A college text-book of chemistry . by meansof dialysis. It has been found that solutions of differentsubstances pass with different degrees of ease throughporous membranes, just as gases differ as regards the easewith which they pass through porous diaphragms. Thisfact concerning gases was referred to in connection withhydrogen. Now, while some solutions pass readily throughparchment paper, others pass through with difficulty, andsome do not pass through at all. A dialyser, or anapparatus used in dialysis, may be made by tying a pieceof parchment paper over the mouth of a ring-formed glassor r
A college text-book of chemistry . by meansof dialysis. It has been found that solutions of differentsubstances pass with different degrees of ease throughporous membranes, just as gases differ as regards the easewith which they pass through porous diaphragms. Thisfact concerning gases was referred to in connection withhydrogen. Now, while some solutions pass readily throughparchment paper, others pass through with difficulty, andsome do not pass through at all. A dialyser, or anapparatus used in dialysis, may be made by tying a pieceof parchment paper over the mouth of a ring-formed glassor rubber vessel, and placing this in another shallowvessel. Pure water is put in the outer vessel, and the 396 COLLEGE CHEMISTRY. solution for dialysis in the inner one. The arrangementis illustrated in Pig. 76. In the figure aa is the hoop of gutta-percha, and b isthe parchment paper. When now the solution containinghydrochloric acid, sodium chloride, and silicic acid is putin the dialyser, the hydrochloric acid and sodium chloride. Fig. 76. pass readily through the membrane, while the silicic acidis left- behind, and in the course of a few days, if the waterin the outer vessel is renewed, the solution of silicic acidin the inner vessel will be found to be free from the othersubstances. This solution can be evaporated to someextent by boiling, but when a certain concentration isreached the acid separates. In a vacuum such a solutioncan be evaporated further without the formation of adeposit. Finally, there is left a transparent mass whichhas approximately the composition represented by theformula H2SiOs. The dialysed solution of silicic acid iscoagulated by a very dilute solution of sodium or potassiumcarbonate, and by carbon dioxide itself. When the solutions containing silicic acid are evaporatedto complete dryness the acid is converted into silicondioxide and insoluble hydrates. This residue is calledinsoluble silicic acid. When this is treated with hydro-chloric acid and wate
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