666 ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
fies into the normal salt than partially into a salt of a modified character. The formation 
of the modified salt, which has given rise to much speculation, will next be considered. 
IV. On the Formation of the modified Salt. 
Two ounces of zincic sulphate and one ounce of water were heated in a clean flask 
until the solution boiled briskly. The solution was then filtered into chemically clean 
test-tubes, made so by washing them out with strong sulphuric acid. The tubes were 
plugged with cotton-wool and set aside to cool ; they were bright and transparent ; but 
on cooling to about 7 0° they became troubled by the formation of a monohydrated salt, 
which gradually subsided and formed a small deposit at the bottom of the tubes. Upon 
this deposit, after some hours, a fine crop of acicular crystals had sprung up, leaving the 
mother-liquor clear, but still supersaturated ; for on taking out the cotton-wool from 
one tube, crystallization set in from the surface. On gradually raising the other tube to 
the temperature of 100°, or from that to 106°, the acicular crystals melted down and 
formed a dense lower stratum. 
This experiment throws much light on the behaviour of those supersaturated saline 
solutions that form salts of a modified character, and of greater solubility than the 
normal salt. When the richly hydrated salts, which best display the phenomena of 
supersaturation, enter into solution at high temperatures, they give up all, or nearly all, 
their water of crystallization to the aqueous solvent. In the case of the sodic sulphate, 
for example, I quite agree with the accepted theory, that from the boiling-point of an 
aqueous solution down to about 60° it is the anhydrous salt that is held in solution ; 
but I differ from that theory which supposes a molecular change to take place in the 
supersaturated solution at and below 60°, and the molecules to arrange themselves into 
the constitution of the 7-atom hydrate. There is no experimental proof that such is 
the case. We can get the 7-atom hydrate, it is true, when a strong solution is cooled 
down below 60° ; but this is no proof that the solution passed through the molecular 
change which has been insisted on. 
I have already given a case in which a strong solution of the sodic sulphate, contained 
in a tube made chemically clean with special precautions, was cooled down to 10° F. 
without any separation of the salt, in consequence of the absence of a nucleus, and the 
strong adhesion of the solution as a whole to the sides of the tube. If a strong solu- 
tion of sodic sulphate be filtered into a tube washed without any special precautions to 
make it chemically clean, and the mouth be closed with cotton-wool, such a solution may 
be cooled down to 50° or less without depositing any salt. If we now place this tube in 
ice-cold water, and watch it carefully, a small number of octahedral crystals (the form 
of the anhydrous salt) will separate from the solution and subside, liberating heat-cur- 
rents as they fall and when resting at the bottom. This experiment has been performed 
with supersaturated solutions of sodic sulphate of varying strengths and with the same 
result, unless crystallization set in from the surface by the dragging in of a nucleus con- 
