224 DIFFUSION AND OSMOSIS. 



are held in solution by a most feeble force. The}' appear singu- 

 larlj- inert in the capacitj- of acids and bases, and in all the ordi- 

 nary chemical relations. But, on the other hand, their peculiar 

 physical aggregation with the chemical indifference referred to, 

 appears to be required in substances that can intervene in the 

 organic processes of life. The plastic elements of the animal 

 bodj- are found in this class." 



607. Osmose, or Osmosis. Diffusion of liquids through mem- 

 branes. The interposition of a permeable septum between mis- 

 cible liquids does not prevent diffusion. Thus if a solution of 

 sodic chloride is sepai'ated from pure water by an Intervening 

 membrane, as one of bladder or of vegetable parchment (see 

 page 32), diffusion takes place in about the same time as if no 

 membrane were present. 



608. For most experiments in osmosis the simple apparatus 

 known as an osmometer answers very well. It consists of a 

 small reservoir furnished with a membrane bottom, and a gradu- 

 ated tube at its upper part. A very good osmometer can be 

 prepared from a, short-necked bottle from which the bottom has 

 been carefully removed. After the edges at the bottom have 

 been made smooth, a piece of wet parchment paper is tightly 

 fastened on bj' waxed thread. Great care must be taken to 

 select parchment or parchment paper which is free from perfora- 

 tions,^ and the tube at the neck must be well fitted to a velvet 

 cork, so that no escape of liquid can take place in anj- waj'. A 

 film of ordinary unsized paper evenl}- covered with a solution of 

 warm gelatin, which cools to form a firm mass upon its surface, 

 makes a good substitute for parchment in this apparatus. A 

 thin film of white of egg coagulated hj- heat will also serve well 

 for a covering. 



609. The osmometer, filled to a certain point on the tube with 

 the Hquid to be experimented upon, is suspended in pure water so 

 that the liquid in the apparatus is on exactly the same level as 

 the water. It will be seen bj- the experiment that not only does 

 diffusion take place, but that there is a change in the level of 

 the liquid in the tube. 



610. When any of the more diffusible substances are placed 

 in a state of solution in the reservoir, a small amount of the 

 crj-stalloid passes outwards, while a much larger amount of 



1 The existence of actual perforations in good parchment can be demon- 

 strated by subjecting the apparatus to pressure, or even by repeatedly wiping 

 the exposed surface of the parchment with filtering-paper. 



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