LIQUID DIFFUSION. 607 



or a graduated jar is filled, up to the highest mark but one, with water, and 

 then, by means 'of a pipette, the solution 'to be tried is poured in at the bottom 

 of the jar, so as to elevate the water to the top of the scale. On leaving phials 

 or jars, so prepared, standing, without agitation, or change of temperature, 

 the substance in solution ascends in the water, against the influence of gravity, 

 as if it were volatile. In other words, it diffuses ; hence the term liquid diffu- 

 sion. All soluble substances diffuse in this way, but they are not equally 

 diffusible. Thus, in phial experiments, the relative quantities of the following 

 substances, diffused through the water above, from solutions of like concentra- 

 tion, in the same time, are as follow : chloride of sodium 58, nitrate of soda 57, 

 sulphate of soda 27, cane sugar 26, gum 13, and albumen 3. In jar experi- 

 ments, the relative times of diffusion of equal quantities of different substances 

 are these : hydrochloric acid, the most diffusible substance hitherto tried, 1 ; 

 chloride of sodium 2.3 ; sugar 7 ; sulphate of magnesia 7 ; albumen 49 ; and 

 caramel, or burnt sugar, 98. The rate of diffusion of different substances is, 

 therefore, remarkably different, being very high for hydrochloric acid and 

 chloride of sodium, but low for gum, albumen, and caramel. So distinct and 

 constant is the diffusive power of different substances, that, from mixed solu- 

 tions of these, chloride of potassium ascends more rapidly than common salt, 

 and this, faster than sulphate of soda ; with salt and albumen, the difference 

 is still more marked. Weak chemical compounds may even be decomposed 

 through the different diffusive power of their constituents ; thus, alum, a double 

 sulphate of alumina and potash, is decomposed, in a phial diffusion experiment, 

 by some of the sulphate of potash rising away from its associated sulphate of 

 alumina. 



The rate of diffusion, in proportion to the quantity of the substance diffused, 

 is greater when the solution is weak ; but the absolute quantity diffused is 

 greater with strong solutions. Heat increases the rate of diffusion, common 

 salt, e. g., diffusing 2 times more rapidly at 120 than at 60. 



From various points of contrast, including their behavior as diffusible bodies, 

 chemical substances are arranged by Graham into crystalloids and colloids. 



Crystalloid bodies are hard, rigid, and quickly soluble ; their solutions are 

 never viscous ; they are always more or less sapid ; their chemical reactions 

 are quick and energetic, but in the molecular sense, they are, if left to them- 

 selves, static, or little liable to molecular changes. This class includes every 

 crystallizable body, and every substance capable of entering into the formation 

 of a crystalline body. 



Colloid substances do not crystallize, but are amorphous ; they have, when 

 dry, a vitreous structure, and instead of being hard and brittle, are soft or 

 tough ; they dissolve freely but slowly, their solutions being more or less 

 viscous, and they gelatinize on cooling, or by concentration. Hence they are 

 named colloids, from collin or gelatin, arid sometimes pectoids, from their 

 gummy character ; they are tasteless or insipid, but they may give rise to 

 sapid crystalloids ; their combining equivalents are high, and their molecules 

 accordingly heavy ; as acids, or bases, they are chemically inert, but they are 



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thrown 

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from a state of solution into the gelatinous, pectous, or solid condition, and, 

 with time, even into the crystalloid state, either spontaneously, or by the 

 slightest contact with extremely minute portions of other substances ; thus a 

 solution of silicic acid is gelatinized by TF j^ c part of an alkaline or earthy carbon- 

 ate. Lastly, in their soft condition, they form, like water, media for liquid 

 diffusion, a crystalloid body diffusing itself through a jelly, almost as readily as 

 through water itself. Colloid substances include gelatinized starch, dextrin, 

 gum, caramel, gelatin, albuminoid bodies, vegetable and animal bodies, ex- 

 tractives, and a number of soluble hydrated mineral substances, as, for exam- 

 ple, silicic acid and peroxide of iron. 



Of the two great classes of substances thus distinguished, crystalloids are 

 highly diffusible, whilst colloids are of low diffusibility. 



Finally, liquid diffusion is to be regarded, not as a purely physical process, 

 like the diffusion of gases, which depends on a tendency of those elastic fluids 



