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X. Liqiiid Diffusion applied to Analijsis. 
Dy Thomas Geaham, F.B.S., Master of the Mint. 
Eeceived IMay 8, — E«ad June 13, 1861. 
The property of volatility, possessed in various degrees by so many substances, affords 
invaluable means of separation, as is seen in the ever-recurring processes of evaporation 
and distillation. So similar in character to volatility is the Diffusive power possessed 
by all liquid substances, that we may fairly reckon upon a class of analogous analytical 
resources to arise from it. The range also in the degree of diffusive mobility exhibited 
by different substances appears to be as wide as the scale of vapour tensions. Thus 
hydrate of potash may be said to possess double the velocity of diffusion of sulphate of 
potash, and sulphate of potash again double the velocity of sugar, alcohol, and sulphate 
of magnesia. But the substances named belong all, as regards diffasion, to the more 
“volatile” class. The comparatively “fixed” class, as regards diffusion, is represented 
by a different order of chemical substances, marked out by the absence of the power to 
crystallize, which are slow in the extreme. Among the latter are hydrated silicic acid, 
hydrated alumina, and other metallic peroxides of the aluminous class, when they exist 
in the soluble form ; wdth starch, dextrin and the gums, caramel, tannin, albumen, 
gelatine, vegetable and animal extractive matters. Low diffusibility is not the only 
property which the bodies last enumerated possess in common. They are distinguished 
by the gelatinous character of their hydrates. Although often largely soluble in water, 
they are held in solution by a most feeble force. They appear singularly inert in the 
capacity of acids and bases, and in all the ordinary chemical relations. But, on the 
other hand, their peculiar physical aggregation with the chemical indifference referred 
to, appears to be recpiired in substances that can inteiwene in the organic processes of 
life. The plastic elements of the animal body are found in this class. As gelatine 
appears to be its type, it is proposed to designate substances of the class as colloids, and 
to speak of their peculiar form of aggregation as the colloidal condition of matter. 
Opposed to the colloidal is the crystalline condition. Substances affecting the latter 
form will be classed as crystalloids. The distinction is no doubt one of intimate mole- 
cular constitution. 
Although chemically inert in the ordinary sense, colloids possess a compensating 
activity of their own arising out of their physical properties. While the rigidity of the 
crystallme structure shuts out external impressions, the softness of the gelatinous colloid 
partakes of fluidity, and enables the colloid to become a medium for liquid diffusion, 
like water itself. The same penetrability appears to take the form of cementation in 
