2T6 



INDUCTION. 



The accumulated proof of which 

 the Theory of Dew has been found 

 susceptible is a striking instance of 

 the fulness of assurance which the 

 inductive evidence of laws of causa- 

 tion may attain in cases in which 

 the invariable sequence is by no means 

 obvious to a superficial view. 



§ 4. The admirable physiological 

 investigations of Dr. Brown-Sequard 

 afford brilliant examples of the appli- 

 cation of the Inductive Methods to a 

 class of inquiries in which, for reasons 

 which will presently be given, direct 

 induction takes place under peculiar 

 difficulties and disadvantages. As 

 one of the most apt instances, I select 

 his speculation (in the Proceedings 

 of the Royal Society for May 16, 

 1 861) on the relations between mus- 

 cular irritability, cadaveric rigidity, 

 and putrefaction. 



The law which Dr. Brown-S^quard's 

 investigation tends to establish is the 

 following : — " The greater the degree 

 of muscular irritability at the time of 

 death, the later the cadaveric rigidity 

 sets in, and the longer it lasts, and 

 the later also putrefaction appears, 

 and the slower it progresses." One 

 would say at first sight that the 

 method here required must be that 

 of Concomitant Variations. But this 

 is a delusive appearance, arising from 

 the circumstance that the conclusion 

 to be tested is itself a fact of con- 

 comitant variations. For the estab- 

 lishment of that fact any of the 



and can form, therefore, no suflBcient judg- 

 ment what portion of the effects may be 

 due, not to the supposed cause, but to some 

 unknown agency <>f the means by which 

 that cause was produced. In the natural 

 experiment which we are speaking of, the 

 means used was the clearing off a canopy 

 of clouds ; and we certainly do not know 

 sufficiently in what this process consists, 

 or on what it depends, to be certain d priori 

 that it might not operate upon the deposi- 

 tion of dew independently of any ther- 

 mometric effect at the eartii's surface. 

 Even, therefore, in a case so favourable as 

 this to Nature's experimental talents, her 

 experiment is of little value except in 

 corroboration of a conclusion already at- 

 tained through other meani. 



Methods may be put in requisition, 

 and it will be found that the fourth 

 Method, though really employed, has 

 only a subordinate place in this par- 

 ticular investigatioa 



The evidences by which Dr. Brown - 

 S^quard establishes the law may be 

 enumerated as follows : — 



1st. Paralysed muscles have greater 

 irritability than healthy muscles. 

 Now, paralysed muscles are later in 

 assuming the cadaveric rigidity than 

 healthy muscles, the rigidity lasts 

 longer, and putrefaction sets in later, 

 and proceeds more slowly. 



Both these propositions had to be 

 proved by experiment; and for the 

 experiments which prove them science 

 is also indebted to Dr. Brown-Sdquard. 

 The former of the two — that paralysed 

 muscles have greater irritability than 

 healthy muscles — he ascertained in 

 various ways, but most decisively by 

 "comparing the duration of irrita- 

 bility in a paralysed muscle and in 

 the corresponding healthy one of the 

 opposite side, while they are both 

 submitted to the same excitation." 

 He "often found in experimenting 

 in that way that the paralysed muscle 

 remained irritable twice, three times, 

 or even four times as long as the 

 healthy one." This is a case of in- 

 duction by the Method of Difference. 

 The two limbs, being those of the 

 same animal, were presumed to differ 

 in no circumstance material to the 

 case except the paralysis, to the pre- 

 sence and absence of which, therefore, 

 the difference in the muscular irrita- 

 bility was to be attributed. This 

 assumption of complete resemblance 

 in all material circumstances save 

 one, evidently could not be safely 

 made in any one pair of experiments, 

 because the two legs of any given 

 animal might be accidentally in very 

 different pathological conditions ; but 

 if, besides taking pains to avoid any 

 such difference, the experiment was 

 repeated sufficiently often in different 

 animals to exclude the supposition 

 that any abnormal circumstance could 

 be present in them all, the conditions 



