252 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



Spore formation takes place at temperatures between 16° and 45° 0., 

 and these are in general the extreme limits. Bacilli which do not 

 form spores — for instance, those of typhoid fever, glanders, and fowl 

 cholera — are easily killed outside the body by a number of natural and 

 artificial agencies. Among these agencies the most efficacious are 

 drying, exposure to dry air and oxygen, high temperature, sunlight, 

 the presence of other species of microbes, the poisons evolved by 

 themselves or by other species, cold weather, exhaustion of their 

 appropriate nutriment, and various inimical substances which inhibit 

 growth or actually kill. In the very fatal diseases of cattle known as 

 anthrax, and when transferred to mankind, as wool sorter's disease, the 

 bacilli which infect the blood of the dead animal are killed by mere 

 drying, without exposure to air; but if the blood be for some little time 

 exposed to the air, spores are formed which may remain upon the pas- 

 ture, or upon wool, or hides, or elsewhere, and infect fresh cattle or 

 human beings at some distant date. The putrefactive process in the 

 carcass also kills the bacilli, but will not kill the spores if these are 

 allowed to be formed. 



Anthrax is known to be in many cases communicated through the 

 air from one animal to another or to man,Wid among wool sorters, 

 butchers, and others enters the body through a wound, or by the lungs, 

 or by the alimentary canal. 



Spore formation is generally favored by a copious supply of oxygen. 

 It is a process by which the degeneration and destruction which takes 

 place in a colony of nonspore-bearing bacilli is prevented, and by which 

 the seeds are set adrift, to be planted and grow again into bacilli in 

 more favorable surroundings. 



The process of growth from a spore into a bacillus has been experi- 

 mentally observed in favorable conditions to be completed in periods 

 varying from half an hour to two hours. The bacillus introduced into 

 an appropriate medium multiplies by fission at an enormous rate, so 

 that, for instance, 248 microbes of the pathogenic species Staphylococcus 

 pyogenes aureus in a cubic centimeter increased to 20,000,000 in twenty- 

 four hours, and 20,000 bacilli of fowl cholera multiplied in the blood of 

 a rabbit to about 1,200,000,000 in twenty hours. 



Microbes vary greatly in size not only between classes and species, 

 but between individuals, according to the medium and circumstances 

 of growth. Ordinary dimensions lie between about 0.5 and 5 micro- 

 millimeters in length and 0.1 to 0.5 in breadth. The spores are in 

 many cases much smaller. Clearly, an organic living dust of less than 

 one thousandth of a millimeter in diameter is capable of existing in 

 great numbers on very small areas, even on small, almost invisible, dust, 

 and of being wafted long distances by gentle aerial movements with- 

 out sinking. In perfectly still air inclosed in a box in the laboratory 

 Tyndall found that all visible dust sank within three days, and nutri- 

 ent media then exposed were unaffected by bacterial growths, so that 



