EXAMPLES OF THE FOUR METHODS. 



267 



CHAPTER XI. 



MISCELLANEOUS EXAMPLES OF THE 

 FOUR METHODS. 



§ r. I SHALL select, as a first ex- 

 ample, an interesting speculation of 

 one of the most eminent of theoretical 

 chemists, Baron Liebig. The object 

 in view is to ascertain the immediate 

 cause of the death produced by metal- 

 lic poisons. 



Arsenious acid and the salts of lead, 

 bismuth, copper, and mercury, if in- 

 troduced 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 generalisation ; 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, pointing out what 

 property, common to all tliese dele- 

 terious substances, is the really operat- 

 ing 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 mem- 

 branes, the acid or salt leaves the 

 water in which it was dissolved, and 

 enters into combination with the ani- 

 mal substance : 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 sub- 

 stances have been brought into con- 

 tact do not afterwards putrefy. 



And, finally, when the poison has 

 been supplied in too small a quantity 

 to destroy life, eschars are produced, 

 that is, certain superficial portions of 

 the tissues are destroyed, which are 

 afterwards thrown off by the repara- 

 tive process taking place in the healthy 

 parts. 



These three sets of instances admit 

 of being treated according to the 



Method of Agreement. In all of 

 them the metallic compounds are 

 brought into contact with the sub- 

 stances which compose the human 

 or animal body ; and the instances do 

 not seem to agree in any other cir- 

 cumstance. The remaining antece- 

 dents are as different, and even oppo- 

 site, as they coiild possibly be made ; 

 for in some the animal substances 

 exposed to the action of the poisons 

 are in a state of life, in others only 

 in a state of organisation, in others 

 not even in that. And what is the 

 result which follows in all the cases ? 

 The conversion of the animal sub- 

 stance (by combination with the 

 poison) into a chemical compound, 

 held together by so powerful a force 

 as to resist the subsequent action of 

 the ordinary causes of decomposition. 

 Now, organic life (the necessary con- 

 dition of sensitive life) consisting in 

 a continual state of decomposition 

 and recomposition of the different 

 organs and tissues, whatever inca- 

 pacitates them for this decomposition 

 destroys life. And thus the proxi- 

 mate cause of the death produced by 

 this description of poisons is ascer- 

 tained, as far as the Method of Agree- 

 ment can ascertain it. 



Let us now bring our conclusion to 

 the test of the Method of Difference. 

 Setting out from the cases already 

 mentioned, in which the antecedent 

 is the presence of substances forming 

 with the tissues a compound incap- 

 able of putrefaction, (and a fortiori 

 incapable of the chemical actions 

 which constitute life,) and the con- 

 sequent is death, either of the whole 

 organism, or of some portion of it ; 

 let us compare with these cases other 

 cases, as much resembling them as 

 possible, but in which that effect is 

 not produced. And, first, " many in- 

 soluble basic salts of arsenious acid 

 are known not to be poisonous. The 

 substance called alkargen, discovered 

 by Bunsen, which contains a very 

 large quantity of arsenic, and ap- 

 proaches very closely in composition 

 to the organic arsenious compoundi 



