a Chemically Clean Surface. 249 



noticed, and act as nuclei in liberating gas, because such film 

 pi-events the adhesion of water, but does not prevent the adhesion 

 of gas. 



A precisely similar explanation may be applied to supersatu- 

 rated saline solutions. In chemically clean vessels many of them 

 may be preserved unchanged during a long time, and their very 

 existence as supersaturated solutions depends in many cases on 

 the absence of nuclei. Previous observers remark how sensitive 

 some of these solutions are to cold ; solutions that could be kept 

 during the summer months begin to deposit crystals when the 

 first touch of winter's cold is felt. I have kept such solutions 

 unchanged during winter, and have put them into freezing- 

 mixtures at 20°, 10°, and even at 0° F., and have frozen some 

 of them without any separation of the salt. Previous observers 

 are constantly referring to the supposed molecular changes un- 

 dergone by the solution, or by the sides of the vessel containing 

 it, when many of the results thus obtained are traceable to un- 

 clean tubes and flasks and thermometers. It seems tome to be 

 impossible to study fairly or with profit the phenomena of super- 

 saturation without reference to chemically clean conditions; and 

 I believe the time is not far distant when such conditions will 

 be considered essential in various chemical and physico-chemical 

 inquiries. 



Take a case of clean as compared with unclean. Five parts of 

 sodic acetate and one part of water were boiled, filtered, and 

 boiled again at 240° F. in a flask containing a thermometer 

 which passed through a plug of cotton with which the flask was 

 closed when the lamp was removed. The solution was left in a 

 cold room during twenty hours, and then placed in a freezing- 

 mixture during some hours. The solution had been some time 

 at 14°, when, the thermometer being gently lifted up without 

 being taken from the flask, the solution ran off from the bulb 

 like a thickish syrup. On touching the solution with a wire it 

 immediately solidified, and the thermometer rose from 14° to 

 104° F. 



Such a solution as is here described crystallizes at 150° in an 

 open evaporating-dish ; and yet by observing the conditions re- 

 quired for chemical purity it may be cooled down to 14°, and 

 probably to 0°, without crystallizing. It may be cooled at least 

 140° below its point of crystallization and yet not crystallize, 

 simply because there is no nucleus to start the action. In the 

 open evaporating-dish, on the contrary, no sooner is the boiling 

 solution in contact with the air than its surface becomes dotted 

 here and there with some of the specks and filaments that are 

 floating about. If these be watched, one or more of them will 

 be seen to increase in size by the aggregation of saline particles 



