CHAP. xvin. VACCINATION A DELUSION. 251 



system, and with it the rapid growth of London, from 

 a population of two millions in 1844 to one of four mil- 

 lions in 1884. This rapid growth of population was at 

 first accompanied with overcrowding, and, as no ade- 

 quate measures of sanitation were then provided, the 

 conditions were prepared for that increase of zymotic 

 disease which constitutes so remarkable a feature of the 

 London death-rates between 1848 and 1866. But at 

 the latter date commenced a considerable decline both in 

 the total mortality and in that from all the zymotic dis- 

 eases, except measles and small-pox, but more especially 

 in fevers and diphtheria, and this decrease is equally well 

 explained by the completion, in 1865, of that gigantic 

 work, the main drainage of London. The last marked 

 decline in small-pox, in fevers, and to a less marked de- 

 gree in whooping-cough, is coincident with a recognition 

 of the fact that hospitals are themselves often centres of 

 contagion, and the establishment of floating hospitals for 

 London cases of small-pox. Perhaps even more benefi- 

 cial was the modern system of excluding sewer-gas from 

 houses. 



We thus see that the increase or decrease of the chief 

 zymotic diseases in London during the period of regis- 

 tration is clearly connected with adverse or favorable 

 hygienic conditions of a definite kind. During the 

 greater part of this period small-pox and measles alone 

 showed no marked increase or decrease, indicating that 

 the special measures affecting them had not been put in 

 practice, till ten years back the adoption of an effective 

 system of isolation in the case of small-pox has been fol- 

 lowed by such marked results wherever it has been 

 adopted as to show that this is the one method yet tried 



