1879.] Mr.' C. Tomlinson on the Action of Solid Nuclei. 



25 



Some eminent observers have denied the accuracy of these state- 

 ments. Thus, M. Viollette concludes from his experiments that 

 porous bodies, such as pumice, calcined baryta, lime, and gypsum, and 

 bodies greedy of water, and capable of being hydrated, such as cal- 

 cined sulphate of iron and of copper, have no influence on supersatu- 

 rated solutions. 



This observer, believing, as he did, that the only nucleus is a salt of 

 the same kind as that in solution, or one isomeric therewith, and that 

 such nuclei are generally floating in the atmosphere, so ingeniously 

 arranged his apparatus as to exclude the action of the air, and hence 

 his results were of a negative character. 



Mr. Liversidge ("Proceedings," vol. xx), actuated by the same 

 idea, conducted his experiments on a similar principle, namely, that 

 of excluding the air, and he also arrived at negative results. He 

 employed several solid dehydrating substances, such as freshly ignited 

 quicklime, and placed them in thin glass bulbs, which, being sealed, 

 were heated nearly to redness, and dropped into the supersaturated 

 solutions. The flasks were plugged with cotton wool, through which 

 a glass rod was passed, and were then boiled up and allowed to cool 

 during some hours ; when quite cold, the bulb was broken by means 

 of the glass rod, and its contents were set free, but in no case with any 

 result. He also found by his mode of experimenting that porous 

 bodies and absolute alcohol are inactive. 



In the " Proceedings " vol. xxvii, p. 122, some account is given by 

 me of a series of observations, extending over several months, on the 

 action of essential oils on a supersaturated solution of sodic sulphate. 

 It was found that when the wind at Highgate was in a southerly or 

 westerly quarter, the oils were inactive ; but that they quickly became 

 active when the wind was northerly or easterly. 



A similar course of observations, though not extending over so 

 long a time, has led to a similar conclusion respecting solid porous, 

 and dehydrating bodies. 



The solutions chiefly employed in the experiments that led me to 

 this conclusion were those of sodic sulphate, sodic acetate, and alum, 

 of various degrees of supersaturation. The solid nuclei employed 

 were chiefly pumice and plaster of Paris ; but others were also used, 

 such as meerschaum, marble, lime from marble and chalk, coke, 

 charcoal, and bread. 



The flasks containing the solutions were sometimes carried into the 

 open air of my garden before being uncovered. The plaster of Paris 

 was taken out of a bottle by means of a small, clean, platinum spoon, 

 and a fragment of pumice, about the size of a pea, was taken up with 

 forceps, and thus dropped into the solution, the cover being imme- 

 diately restored to the flask. 



The active solid nuclei could be immediately rendered inactive by 



