ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 661 
surface, while there is strong adhesion between a salt or a gas and such surface, the 
presence of such a surface in a supersaturated solution will cause a separation of the 
salt, or of the gas. 
3. Hence a surface contaminated ever so slightly with oily, fatty, or greasy matter 
will act as a nucleus in such solutions. 
4. A nucleus may be defined as a body that has a stronger adhesion for the salt or 
the gas than for the liquid which holds it in solution. 
5. Bodies that have been exposed to the products of respiration or of ordinary com- 
bustion, or that have been handled or wiped with a cloth, are covered more or less with 
a greasy film, and act as nuclei in aqueous solutions. So also the dust of the room 
raised into the air is more or less contaminated with greasy or organic matter, and acts 
as a nucleus. 
6. By washing such bodies in a solution of caustic alkali, in sulphuric acid, in ether, 
&c., such oily or greasy film is got rid of. In this way the sweepings of a room may be 
made inactive as nuclei. It is proposed to name bodies or surfaces that have been 
treated in this way catJiarized , from KaOapoc. 
III. Action of Nuclei. 
This action may first be illustrated by taking care to ensure the absence of nuclei. 
Previous observers insist on the great sensitiveness to cold of sodic sulphate solutions ; 
but if precautions be taken to ensure chemical purity, this sensitiveness disappears. For 
example : — Two ounces of large crystals of this salt (which had been purified by repeated 
crystallization) in a dry and slightly efflorescent state were put into a clean flask with 
one ounce of distilled water, and the flask was carefully heated so as to avoid the deposit 
of anhydrous salt, and then boiled. Two thin glass test-tubes, each of 1^ oz. capacity, 
and about f inch in diameter, were filled with strong sulphuric acid, emptied, and left 
to drain with their mouths under water. A glass funnel washed with spirits of wine, 
had a clean filter adapted to it, and the boiling solution was passed through the filter 
into the two tubes. These were immediately plugged with cotton-wool, and passed over 
the flame of a spirit-lamp, when one of the tubes gave a kick, and threw down a small 
quantity of anhydrous salt. Both tubes were now placed in a vertical position in a rack, 
and left to cool. At about 50° F. they were put into a freezing-mixture in which a 
thermometer rapidly sank to 10° ; the tubes were moved about in the mixture, and then 
left in it during some hours, while the temperature slowly rose from 10° to 40°, when 
the solution in one tube was still liquid, bright, and transparent ; and equally so in the 
other tube, only at the bottom there was a small group of 7-atom crystals, built, as it 
were, on the deposit of anhydrous salt already referred to. The cotton-wool was now 
taken out so as to admit a nucleus from the air, and crystallization instantly set in at 
the surface, spreading rapidly downwards, and carrying with it sufficient water to convert 
the whole solution into the 10-atom hydrate, and also to impart to the 7-atom hydrate 
three additional equivalents of water. There is a considerable rise of temperature 
4x2 
