EXAMPLES OF THE EXPLAIN ATION OF LAWS. 279 



more limltod kind, reducing the jilieiiomena concerned in those gener- 

 alizations into mere cases pt" itself. 



The contagious inHuencc ot' clumucal action is not a powerful force, 

 and is only capable of overcoming weak affinities : we may, therefore, 

 expect to tind it principally exeuiplitied in the decomposition of sub- 

 stances which are held together by weak chemical forces. Now the 

 force which holds a compound eubstauce together is generally weaker, 

 the more compound the substance is ; and organic products are the most 

 compound substances known, those which have the most complex 

 atomic constitution. It is, therefore, upon such substances that the 

 self-j>ropagating power of chemical action is likely to exert itself ill 

 the most markexl manner. Aecoi'dingly, first, it explains the remark- 

 able laws of fermentation, and some of those of putrefaction. "A little 

 leaven," that is, dough in a certain state of chemical- action, impresses 

 a similar chemical actioniipon " the wliole hnnp." The contact of any' 

 decaying substance, occasions the decay of matter previously sound. 

 Again, yeast is a substance actually in a process of decomposition from 

 the action of air and water, evolving carbonic acid gas. SQgar is a 

 substance which, from the complexity of its composition, ha^ no great 

 energy of coherence in its existing form, and is capable of being easily 

 converted (by combination with the elements of water) into carbonic 

 acid and alcohol. Now the mere presence of yeast, the mere proxim- 

 ity of a substance of which the elements are separating from each 

 other, and combining with the elements of water, causes sugar, to un- 

 dergo the same change, giving out carbonic acid gas, arid becoming 

 alcohol. It is not the elements contained-in the yeast which do this. 

 " An acpieous infusion of yeast may be mixed with a solution of sugar, 

 and preserved in vessels from which the air is excluded, without either 

 experiencing the slightest change." Neither does the insoluble resi- 

 due of the yeast, after being treated with water, possess the power 

 of exciting fermentation. It is not the yeast itself, therefore ;■ it is 

 the yeast in a state of decomposition. The sugar, which would noC 

 decompose and oxidize by the mere presence of Oxygen and waterj 

 is induced to do so when another oxidation is at work in the midst 

 of it. ' 



J3y the same principle Liebig is enabled to explain rrialaria; the 

 pernicious influence of putrid substances ; a variety of poisons; conta- 

 gious diseases ; and other phenomena. Of all substances, those com- 

 -posing the animal body are the most complex in their Composition, and 

 in the least stable condition of union. The blood, in particular, is the 

 most unstable compound known. What, therefore, can be less. sur- 

 prising than that gaseous or other substances, in the act of undergoing 

 the chemical changes which constitute, for instance, putrefaction, 

 should, when brought into contact with the tissues by respiraticjn or 

 otherwise, and still more when introduced by inoculation into the blood 

 itself, impress upon some of the particles a chemical action similar to 

 its own ; which is propagated in like manner to other particles, until 

 the whole system is placed in a state of chemical action more or less 

 inconsistent with the chemical conditions of vitality. 



Of the three modes in which we observed in the last chapter that the 

 resolution of a special law into more general ones may take place, this 

 speculation of Liebig exemplifies the second. The laws explained 

 are such as this, that yeast puts sugar into a state of fennentation. 



