ARTIFICIAL SEMIPERMEABLE MEMBRANES 403 



over one end of a glass tube by dipping it in 20 per cent, gelatine 

 to which a little potassium bichromate has been added ; the 

 latter has the effect of rendering the gelatine insoluble, if it is 

 allowed to set in the light. After this the tube is again filled 

 with the copper sulphate and the closed end allowed to dip into 

 the potassium ferrocyanide solution until the gelatine has acquired 

 the brown colour of the precipitate. 



Still another method consists in allowing the copper ferro- 

 cyanide precipitate to form in a membrane of celloidin. This 

 can be obtained by pouring a celloidin solution on a clean mercury 

 surface contained in a Petri dish and allowing the solvent to 

 evaporate away. The membrane which remains is then fitted 

 over the open end of a thistle funnel, the overlapping portion 

 being tied securely round the flange of the bulb. In drying, the 

 membrane contracts slightly and becomes stretched taut. A 

 very strong combination is obtained if two membranes of this 

 kind are fitted over one another. 



A piece of pig's bladder, which should be thoroughly dried 

 before use, or a piece of parchment, stretched over the end of a 

 thistle funnel, is often quite effective as a semipermeable mem- 

 brane. In all cases the solution whose osmotic pressure is to 

 be determined is placed in one of these osmometers, and the whole 

 is immersed in water in such a way that diffusion between the 

 two solutions can only take place by way of the semipermeable 

 membrane. The pressure can then be calculated from the 

 height to which the solution rises within a glass tube fitted on 

 to the osmometer, or by means of a suitable manometer. 



V. Equivalent Osmotic Pressures of Sea-water. — For the fol- 

 lowing data, which refer to strengths of Tidman's sea-salt, we 

 are indebted to Mr. F. M. Haines, B.Sc. ; 



