EXAMPLES OF TUE FOUR METHODS. 239 



The disease called painter's cqUc/ so common in manufactories of 

 white lead, is unknown whpre the workmen are accu&tomed to take, 

 as a preservative, sulphuric-acid-lemonade (a solution Of sugar ren- 

 dered acid by sulphuric acid). Now diluted sulphuric acid has the 

 property of decomposing all compounds of lead with organic matter, and 

 (of course) of preventing them from being formed. 



There is another -class of instances, of the nature required by the 

 Method of Pifference, which seem at first sight to conflict with the 

 theory. Soluble salts of silver, such for instance as the nitrate, have 

 the same stiftcning antiseptic effect on decomposing anifnal substances 

 as corrosive sublimate and the most deadly metallic poisons ; and when 

 applied to the external parts^of the body, tlie nitrate is a powerM 

 criustic, depriv.inc" tho^o ports of rdl ertivo vitnlity/nnd musing thorn to 

 be tlu'own otf by the neighboring living structures, in the form of an 

 eschar. The nitrate and the other salts of silver ought, then, it would 

 seem, if the theory be correct, to be poisonous; yet they may b^ ad- ' 

 ministered "internally with perfect imj)unity. From this apparent 

 exception arises the strongest confirmation which this theory of Liebig 

 has yet received. Nitrate of silver, in spite of its chemical properties, 

 does not poison when introduced into the stomach ; but in the stomach, 

 as in all animal liquids, there is common salt ; and in the stomach 

 there is also free muriatic acid. These substances operate as natural 

 antidotes, combining with the nitrate, and if its quantity is not too great, 

 immediately converting it into chloride of silver ; a substance very 

 slightly soluble, and therefore incapable of combining with the tissues, 

 although to the extent of its solubility it has a medicinal influence, 

 through an entirely different class of org-anic actions. 



§ 2. The preceding instances have afforded an, induction of "a high 

 order of conclusiveness, illustrative of the two simplest of our four 

 methods ; although not rising to the maximum of certainty which the 

 Method of Difference, in its most perfect exemplification, is capable of 

 affording. For (let us not forget) the positive instance and the neg- 

 ative one which the rigor of that method requires, ought to differ only 

 in the presence or absence of one single circumstance. Now, in the 

 preceding argument, tliey differ in the presence or absence not of a sin- 

 gle circumstance, but of a single substance : and as every substance has 

 innumerable properties,, there is no knowing what number of real dif- 

 ferences are involved in what is nominally and apparently only one 

 difference. It is conceivable that the antidote, the peroxide of iron for 

 example, may counteract the poison through some other of its proper- 

 ties than that of forming an insoluble compound with it ; and if so, the 

 theory would fall to the ground, so far as it is supported by that in- 

 stance. This source of uncertainty, which is a serious hindrance to 

 all extensive generalizations in chemistry, is however reduced in tho 

 present case to almost the lowest degree possible, when we find that 

 not only one substance, but many substances, possess the capacity of 

 acting as antidotes to metallic poisons, and that all these agree in the 

 property of forming insoluble compounds with the poisons, while they 

 cannot be ascertained to agree in any other property whatsoever. We 

 have thus, in favor of the theory, all the evidence which can be ob- 

 tained by what we termed the Indirect Method of Difference, or the 

 Joint Method of Agreement and Difference ; the evidence of which, 



