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partition. The parchment paper which is manufactured by im- 

 mersing unsized paper in sulphuric acid, and then washing out the 

 acid, is a good material of which to form this partition. During 

 the diffusion of mixed solutions through this parchment paper 

 some substances pass much more quickly than others, and so their 

 separation can be effected. Very remarkable relations have been 

 discovered to exist between the laws of the diffusion ol fluids into 

 one another, and the laws of the diffusion of heat through solids. 

 In fact, when quantities or degrees of temperature communicated 

 by conduction of heat, are duly compared with quantities of one 

 diffusible substance, transferred by diffusion into another, the law 

 of diffusion in liquids is found to be identical with that of thermal 

 conductivity. This discovery was first published by Fick. Pro- 

 fessor Thomson showed to the Society a mode which, during some 

 years past, had been practised by his brother, Sir William Thom- 

 son, for illustrating diffusion, and exhibiting its great slowness in 

 liquids, when circulating currents are avoided. Some glass tubes 

 of about a quarter of an inch or half an inch bore, and from two 

 to four feet in length, are hermetically sealed at one end. Into 

 each two liquids are introduced, the heavier one being poured in 

 first, and the lighter one being poured in so as to rest upon the top 

 of it. The upper end is then hermetically sealed. The liquids 

 may be sulphuric acid below, with water above ; or water below, 

 with alcohol above ; or a solution of sulphate of copper below, 

 with water above ; and various others may be taken. The 

 tubes so prepared are kept in an upright position, and diffu- 

 sion proceeds. At first, the region of gradual transition from 

 the one fluid to the other occupies a very short space in the 

 length of the tube, but, as time goes on, the region of transi- 

 tion spreads itself. Even after years have elapsed, however, 

 during which diffusion has been constantly going on, the liquid at 

 top and the liquid at bottom are found to be still remaining dis- 

 tinct from one another. With the sulphuric acid and water tube 

 this may be made very evident after the lapse of a year or two by 

 inverting the tube several times so as to thoroughly mingle its 

 contents, when it will be found to become very hot through the 



