298 THE WONDERFUL CENTURY. CHAP. xvm. 



fire ; yet one of these accidents does not confer immunity 

 against its happening a second time. The taking it for 

 granted that second attacks of small-pox, or of any other 

 zymotic disease, are of that degree of rarity as to prove 

 some immunity or protection, indicates the incapacity of 

 the medical mind for dealing with what is a purely statis- 

 tical and mathematical question. 



Quite in accordance with this influence of small-pox 

 in rendering the patient somewhat more liable to catch 

 the disease during any future epidemic is the body of 

 evidence adduced by Professor Yogt, showing that vac- 

 cination, especially when repeated once or several times, 

 renders the persons so vaccinated more liable to take the 

 disease, and thus actually increases the virulertce of 

 epidemics. This has been suspected by some anti- 

 vaccinators; but it is, I believe, now for the first time 

 supported by a considerable body of statistics. 



The other important feature in Professor Yogt's 

 memoir is the strong support he gives to the view that 

 small-pox mortality is really other things being ap- 

 proximately equal a function of density of population. 

 All the evidence I have adduced goes to show this, espe- 

 cially the enormously high small-pox death-rate in 

 crowded cities in approximate proportion to the amount 

 of crowding. Professor Vogt adds some remarkable 

 statistics illustrating this point, especially a table in 

 which the 627 registration districts of England and 

 Wales are grouped according to their density of popu- 

 lation, from one district having only sixty-four per- 

 sons to a square mile to six which have 20,698 per square 

 mile; another column showing in how many of the years 

 during the period 1859-82 there were any small-pox 



