532 INDUCTION. 



palate " are probably protected by a colloidal membrane," im- 

 permeable to other colloids, a colloid, when tasted, probably 

 never reaches those nerves. Again, " 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." One of the mysterious processes 

 accompanying digestion, the secretion of free muriatic acid by 

 the coats of the stomach, obtains a probable hypothetical ex- 

 planation through the same law. Finally, much light is thrown 

 upon the observed phenomena of osmose (the passage of fluids 

 outward and inward through animal membranes) by the fact 

 that the membranes are colloidal. In consequence, the water 

 and saline solutions contained in the animal body pass easily 

 and rapidly through the membranes, while the substances 

 directly applicable to nutrition, which are mostly colloidal, are 

 detained by them.* 



The property which salt possesses of preserving animal 

 substances from putrefaction is resolved by Liebig into two 

 more general laws, the strong attraction of salt for water, 

 and the necessity of the presence of water as a condition of 

 putrefaction. The intermediate phenomenon which is interpo- 

 lated between the remote cause and the effect, can here be not 

 merely inferred but seen ; for it is a familiar fact, that flesh 

 upon which salt has been thrown is speedily found swimming 

 in brine. 



The second of the two factors (as they may be termed) 

 into which the preceding law has been resolved, the necessity 

 of water to putrefaction, itself affords an additional example 

 of the Eesolution of Laws. The law itself is proved by the 

 Method of Difference, since flesh completely dried and kept 

 in a dry atmosphere does not putrefy ; as we see in the case of 

 dried provisions, and human bodies in very dry climates. A 

 deductive explanation of this same law results from Liebig's 

 speculations. The putrefaction of animal and other azotised 



* Vide Memoir by Thomas Graham, F.K.S., Master of the Mint, "On 

 Liquid Diffusion Applied to Analysis," in the Philosophical Transactions for 

 1862, reprinted in the Journal of the Chemical Society, and also separately as a 

 pamphlet. 



