ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 253 



the microorganisms originally present must have settled down. But iu 

 nature not only is such a calm unknown, but processes are continually 

 taking place which launch fresh organisms into the atmosphere. More- 

 over, there is good reason to suppose that several disease microbes or 

 their spores are still lighter than those which have been subject to 

 similar experiments. The influenza microbe is extremely light. Its 

 length has been given at 5o"ooo aD( l its breadth at 250W0 °f au inch. 

 Disease microorganisms have in the laboratory passed from room to 

 room through the air, and accidentally infected animals inoculated with 

 other kinds. Light dust falls at so slow a rate through the viscous air 

 that even iu a room the downward motion is scarcely perceptible; yet 

 in a few hours all the grosser particles are deposited if drafts, move- 

 ment, and shaking of the room are prevented. Most pathogenic 

 microbes are carried down with this dust or sink of their own gravity, 

 and soon subside, but in ordinary conditions there is too much dis- 

 turbance to permit effective purification by subsidence. The light dust 

 of the volcano Krakatoa, which was visible as a haze, took a year to 

 fall even out of the rare upper strata, and many disease microbes are 

 equally small, and fall still more slowly through the dense strata near 

 the ground. Particles of smoke may perhaps be compared with the 

 spores of bacteria, and tobacco smoke not only floats long in the air of 

 a room, but passes through passages and through chinks into rooms 

 above and below. 



Among animal diseases of an intensely infectious character and dis- 

 astrous to agriculture, cattle plague, pleuropneumonia, and foot-and- 

 mouth diseases are perhaps foremost. Two, at least, of these are com- 

 municated not only by infected articles, but by transmission through 

 air for a short distance of particles derived from an actual or previous 

 case. These diseases, or some of them, have formerly been widely held 

 to come from some unusual epidemic constitution of the air, but they 

 are now thoroughly proved to be preventable by the admission of 

 plenty of external air and rigid precautions against contact or prox- 

 imity of infected articles. They are frequently spread by attendants 

 passing from one herd to another without complete systematic disin- 

 fection; frequently also by imperfectly disinfected sheds. No animal 

 plague has been proved to be capable of passing effectually through a 

 long stretch of atmosphere, and the free atmosphere in all cases tends 

 to diffuse and destroy the poison. There is reason to regard certain 

 low alluvial lands and swamps as the original breeding grounds of the 

 saprophytic microbes which cause some of the worst animal plagues, 

 for these plagues have followed immediately the subsidence of floods 

 and the drying up of marshes. Since the neighborhood of these places 

 is not exempt, the organisms concerned must be capable of transport 

 in a potent state for a short distance by moist air. The filthy condi- 

 tion and foul, unventilated air in which cattle are kept have also been 

 shown to be the cause of their gravest maladies. Tuberculosis in 



