216 LIQUID DIFFUSION. 



which is heavier than pure water and becomes heaviei 

 the stronger it is. An equal quantity of the blu< 

 crystals is placed in a little bag of muslin (or 

 other thin fabric), and suspended in the water coi 

 tained in the second test-tube by means of a thread tie< 

 to the bag and fixed to a small piece of wood whicl 

 is placed across the top of the test-tube, so that the 

 bag may be just immersed in the water. In both test- 

 tubes the crystals at once begin to be dissolved. From 

 the suspended crystals, the solution, being heavier than 

 the water, descends to the bottom of the tube, while 

 a fresh quantity of pure water takes its place, dissolves 

 a portion of the solid crystals, becomes heavier and 

 sinks, giving in its turn room for a lighter portion 

 of the fluid, and so on: the greater specific gravity of 

 the solution produces currents which do not cease until 

 the contents of the bag are completely dissolved, and 

 the liquid is coloured uniformly blue. In the oth< 

 test-tube, solution also begins immediately, but in coi 

 sequence of its greater specific gravity the blue liqui< 

 remains at the bottom of the tube covering the undii 

 solved portion of the crystals; the liquid gradual!] 

 becomes deep blue and completely saturated, while the 

 upper portions ol the liquid remain for a considerable 

 time pure water; only very slowly and gradually, in 

 the course of days and even weeks, diffusion tab 

 place, and months must elapse before the liquid is ui 

 formly blue and the cupric salt is diffused equally 

 through the whole. 



It is a remarkable fact that when walls of certain 

 substances, such as gypsum, unglazed and slightly 

 burnt clay, parchment-paper, animal membranes, el 



kes 

 mi- 



