532 



Mr. Charles Tomlinson on 



[Dec. 20, 



to cover the surface like a fan, while an opaque appearance was seen to 

 descend throughout the depth of the mass, very much after the behaviour 

 of the 7-atom sodic sulphate when touched by a nucleus. The watch- 

 glass and the metal pan on which it rested became very hot (the tempera- 

 ture rising, as was afterwards found with the remainder of the' solution, 

 from 60° to 129° E.) * but still with no loss of weight, seeing that the 

 whole of the water of the solution had again become water of crystalliza- 

 tion. If there is any water to spare it is driven off by the heat of com- 

 bination, and there is a corresponding loss in weight. 



Some of the drops, when exposed to the air, form pulpy masses, such 

 as those of sodic, zincic, and magnesic sulphates ; but by continued ex- 

 posure they soon develop into hard w T ell-formed crystals. This is, I 

 suppose, the explauation of an effect that I obtained some years ago. A 

 spoon of white metal shaped like a denagrating-spoon, with its wire 

 run through a flexible porous plug, was lowered into a boiling solution of 

 sodic sulphate : the plug was then fitted into the neck of the flask, which 

 it closed accurately, and the solution was left to cool. The spoon was 

 next drawn up full of the solution and left in the air of the flask. In 

 the course of some days the liquid in the spoon evaporated and left a salt 

 which, on being lowered into the solution, was inactive ; but if the spoon 

 containing the salt were first taken out and exposed for a few seconds to the 

 open air and then lowered into the solution, it was powerfully nuclear. 

 This exposure to the open air or to the air of a room seems to be necessary, 

 in some cases, to the formation of a crystalline salt, either by evaporation 

 or by the action of some other force. 



Schroder insists on the powerful action of the air in connexion with 

 the phenomena of supersaturated saline solutions ; and Lowel asks, " "What 

 is this mysterious action of the air which endows bodies with a nuclear 

 action?" He supposed it to be catalytic in its mode. The French 

 theory supposes the air to act as a carrier of salts of the same kind, which, 

 it says, are alone capable of acting as nuclei. In the course of my experi- 

 ments the air has had abundant opportunities of becoming charged with 

 salts of the same kind as those I was working with in solution. For 

 example : during all the time that I was weighing the solutions of sodic 

 acetate, so as to note their increase in weight by the absorption of mois- 

 ture from the air, an evaporating-dish, containing sodic acetate in crystals, 

 was within a foot and a half of my balance ; but although for days 

 together the solution caught moisture from the air, it never caught a 

 crystal of the salt from the air. When, however, the wind got round to 

 the north, or to the east, the solutions of this and of other salts crystal- 

 lized, not only when exposed as drops, but also often in the covered flasks 

 which supplied the drops. 



"When the air is from a dry quarter the results are tolerably uniform ; 



* In my first paper (Phil. Trans. 1868, p. 663) a case is given in which sodic acetate 

 solution in solidifying rose from 14° to 104° F. 



