104 



SCIENTIFIC AMUSEMENTS. 



Instead of bladder or sausage skins immersed in water, the 

 following arrangement may be conveniently employed for experi- 

 ments in dialysis. Procure a wooden hoop, or better still the 

 wooden drum-like cylinder of a sieve, and strain over it, tambourine 

 fashion, a sheet of parchment paper or a flat circular piece of 

 bladder, tying it down with string as though you were covering a 

 jam-pot (fig. 53). Fill a basin with water, and on the surface of 

 this float the hoop dialyser thus formed ; pour the mixed solution 

 into the tambourine, and let the whole stand for twenty-four 

 hours : by this time the majority of the crystalloids present in the 

 mixed fluid in the tambourine will have passed through the 

 membrane, and become diffused through the water in the basin, 

 whilst the colloids will be still retained in the tambourine. 

 Instead of a hoop dialyser, the arrangement shown in fig. 54 

 may be used, consisting of a bell-shaped glass, with a piece of 



bladder tied tightly over 

 the wide mouth, and sus- 

 pended by strings inside a 

 beaker of water. 



Expt. 97. Endosmosis 

 and Exosmosis. If in 

 Expt. 92, instead of brine 

 (watery solution of common 

 salt, or chloride of sodium), 

 an alkaline fluid (Expt. 

 138), such as solution of 

 carbonate of potassium or 



Fig. 53. Hoop 

 Dialyser. 



Fig. 54. Bell- 

 Glass Dialyser. 



carbonate of sodium, be used, the rate of inward flow of water 

 into the bladder by endosmosis is greatly increased; on the 

 other hand, if an acid solution (such as highly diluted sulphuric or 

 hydrochloric acid) be employed, the flow of fluid is reversed in 

 direction, exosmosis now taking place, or passage of fluid through 

 the membrane from within outwards, so that the level of the liquid 

 sinks in the glass tube instead of rising. The rate of passage of 

 the fluid is greatly influenced by the character of the membrane ; 

 the finer its pores the more rapid is the action, so that a thin 

 fresh animal membrane will cause a less rapid flow of liquid than 

 the same membrane partially tanned or acted on in analogous fashion 

 by chemical agents (e.g., chromic acid solution), which tend to 



drops of solution of iodine, when, if starch be present, the blue colour of 

 iodide of starch will appear. 



Should plaster of Paris be present (which sometimes happens with icings 

 of sugar on certain kinds of cakes, &c. ), it will not be gelatinised by the hot 

 water, and will subside as a white powder from the hot liquid on standing. 



