Human Physiology. 351 



may cost a white man his life, though the acclimated negro 

 may breathe the same air with impunity; as one habituated to 

 them may take what to others would be fatal doses of opium, 

 tobacco, or arsenic. A ship with yellow-fever on board, lying 

 at quarantine, has her hatches open when the wind is blowing 

 on shore, and in a few days twenty or thirty cases of yellow- 

 fever break out in a healthy sea-side village. The small-pox 

 contagion fills the air of a whole district, and the air carries 

 the diseasing influence, or vaporous virus, across wide rivers. 

 Scarlet-fever spreads by such virus, germs, or diseasing emana- 

 tions, floating in the atmosphere. There can be little doubt 

 that the exciting and determining cause of cholera is atmos- 

 pheric. It rests like a cloud over certain localities, and affects 

 all who are susceptible to its influence. 



But ordinarily impure air air unaffected by the virus of any 

 special pestilence is an abundant cause of disease. As health 

 consists of vigour and purity, weakness and impurity are the 

 conditions of disease; and foul air fails to invigorate, and at 

 the same time fills the system with its impurities. And when 

 the vital force is lowered by the lack of pure air, and the blood 

 filled with the impurities of foul air, the filth and abominations 

 of decaying vegetation, putrifying animal matter, reeking gut-, 

 ters, sewers, and cesspools, and the foul emanations of the 

 lungs and skins of a crowded population in all stages of filth 

 and disease, the person in such a condition is ready to be 

 affected by any kind of contagious matter that of scarlet-fever, 

 small-pox, yellow-fever, typhus, or cholera. These atmospheric 

 poisons do not produce their effects upon all. The strong 

 resist, the weaker fall before them. In pure bodies the germs 

 of disease find no lodgment they find their nidus or condition 

 of propagation in bodies already diseased. We see that in 

 a great city like London or New York, the cholera or other 

 epidemic may attack one person in a hundred in the cleaner, 

 more airy districts, while it destroys one in ten in the more 



