190 



Mr. Charles Tomlinson on 



[Mar. 14, 



Lowel,* whose researches extend over about ten years, namely, 

 from 1848 to 1867, makes frequent reference, in the course of his 

 six memoirs, to the sides of the vessel. He is of opinion that the aspe- 

 rities of the sides exert no action in determining crystallisation ; that 

 nothing certain can be affirmed as to the size of the tubes in producing 

 such an effect ; that it is not a mechanical action, as Gay-Lussac sup- 

 posed ; that heat deprives the sides of their active, or as he calls it, 

 dynamic power; that even in an open tube the sides are not in a 

 passive state ; that nothing is positively decided as to the action of 

 the sides. He also refers to that mysterious unknown force which 

 holds the crystals in an abnormal state ; he also speaks of the inner 

 surface of the flasks recovering that particular property of determin- 

 ing crystallisation which heat had deprived it of, the cause of which is 

 unknown ; and in his last memoir he supposes that the sides determine 

 the formation of the normal salt by an action of contact. 



I have already expressed my opinionf that had Lowel worked with 

 chemically clean flasks and tubes, he would not have encountered so 

 many contradictory results, or have expressed so many uncertain and 

 conflicting opinions as those above quoted. 



In 1866 Jeannel^ threw out an opinion that the state of supersatura- 

 tion is maintained in closed vessels by reason of the attraction of the 

 sides and the saturation of the interior air. 



I had long ago formed an opinion that the adhesion of the solution 

 to the walls of the vessel is an important function in maintaining 

 the state of supersaturation ; and it seemed probable if this were so, 

 that any force that could effectually detach the solution from a small 

 portion of the side, below the surface, would cause the whole system 

 to break down. 



The most obvious mode of experiment seemed to consist in rubbing 

 the interior surface with a clean wire. About five years ago I obtained 

 a number of results in this way, but did not think them sufficiently 

 important or trustworthy for publication. One source of fallacy soon 

 became obvious. On moving the wire up and down in a nearly vertical 

 direction, a film of the solution is being constantly dragged above the 

 surface, which film, in consequence of rapid evaporation, disengages a 

 molecule of the salt, and this acts as a nucleus to the whole of the 

 solution. Attempts were made to prevent such an effect by tying a piece 

 of linen dipped in hot water loosely round the neck of the tube, and pass- 

 ing the wire through the cloth into the tube, but the result was not 

 satisfactory; for unless the crystals could be seen actually to start 

 from the place rubbed and to spread from this point alone through the 

 solution, no fair conclusion could be formed as to the effect of the 

 rubbing. Friction with platinum, brass and steel wires seemed to 

 * Ann. de Ch. et de Phy. 

 f Proc. Eoy. Soc, xvi, 408. J Comptes Eendus, 2nd January. 



