500 Mr. A. Livcrsid^e on Supersaturated Saline Solutions. [Jane 20, 



It should perhaps here be mentioned that each flask was always proved 

 to be thoroughly supersaturated by dropping in a crystal of the salt or 

 touching the solution with a dirty rod, after the substance made trial of 

 was found to be wanting in nuclear power. 



From the foregoing it appears that the crystallization of supersaturated 

 saline solutions is not determined by the removal of water by chemical 

 agency; neither do porous bodies, like wood, charcoal, sponge, spongy plati- 

 num, earthenware, &c, determine the solidification of solutions by me- 

 chanical absorption of the water. 



Concerning the action of thin films. 



In the same paper it is stated that while oils, fats, and greasy bodies 

 generally do not act as nuclei when chemically clean and in the bulk, i. e. 

 in the form of a solid mass, lens, or drop, yet these identical bodies when 

 in the form of thin films do act as nuclei, and that any substance which 

 possesses a nuclear action has derived such power from having become 

 contaminated with a thin film of greasy matter, which it acquires by hand- 

 ling, wiping with a dirty cloth, or by mere exposure to the air containing 

 the products of respiration and other excretions, &c. 



Thus in the series of experiments detailed it was found that such bodies 

 as ether, absolute alcohol, naphtha, turpentine, herring-oil, sperm-oil, 

 castor-oil, and many others, while in the form of a lens or globule, did not 

 act upon a supersaturated solution, but did immediately when spread out 

 into a thin film. 



It should be noticed that the oil was added to the solution bv removing 

 the cover of the flask, delivering the drop, and then replacing the cover ; 

 or a glass tube was used provided with a shield covering the mouth of the 

 flask : both methods have the great objection of exposing the solution to 

 the air, and so allowing nuclei to gain access. 



It is stated that if the finger be cleaned by washing it in alcohol or 

 caustic potash, or by passing it through the flame of a spirit-lamp, it may 

 be held in a supersaturated solution for some time without causing crys- 

 tallization ; but that if it be rubbed against the sides of the flask, a greasy 

 smear is produced which at once acts. 



The writer has repeated this form of experiment several times, but never 

 with the above result when sufficient care had been taken to free the finger 

 from nuclei. 



Exp. The finger was made greasy by dipping it into oil and imperfectly 

 wiping it with a cloth ; it was then passed many times through the flame 

 of a spirit-lamp, and finally, while still far above its normal temperature, 

 inserted into a flask of supersaturated solution : the flask was chosen with 

 a neck such that it could be entirely closed by the thicker part of the 

 finger. The flask was then transferred to a vessel of water, lowered arti- 

 ficially to 38° F., and there kept, with the finger still in it, for several 

 minutes, varying in different experiments from 10, 15, 20, 25, 30, and 35 



