1868.] Mr. C. Tomlinson on Supersaturated Saline Solutions. 403 



Thus we see that the same periods of certain disturbances are mani- 

 fested very differently in two stations so near each other as Kew and 

 Lisbon. The modification is the greatest, particularly in the periods which 

 depart from the general rule at Lisbon, and are doubtless also abnormal 

 at Kew. 



From the examples here quoted, it is evident that a great value would 

 be attached to the curves from another intermediate station ; for the little 

 vertical- force peaks and hollows, being opposed at Lisbon and Kew, it 

 would be very interesting to see if these peaks would be wholly or nearly 

 absent at some intermediate station. 



With a certain number of these magnetographs very discreetly placed, 

 we may one day analyze the different forces acting on the needle in the 

 different places on the earth — a manifest desirability. 



IL "On Supersaturated Saline Solutions.^^ By Charles Tom- 

 linson, F.R.S. Received April 21, 1868. 



(Abstract.) 



This memoir is divided into six parts. The first part contains a defini- 

 tion of the subject ; the second an historical sketch ; the third is on the 

 action of nuclei in inducing crystallization, and the effect of low tempera- 

 tures on a number of supersaturated solutions contained in chemically clean 

 vessels ; the fourth is on the formation of a modified salt, as in the case of 

 zinco-sulphate and sodic sulphate ; the fifth contains an inquiry as to 

 whether anhydrous salts form supersaturated solutions ; and the sixth and 

 last part is a summary with a classified list of the salts examined. 



1 . Definition. — When water at a high temperature is saturated with a 

 salt, and, on being left to cool in a closed vessel, retains in solution a larger 

 quantity of the salt than it could take up at the reduced temperature, the 

 solutionis said to be supersaturated. 



2. History. — During many years the phenomena of supersaturation 

 were studied with reference to solutions of Glauber's salt. In 1809, Ziz of 

 Mayence* showed that the sudden crystallization of these solutions is not 

 due to agitation ; that the vessels containing the solutions do not require to 

 be hermetically sealed ; but if put under a bell-glass, or loosely covered as 

 with a capsule, they can be preserved during a long time ; that solids 

 brought into contact with the solutions act as nuclei and produce instant 

 crystaUization, but that such solids act best as nuclei when dry ; if wet or 

 boiled up with the solution they become inactive. The most efiicient nu- 

 cleus is a crystal of the salt itself. Air, if artificially dried, ceases to be a 

 nucleus. Three varieties of the sodic sulphate are noticed, i. e. the anhy- 

 drous and the ordinary lO-atom hydrate, and also a peculiar salt formed 

 when supersaturated solutions in closed vessels are left to cool down. This 



* Scliweiggcr, * Journal,' 1815, vol. xv. 

 VOL. XVI. 2 o 



