132 AN INTRODUCTION TO SCIENCE 



heat), broth, serum of blood, etc. Every housekeeper 

 knows that, in summer, a calves-foot jelly begins to liquify 

 and to give off an unpleasant odour within forty-eight hours. 

 It is cultivating bacteria on an extensive scale. Suppose 

 that it is desired to determine whether a particular kind of 

 germ is present in the air of a London cowshed, a solution 

 of gelatin is spread upon a plate of glass. This is sterilized 

 by heating to a point at which all bacteria are killed. It is 

 then taken out of the jar (in which it is enveloped by 

 sterilized air), exposed for a few minutes in the cowshed, 

 and put back into its jar. In two or three days the plate is 

 covered with colonies of bacteria. A gardener's next 

 operation would be the weeding of his bed. This is im- 

 practicable in microphytology ; but since the several colonies 

 have distinctive forms, it is possible to reverse the process 

 and, so to speak, to "flower" it. A colony, known to be 

 of the required kind, is transferred with a sterilized needle 

 to a tube of sterilized gelatin and grown by itself. After 

 several transfers a pure growth is obtained which may be 

 cultivated, if desired, upon a large scale. 



2. When the pathologist is seeking for the specific germ 

 of a certain disease, he makes pure cultures of every kind of 

 bacteria which he can obtain from the diseased animal or 

 person. If a certain microbe is invariably present, he has 

 good ground for suspecting it of being the cause ; but there 

 is only one way of proving that his suspicion is correct. It 

 must produce the disease when injected into some animal 

 which is capable of taking it. Having ascertained the 

 nature of the germ which causes the disease, it next 

 becomes the duty of the microphytologist to investigate its 

 life-history. There are two points in particular upon which 

 he needs to obtain information : (A ) Can the microbe live 

 out of the animal body, or out of its special medium ; and, 

 if so, is there any situation, such as water or the soil, in 

 which it is commonly to be found ? Does its life as a para- 

 site, that is to say, alternate with a free existence? (B) 



