Examples o^ the explanation of laws. 313 



mixture, by passing them through a 

 thin septum of colloidal matter, which 

 does not sufifer anything colloidal to 

 pass, or suffers it only in very minute 

 quantity. This property of colloids 

 enabled Mr. Graham to account for a 

 number of special results of observa- 

 tion not previously explained. 



For instance, "while soluble crys- 

 talloids are always highly sapid, 

 soluble colloids are singularly in- 

 sipid," as might be expected ; for, as 

 the sentient extremities of the nerves 

 of the palate "are probably protected 

 by a colloidal membrane," imperme- 

 able 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 col- 

 loids." One of the mysterious pro- 

 cesses accompanying digestion, the 

 secretion of free muriatic acid by 

 the coats of the stomach, obtains a 

 probable hypothetical explanation 

 through the same law. Finally, 

 much light is thrown upon the ob- 

 served 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 in- 

 termediate phenomenon which is in- 

 terpolated between the remote cause 



* Vide Memoir by Tliomas Graham, 

 F.R.S., Master of the Mint, "On Liquid 

 Diffusion Applied to Analysis ; " in the 

 Philosophical Transactions for 1862, re- 

 printed in the Journal of the Chemical 

 •Society, and also separntely as a pamphlei. 



anct 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 swim- 

 ming 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 Resolution of Laws. The law 

 itself is proved by the Method of Dif- 

 ference, 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 re- 

 sults from Liebig's speculations. The 

 putrefaction of animal and other azo- 

 tised bodies is a chemical process, by 

 which they are gradually dissipated 

 in a gaseous form, chiefly in that of 

 carbonic acid and ammonia ; now to 

 convert the carbon of the animal sub- 

 stance into carbonic acid rec^uires oxy- 

 gen, and to convert the azote into 

 ammonia requires hydrogen, which 

 are the elements of water. The ex- 

 treme rapidity of the putrefaction of 

 azotised substances, compared with 

 the gradual decay of non-azotised 

 bodies (such as wood and the like) 

 by the action of oxygen alone, he 

 explains from the general law that 

 substances are much more easily de- 

 composed by the action of two dif- 

 ferent afl&nities upon two of their ele- 

 ments than by the action of only one. 



§ 3. Among the many important 

 properties of the nervous system which 

 have either been first discovered or 

 strikingly illustrated by Dr. Brown- 

 Sequard, I select the reflex influence 

 of the nervous system on nutrition 

 and secretion. By reflex nervoias 

 action is meant action which one part 

 of the nervous system exerts over 

 another part, without any interme- 

 diate action on the brain, and con- 

 sequently without consciousness ; or 

 which, if it does pass through the 

 brain, at least produces its effects in- 



