160 Life ofDw Jenner. 



Jenner should have acknowledged the full value of the po- 

 pular opinions regarding the security of cow-pox, but should 

 have neglected, or at least treated lightly, the qualifying 

 clauses. Experience has now shown that common observa- 

 tion was pretty nearly as successful in the one case as in the 

 other. But the question naturally occurs, how did Dr. 

 Jenner meet these objections 1 It was in the following man- 

 ner : — 1st, By adopting the notion of a true and spurious 

 cow-pox, one of which afforded full protection, and the other 

 little or none. (p. 132.) — 2d, When he afterwards learned that 

 the true cow-pox itself could not always be depended on, he 

 adopted the theory of deterioration of the virus, and contended 

 that cow-pox, in its deteriorated state, may have produced a 

 local disease, but *' no such influence upon the constitution as 

 is requisite to render the individual unsusceptible of vario- 

 lous contagion." (p. 133.) With these two qualifying expla- 

 nations, Dr. Jenner confidently announced the completeness 

 and the permanency of vaccine protection. The preamble 

 of the petition to parliament, formerly alluded to, runs 

 thus: — " Your petitioner having discovered that a disease, 

 known by the name of cow-pox, admits of being inoculated 

 on the human frame, and is attended with the singularly be- 

 neficial effect of rendering, throughlife, the persons so inocu- 

 lated perfectly secure from the infection of the small-pox, 

 therefore," &c. The same sentiment is no less strongly ex- 

 pressed at p. 294, in a letter to Dr. Ingenhousz, dated in 

 J 798. *' At present, says he, 1 have not the most distant 

 ■doubt that any person who has once felt the influence of 

 perfect cow-pox matter, would ever be susceptible of the 

 small-pox. 



In process of time, however, when some of those persons 

 vaccinated even by himself, took small-pox, a third explana- 

 tory principle was adopted, which, however, is not to be found 

 in any of Dr. Jenner's early letters. This we would term 

 the principle of variolous diathesis ; in other words, the 

 occasional imperfection in the security afforded by small-pox 

 itself. To give a strong instance, Louis XV. died of a se- 

 oond attack of small-pox, at the age of sixty-four, having 

 undergone that disease at the age of fourteen years. This 

 event made a great sensation throughout Europe." (Baron's 

 Life, p. 226.) Much importance has always been attached 

 to this circumstance as a source of vaccine failure. We do 

 not disregard it, but we think it very much overrated. Its 

 validity proceeds upon the assumption that small-pox and 

 cow-pox are essentially the same diseases, a position which 



