EXAMPLES OF THE TOUR METHODS. 237 



the relation wlilcli wo liavo obsoivotl to exist betweeit the variations 

 of A and a, will hold true in uU cases which fall between the same 

 extreme; limits ; that is, wherever the ntnu)st increase or diminution in 

 which the result has been found' by observation to coincide with thq 

 law, is not exceeded. 



The four methods which it has now. been attempted to describe, arc 

 tlie only possible modes of experimental inquiry, of direct induction 

 d posteriori, as distinguished from deduction : at least I know not, nor 

 am able to conceive, any others. And even of these, the Method of 

 Residues, as\ve have seen, is not independent of deduction; though, as 

 it retpures specific experience in addition, it may, without impropriety, 

 be included among methods of direct observation and experiment. 



These, then, with such assistance as can be obtained from Deduction, 

 co;mpose the available resources of the human mind for ascertaining 

 the laws of the succession of phenomena. Before proceeding to point 

 out certain circumstances, by which the employment of these methods 

 is subjected to an immense increase of complication and of difficulty, 

 it is expedient to illustrate- the use of the methods, by suitable 

 examples, drawn from actual physical investigations. These, accord- 

 ingly, will form the subject of the succeeding chapter. 



CHAPTER IX. 



MISCELLANEOUS EXAMPLES OF THE FOUR METHODS. 



• ■ § 1. I SHALL select, as my first example, an interesting, speculation 

 of one of the most eminent theoretical chemists of the present or any 

 age, Dr. Liebig. The object in view, is to ascertain the immediate 

 cause of the death produced by metallic poisons. 



Arsenious acid, and the salts of lead, bismuth, copper, and mercury, 

 if introduced into the animal organism, except in the smallest doses, 

 destroy life. These facts have long been known, as insulated truths of 

 the lowest order of generalization ; but it was reserved for Liebig, by 

 an apt employment of the first two of our methods of experimental 

 inquiry, to connect these truths together by a higher induction, point- 

 ing out what property, common to all these deleterious &ubstances,. ia 

 the really operating cause of their fatal effect. 



When solutions of these substances are placed in sufficiently close 

 contact with many animal products, albumen, milk, muscular fibre, 

 and animal membranes, the acid or salt leaves the water in which 

 it was dissolved, and enters into combination with the animal sub- 

 stance ; which substance, after being thus acted upon, is found to have 

 lost its tendency to spontaneous decomposition, or putrefaction. 



Observation also shows, in cases where death has been produced by 

 these poisons, that the parts of the body with which the poisonous 

 substances have been brought into contact, do not afterwards putrefy. 



And, finally, when the poison has been supplied in too small a quan- 

 tity to destroy life, eschars are produced, tfiat is, certain superficial 

 portions of the tissues are destroyed, which are afterwards thrown off 

 by the reparative process taking jjlace in the healthy parts. 



