1 6 MANUAL OF BACTERIOLOGY. 



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 arrow-poison, 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. The 

 necessity that falls upon bacteria, in common with other 

 fungi, to derive their food from organic matter makes it 

 easy to understand that they should frequently exist as para- 

 sites upon living animals and plants. Pear-blight and some 

 other diseases of plants are caused by bacteria. We find 

 that frogs, birds, cattle and a great number of animals 

 besides men suffer from diseases produced 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-processes, called flagella. 



Often it is expedient to study bacteria after drying them 

 on slips of glass, when they may be made more conspicuous 

 by giving them an artificial color (staining). Some of the 

 substances of which they are composed readily absorb cer- 

 tain dyes. For this purpose the aniline dyes are used, and 

 their employment has been one of the important factors in 

 making progress in bacteriology possible. 



With the microscope alone it is not usually practicable to 

 distinguish accurately between various kinds of bacteria. 



