250 Mr. C. Tomlinson on some Effects of 



about it, and suddenly from this point fans of crystals begin to 

 radiate. In the meantime the sides of the vessel are active in 

 separating the saline molecules, and the more so in proportion 

 to the absence of chemical purity. 



If the thermometer be taken out of the flask that contains a 

 supersaturated solution and held in the air for a moment, it will 

 catch some of the aerial motes, so that, on returning it to the so- 

 lution, crystallization immediately sets in. 



Air is not a nucleus ; for it may be drawn through or shaken 

 up with a highly supersaturated solution without any separation 

 of the salt. The " mysterious action of the air," that several 

 observers refer to, resolves itself into this, — that it acts the part 

 of a carrier of some unclean mote or speck of dust which is the 

 real nucleus. It had been noticed that supersaturated solutions 

 can be kept longer in the liquid state in narrow-necked vessels 

 han in wide ones ; and the reason is obvious : the floating nuclei 

 have a less chance of entering a narrow than a wide aperture. 

 But even in such a case the locality of the experiment is of im- 

 portance. If a flask containing a supersaturated saline solution 

 be opened in a room, the solution will most likely crystallize the 

 moment it is opened ; but if taken into a field in the country, it 

 may be kept for hours uncovered without crystallizing. Even 

 so sensitive a solution as that of sodic sulphate I have kept 

 in an open flask in the garden of my house without its crystal- 

 lizing. 



It is to my present purpose to notice in this place a fact com- 

 municated to me by Mr. Wentworth Scott after the reading of 

 my paper at the last Meeting of the British Association, namely, 

 that, on crystallizing saline solutions on a large scale in chemical 

 manufactories (chlorate of barium for example), the workmen 

 sometimes stretch clean white strings across the large enamelled 

 vessels into which the solution is to be poured ; and they find, 

 practically, that the strings act best as nuclei when they draw 

 them through their hands, "which," as my informant remarked, 

 "are not particularly clean." 



I might here introduce a number of facts respecting the boil- 

 ing of water in vessels of different material, and the action of 

 solid fragments in preventing jumping ebullition; but the sub- 

 ject is so extensive that I have included all I have to say respect- 

 ing it in a separate memoir. 1 may merely mention here what 

 is, I believe, a matter of common experience, that in introducing 

 fragments of platinum, glass, &c. into retorts to prevent bump- 

 ing, the fragments soon cease to be of any use. The reason is 

 that they become chemically clean, and the liquid adheres to 

 them with as much force as to the sides of the retort. 



Dr. Faraday's experiment of firing the mixed gases by means 



