MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 665 
boiling-point without giving any signal whatever, until the liquid bursts into vapour 
with explosive violence, or is simply turned out of the flask without any noise at all. 
Five ounces of sodic acetate were fused in its water of crystallization, boiled, and set 
aside to cool. The liquid contained a number of hairs or motes. At 150° it began to 
crystallize, and the whole solution became filled with crystalline needles. The salt was 
again raised to the boiling-point, and it boiled quickly and readily with the usual symp- 
toms, both before and during the boiling. The solution was now filtered into a chemi- 
cally clean flask wet with acetic acid, and again placed over the lamp. It rose to 240° 
without any sign, and a few degrees above this it suddenly burst into vapour. I have 
not repeated this experiment ; but have no doubt that, instead of crystallizing at 150°, the 
clean solution might be reduced to a very low temperature without crystallizing, simply 
from the absence of the nuclei which were so abundant in the unfiltered solution. 
*[It is said that the most powerful nucleus is a crystal of the salt itself. This state- 
ment must be received with reference to the condition of clean and unclean. When 
supersaturated solutions are lightly covered, so as to allow a portion of the water to 
escape, a crystalline crust often forms on the side of the flask. This crust does not act 
as a nucleus, because it is clean ; and the solution, being supersaturated, does not dis- 
solve it. I have kept such a solution of ammonium-phosphate in contact for days toge- 
ther with the crystalline crust that had formed above it with little or no corroding 
action. These crusts may be readily formed in the case of the magnesic sulphate solu- 
tion (2 salt to 1-|- water) in clean tubes placed in strong sulphuric acid under an ex- 
hausted receiver. A crust will form on the surface of the cold solution in about twenty 
minutes, and this may be allowed to fall through the solution without acting as a 
nucleus. Or a solution of the magnesic sulphate (4 salt to 4 water) may be made in a 
flask, and filtered into a clean flask ; and while this is boiling, perfectly clean crystals 
(made so by washing in sulphuric acid and rinsing in water), contained in a clean, short 
tube, are to be suspended by a clean wire in the neck of the flask while steam is pouring 
off from it. The cotton-wool plug may now be inserted, the lamp removed, and the flask 
left to cool during twelve hours or so. The tube may then be lowered into the solu- 
tion, and the crystals will not act as nuclei in inducing crystallization. 
In a similar experiment with sodic sulphate the salt in the short tube became, of 
course, converted into the anhydrous salt ; but it was quite inactive as a nucleus on the 
cold solution. Exposed to the air for a few moments it acted powerfully as a nucleus.] 
Such cases as the above lead me to the conclusion that the state of supersaturation 
is simply a case of no nucleus. There is nothing to start the action which produces 
change of state, and there is no reason why the state should change, unless indeed, as 
may sometimes happen, mere reduction of temperature is the efficient agent in inducing 
crystallization by bringing the particles so close together that cohesion can prevail over 
adhesion. But in such cases it more often happens that the solution completely solidi- 
* Those paragraphs within [ ] formed the subject of a Note sent in to the Royal Society, August 13, 1868. 
