220 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
soluble poisonous substances, whatever their origin, appear to be crystalloids, and 
accordingly pass through colloidal septa. 
7. Colloidal Condition of Matter. 
I may be allowed to advert again to the radical distinction assumed in this paper to 
exist between colloids and crystalloids in their intimate molecular constitution. Every 
physical and chemical property is characteristically modified in each class. They appear 
like different worlds of matter, and give occasion to a corresponding division of chemical 
science. The distinction between these kinds of matter is that subsisting between the 
material of a mineral and the material of an organized mass. 
The colloidal character is not obliterated by liquefaction, and is therefore more than 
a modification of the physical condition of solid. Some colloids are soluble in water, as 
gelatine and gum-arabic ; and some are insoluble, like gum-tragacanth. Some colloids, 
again, form solid compounds with water, as gelatine and gum-tragacanth, while others, 
like tannin, do not. In such points the colloids exhibit as great a diversity of property 
as the crystalloids. A certain parallelism is maintained between the two classes, 
notnathstanding their differences. 
The phenomena of the solution of a salt or crystalloid probably all appear in the solu- 
tion of a colloid, but greatly reduced in degree. The process becomes slow ; time, in- 
deed, appearing essential to all colloidal changes. The change of temperature, usually 
occurring in the act of solution, becomes barely perceptible. The liquid is always 
sensibly gummy or viscous when concentrated. The colloid, although often dissolved in 
a large proportion by its solvent, is held in solution by a singularly feeble force. Hence 
colloids are generally displaced and precipitated by the addition to their solution of any 
substance from the other class. Of all the properties of liquid colloids, their slow diffu- 
sion in water, and their arrest by colloidal septa, are the most serviceable in distinguish- 
ing them from crystalloids. Colloids have feeble chemical reactions, but they exhibit 
at the same time a very general sensibility to liquid reagents, as has already been 
explained. 
While soluble crystalloids are always highly sapid, soluble colloids are singularly in- 
sipid. It may be questioned whether a colloid, when tasted, ever reaches the sentient 
extremities of the nerves of the palate, as the latter are probably protected by a colloidal 
membrane, impermeable to soluble substances of the same physical constitution. 
It has been observed that vegetable gum is not digested in the stomach. The coats 
of that organ dialyse the soluble food, absorbing crystalloids and rejecting all colloids. 
This action appears to be aided by the thick coating of mucus which usually lines the 
stomach. 
The secretion of free hydrochloric acid during digestion — at times most abundant — 
appears to depend upon processes of which no distinct conception has been formed. 
But certain colloidal decompositions are equally inexplicable upon ordinary chemical 
■views. To facihtate the separation of hydrochloric acid from the perchloride of iron, 
