228 THE POPULAR SCIENCE MONTHLY. 



with the density of the population in the districts considered, 

 until with a population of 65,823 to a square mile, the death- 

 rate for all ages reaches 38'67 per 1,000, and for those under 

 five, 139"52 per 1,000 — that is, while the total death-rate is in- 

 creased a little more than twofold, that under five was nearly- 

 quadrupled. 



Another fruitful cause of death, and one which especially helps 

 to swell the mortality of infancy, is contagion, including in that 

 term all those influences by which disease is communicated from 

 one individual to another, either by direct contact or through the 

 atmosphere. This is the prime cause of the large class of zymotic 

 or germ diseases, which in their various forms carry off nearly 

 one third of all children dying under the age of five years. In 

 general terms it may be stated that, while contagion is the direct 

 cause of these diseases, yet their prevalence and fatality are in in- 

 verse proportion to the general observance of the laws of public 

 sanitation and private hygiene. 



The depressing influences of extreme poverty, filth in all its 

 forms, and the overcrowding of large cities, are great promoters 

 of contagion, resulting in epidemics, plagues, and pestilences ; 

 while strict cleanliness, fresh air, pure water, and hygienic living, 

 tend greatly to restrict its spread and prevent these results. Tem- 

 perature, also, has much to do with the prevalence of zymotic dis- 

 eases, some of which require a certain high degree of average 

 temperature, while others thrive best in cold weather. Extreme 

 degrees of heat and cold (boiling and freezing) destroy the life of 

 most germs, but not of all. Thus, the first sharp frosts of autumn 

 cut short the progress of yellow fever, while diphtheria is some- 

 what more prevalent in winter than in summer. The strict isola- 

 tion of the sick, and careful disinfection of their surroundings, 

 are also essential to the limitation or prevention of contagious 

 diseases. 



The death-rate among infants and young children is especially 

 influenced by the five principal acute contagious or infectious dis- 

 eases — namely, measles, scarlet fever, small-pox, diphtheria, and 

 whooping-cough. According to English life-tables, these five dis- 

 eases were the cause of 18*8 per cent of the entire mortality under 

 five for the ten years from 1860 to 1870, while the average age of 

 all persons dying from whooping-cough was 1'8 years, from measles 

 27 years, from diphtheria and scarlet fever 5'8 years, and from 

 small-pox 11*9 years. 



This latter disease, which was formerly by far the most fatal of 

 the class, has of late years been shorn of its terrors by the benefi- 

 cent discovery of Jenner. Thus, during the sixteenth and seven- 

 teenth centuries, it is estimated that from seven to twelve per 

 cent of all deaths were caused by small-pox ; while since the gen- 



