of wood ; at 
about 149° heat-currents suddenly rose like a cloud from the bottom of the solution to 
the surface, and crystallization instantly set in. If, before crystallization begins, the 
flask be plunged into cold water, the sides become coated with a shell of crystals, while 
the rest of the solution is still hot. 
It might reasonably be expected, from its great solubility, that nitre would form 
supersaturated solutions ; but it behaves as, I believe, anhydrous salts in general behave ; 
its solution cools down until it is exactly saturated, and then it begins to deposit crystals 
of the normal salt without any modification whatever. 
The potassic bichromate is cited, on the authority of Ogden and others, as an example 
of an anhydrous salt that produces supersaturated solutions. My experiments do not 
confirm this view. According to Kremer’s Table, 200 parts water at 140° F. dissolve 100 
parts of the salt. Such a solution in cooling down from 212° ought to deposit salt at 
about 140°, unless it becomes supersaturated. One ounce of newly crystallized dry 
salt was boiled in two ounces of water, filtered into a clean flask and again boiled, and the 
flask was closed. At 138° an abundance of crystalline flakes was thrown down, heat- 
currents ascended, and the temperature continued stationary for a minute or so. The 
crystals formed a lining over about two-thirds of that part of the flask that was occupied 
by the solution, but the deposit was most abundant on the side nearest the window*. 
Sal-ammoniac . — According to Karsten, 100 parts water at 65°*75 dissolve 37*02 of the 
salt, or, what is the same thing, one ounce water (480 grains) dissolves 177*6 grains of the 
salt. Such a solution in cooling from 212° began to deposit. salt at about 66°, and just 
below this the salt separated abundantly. 
Sodic nitrate was also tried. According to Poggiale’s Table, 100 parts of water at 158° 
will dissolve 129*6 of the salt, or - ^~ 0 - of Aip = :622 grains. In such a solution raised 
to the boiling-point and allowed to cool down, the salt began to separate at 160°. 
* Saturated solutions of the potassic bichromate undergo a remarkable deepening in colour under the action 
of heat ; they may even become black and opaque if boiled a second or a third time in the same flask ; but 
they recover their usual colour on cooling. The salt deposited in an open evaporating-dish from a hot saturated 
solution is almost white. If this light- coloured portion he placed on filtering-paper and allowed to dry in the 
open air, it does not deepen in colour ; hut if put in a platinum crucible and gently heated over a spirit-lamp, it 
becomes very dark in colour, and in cooling assumes and retains the usual colour of the salt. The phenomena 
in question resemble those pointed out by Sir David Brewster in the case of nitric peroxide. 
