6 MANUAL OF BACTERIOLOGY. 



articles of food, which was formerly believed by the super- 

 stitious to be blood, deposited by some miraculous agency, we 

 know to be due to the growth of a common organism (bacillus 

 prodigiosus) . The emission of light by decaying substances 

 when seen in the dark is caused by bacteria as well as other 

 organisms. 



It seemed that in some cases in which death has been at- 

 tributed to the suction of air into the veins, because air ap- 

 peared to be present inside the heart, the air was in reality a gas 

 formed by certain bacilli that invaded the body just before or 

 just after death (bacillus aerogenes capsulatus). 



Woodhead tells us that some savages are in the habit of 

 smearing the soil of certain localities upon their arrows for an 

 arrowpoison, which is intelligible in the light of the fact that 

 soil often contains the bacilli of tetanus (lockjaw). 



The comparatively small number of species of bacteria that 

 cause disease are the ones that interest us most, and are those 

 which have been most carefully studied. Since the bacteria in 

 common with other fungi are compelled to derive their food 

 from organic matter, it is easy to understand that they should 

 frequently exist as parasites upon living animals and plants. 

 Pear-blight and some other diseases of plants are caused by 

 bacteria. Bees and other insects, frogs, birds, cattle and a 

 great number of animals besides men suffer from diseases pro- 

 duced by bacteria. 



When bacteria are placed upon slips of glass they may be 

 studied with the microscope while alive. Some of them when 

 living are motionless; others wriggle vigorously. Some dart 

 about like minnows in a stream, or they make their way slowly 

 across the field of the microscope like a boat that is being 

 sculled from the stern. By proper methods it- can be shown 

 that the movements are effected through one or more fine, 

 hair -like processes called flagella. 



Often it is expedient to study bacteria after drying them on 



