102 SCIENTIFIC AMUSEMENTS. 



culty. Accordingly soluble bodies are divided into two classes 

 crystalloids and colloids according as they can or cannot easily 

 undergo the process of dialysis or separation whilst in solution by 

 passage through a septum from &a, through, and Xwis, separation. 



Expt. 93. To illustrate the Difference between Crystalloids 

 and Colloids. Dissolve a little tannin in water, and also prepare 

 some thin glue or gelatine solution by digesting fragments of solid 

 glue with warm water. Pour some of the tannin solution into the 

 gelatine solution, and you will see that the latter becomes pre- 

 cipitated as a soft curdy substance, the tannin combining with the 

 gelatine to form a substance insoluble in water. The process of 

 making leather from skins by tanning depends on this action, the 

 tannin contained in the oak bark, &c., used by the tanner, gradually 

 being absorbed by the gelatinous matter of the fresh hide, and 

 indurating it by forming an insoluble compound therewith. A 

 solution of tannin thus serves as a test for the presence of gelatine in 

 solution. 



Now mix with some of the thin glue a little strong brine, and 

 place the whole in a small bladder, or freshly-cleaned sausage skin, 

 with one end tightly tied up. Tie a string round the neck of the 

 bladder, and suspend the bladder by the string inside a large 

 glass cylinder or beaker filled with distilled water. After some 

 hours, or even one or two days, take out a sample of the water 

 from the beaker, and divide it into two parts ; to one add some 

 tannin solution, and to the other some silver nitrate solution. If 

 the bladder was free from visible leaks and pin-holes, no gelatine 

 will have passed through it, so that the tannin solution will yield 

 no visible precipitate in the first case ; on the other hand, the 

 silver nitrate solution will yield a considerable precipitate showing 

 that the salt present has readily passed through the membrane, the 

 salt being a crystalloid and the gelatine a colloid. The liquid 

 remaining in the bladder will precipitate with tannin, showing 

 that the gelatine is still there. 



Expt. 94. Another Illustration. Rub together in a cup with a 

 tea-spoon as much solid starch as will lie on a sixpence and a little 

 water, so as to make a thin pulp or creamy liquid; pour this into 

 a cupful of boiling water, or pour the boiling water on to the 

 starch. The solid starch will then be apparently dissolved, form- 

 ing a slightly opalescent very thin gelatinous liquid. Muslins, 

 linen, &c., soaked in such a liquid, and then dried and pressed 

 with a hot flat-iron, become stiffened, this being the ordinary 

 process of " starching " employed by the laundress, a little gum 

 being, sometimes added to the starch jelly to give greater stiffness 

 and glossiness to the starched shirts, collars, &c. 



