GASEOUS, SALINE, AND VAPOROUS SOLUTIONS. 
717 
out with strong sulphuric acid or caustic alkali, and copious rinsing with clean 
water. If the soda water be poured into this, the results are very different. 
After the first effervescence is over, the glass looks as if it were fuli of pure 
distilled water : not a bubble of gas is to be seen adhering to the sides, and 
there is no appearance of gas anywhere about it. I plunge into the liquid a 
glass rod that has been made chemically clean, or catharized, as it may be 
called; there is no escape of gas, not a single bubble adheres to the glass rod. 
I put in clean iron filings,—there is no liberation of gas. I pour some iron 
filings into my hand and rub them about, and on putting these into the soda 
-water there is a furious boiling effervescence. 
Hence you see there is a marked distinction between a clean or catharized 
body, and an unclean body, or nucleus , as it may be called. 
The action of nuclei may be similarly traced in the case of certain salts that 
form supersaturated solutions. For example, 100 parts of water at 212° F. 
will take up ohl\ parts of potash alum ; whereas at 32° the water will take up 
only 3 - 9 parts, and yet the solution saturated at 212° may be cooled down to 
32° without any separation of the salt, provided the vessel containing it be 
chemically clean, the solution clean, and means be taken to exclude the dust of 
the air. Such a solution is said to be supersaturated, because it holds more salt 
in solution than it can take up at the reduced temperature. In the case before 
us, a solution of alum saturated at 212° and cooled down to 32°, contains 90 
times more salt in solution than it can take up at 32°. Here is a solution of 
alum saturated at 194°, and containing 53 times more salt than the water at 
32° can take up. It is a perfectly clear and bright solution, and may be shaken 
with impunity. It remains liquid and supersaturated simply because there is no 
nucleus to start the act of crystallization. I take out the plug of cotton wool 
from the flask, and you see how beautifully the surface of the solution becomes 
covered with a crystalline crust, while well-shaped octahedra grow downwards 
until the whole mass is solid, and the flask quite warm with the heat developed 
by the change of state. 
Here is a supersaturated solution of sodic sulphate in a narrow-necked flask. 
I take out the cotton wool and the solution does not crvstallize. I touch the 
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surface of the solution with a glass rod, and crystallization instantly sets in. 
Here are similar solutions of sodic sulphate and sodic acetate, each containing 
a solid such as a bit of charcoal, of pumice stone, of meerschaum, of coke, etc. 
These bodies, which would act as nuclei in inducing crystallization if put into 
the cold solution, become inactive if boiled up with the solution, and allowed to 
cool down with it and in it. They are, in fact, chemically clean. I take the 
cotton wool out of these flasks, and some of the solutions crystallize on the in¬ 
stant, others after a short time, others again after some hours. It depends very 
much on the size of the opening whether a speck of dust enters the flask ; for, 
of course, the wider the opening the more likely is it for the dust to fiud an 
entrance. 
In the case of saline solutions a nucleus may be defined as a body that has 
a stronger attraction for the salt of a solution than for the liquid which holds it 
in solution. 
In general, a body that has been exposed to the air, or handled, or wiped 
with a cloth that has been handled, is covered with an oily, fatty, or greasy film, 
to which the water of the solution adheres either not at all, or less strongly than 
does the salt of the solution, and hence such a body introduced into the solution 
acts as a nucleus. Make the body chemically clean, and the solution adheres to 
it as a whole; that is, there is the same amount of adhesion between the nucleus 
and the water of the solution, as there is between the nucleus and the salt of 
the solution, and hence there is no separation. 
An extreme case of this kind of perfect adhesion on the part of chemically 
