376 Mr. T. Graham on Liquid Diffusion applied to Analysis. 



feeble force. Hence colloids are generally displaced and preci- 

 pitated by the addition to their solution of any substance from 

 the other class. Of all the properties of liquid colloids, their 

 slow diffusion in water, and their arrest by colloidal septa, are 

 the most serviceable in distinguishing 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 insipid. 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. Eut certain 

 colloidal decompositions are equally inexplicable upon ordinary 

 chemical views. To facilitate the separation of hydrochloric 

 acid from the perchloride of iron, for instance, that salt is first 

 rendered basic by the addition of peroxide of iron. The com- 

 paratively stable perchloride of iron is transformed, by such treat- 

 ment, into a feebly-constituted colloidal hydrochlorate. The 

 latter compound breaks up under the purely physical agency of 

 diffusion, and divides on the dialyser into colloidal peroxide of 

 iron and free hydrochloric acid. The superinduction of the 

 colloidal condition may possibly form a stage in many analogous 

 organic decompositions. 



A tendency to spontaneous change, which is observed occa- 

 sionally in crystalloids, appears to be general in the other class. 

 The fluid colloid becomes pectous and insoluble by contact with 

 certain other substances, without combining with these sub- 

 stances, and often under the influence of time alone. The 

 poetizing substance appears to hasten merely an impending 

 change. Even while fluid a colloid may alter sensibly, from 

 colourless becoming opalescent; and whiJe pectous the degree 

 of hydration may become reduced from internal change. The 

 gradual progress of alteration in the colloid effected by the 

 agency of time, is an investigation yet to be entered upon. 



The equivalent of a colloid appears to be always high, although 

 the ratio between the elements of the substance may be simple. 



