APPENDIX E. cxlix 



individuals fully exposed to the contagion of measles, scarlet 

 fever, and small-pox, who yet fail to contract the disease ? 

 Facts of this kind are familiar to all medical men. Sir 

 Thomas Watson has referred to the case of ' an old woman 

 who for -years had been in the habit of going from village to 

 village as a nurse ; and of nursing a great number of persons 

 labouring under small-pox, which she had never had, and 

 against which she (naturally enough) believed herself proof:' 

 but, he adds, ' at length she was taken ill, and died of small- 

 pox in the eighty-fourth year of her age/ Again, he says : 

 ' In 1845, a lady with whom I am acquainted went through 

 an attack of measles, that disease being prevalent in the 

 village where she was then residing. She had never had 

 the measles previously ; yet she had long before personally 

 tended eleven of her twelve children when ill of the same 

 complaint 1 / 



Such facts are quite inexplicable in accordance with the 

 vital or 'germ- theory' of causation of these diseases, but 

 they become much more easy to understand in accordance 

 with the views which have just been enunciated. They are, 

 further, thoroughly harmonious with the results of experi- 

 ments made by myself and others with reference to the causes 

 of fermentation. These results have led me to reject, as 

 too narrow and exclusive, the ' vital theory ' of Pasteur, and 

 to adopt the broader physico-chemical doctrines of Liebig, 

 which appear to be harmonious with all the facts. In en- 

 deavouring to explain the initiation of fermentation in any 

 particular fluid which has been boiled, we have also to 

 consider the influence of intrinsic tendencies in the fluid, 

 in combination with the exciting or external agencies to 

 which it is subjected. In some cases the intrinsic ten- 

 dencies may of themselves be potent enough to initiate 

 the process; whilst in other instances the mere contact- 

 1 ' Principles and Practice of Physic,' vol. ii. p. 782. 



