100 SCIENTIFIC AMUSEMENTS. 



Expt. 91. Diffusion of Perchloride of Iron Solution. Eepeat 



the above experiment, using strong perchloride of iron solution 

 instead of brine. After a day or two the water in the jar will 

 contain enough perchloride of iron diffused into it to give the 

 test for iron in solution described in Expt. 57 ; i.e., if a little 

 of the solution be taken out with a spoon, and there be added to 

 it a few drops of solution of ferrocyanide of potassium, prussian 

 blue will be produced. 



Instead of operating as above described, the occurrence of 

 diffusion may be illustrated thus. Fill a glass jar half full of 

 distilled water, place in it a clean funnel with a long stern, so that 



the lower end of the funnel reaches 

 to the bottom of the jar (fig. 51). 

 !N"ow carefully pour the heavier solu- 

 tion of salt or of perchloride of iron 

 down the funnel; the heavier fluid 

 will form a layer at the . bottom of 

 the vessel, on which the lighter 

 water will float, the water rising in 

 the jar as the heavier fluid is poured 

 in. A sample of the water at the 

 top of the jar taken out now and 

 tested will not indicate the presence 

 of any dissolved matter ; but after a 

 day or two, the same test applied to 

 a fresh sample taken out will show 

 that the substance dissolved in the 



^fU"?. r tiall i become 



into the lighter water. 

 Almost any solution heavier than water may thus be used : 

 if a solution of ferrocyanide of potassium be poured down the 

 funnel, the effect of diffusion after a few days will be shown by 

 testing a sample with perchloride of iron solution, and obtaining 

 prussian blue. A strong solution of nitrate of lead after diffusion 

 has taken place will yield a fluid which will give a yellow 

 precipitate with iodide of potassium (compare Expt. 11). A 

 solution of hydrochloric acid will similarly yield a fluid capable 

 of turning blue litmus-paper red (Expt. 57). 



Intermixture of Fluids through Membranes : Dialysis and 

 Osmosis. 



In Expt. 40 we have seen that whilst water can pass 

 pretty easily through an animal membrane (such as a bladder), 



