CRYPTOGAMS 313 



different kinds of bacteria ; hence the importance, from a 

 sanitary point of view, of a thorough knowledge of their life 

 history. 



355. Disinfection is sterilization on a large scale, and 

 the same principles apply to both. Heat is the safest 

 disinfectant for objects that will bear it, if continued long 

 enough and repeated often enough at a sufficiently high 

 temperature. Freezing will destroy some kinds of germs 

 and check or retard the development of nearly all, but 

 is not to be relied on as a permanent germicide, since 

 even among flowering plants there are many kinds, not 

 only of seeds, but of perennial vegetative forms that are 

 capable of enduring an arctic temperature of many degrees 

 below freezing for long continued periods. 



Chemical disinfectants act usually as microbe poisons, 

 and are unsuitable as sterilizers for food, though valuable 

 in the purification of houses, clothing, and utensils es- 

 pecially the instruments employed in surgical operations. 



The prevention of the growth of bacteria, especially in 

 wounds and surgical incisions, whether by means of chem- 

 ical or physical agencies, is known as antisepsis. 



Practical Questions 



1. Why should a person recovering from an ague continue for some 

 time after to take quinine every third or every seventh day? (350, 354.) 



2. Name some of the principal diseases produced by bacteria. 



3. What is the principle to be acted on in the avoidance of such dis- 

 eases? (Exps. 94, 95.) 



4. Are the same means equally effective for prevention and for cure ? 

 (354, 355; Exps. 93-95.) 



5. Why is "fresh air" beneficial in a sick room? (352; Exp. 94.) 



6. Does it act as a disinfectant, or as a mere diluent of the infected 

 air of the room ? (352.) 



7. Why ought preserved fruits and vegetables to be scalding hot when 

 put into the can ? (355.) 



8. Why is it necessary to exclude the air from them? (Exps. 93, 

 94.) 



9. Reconcile question 8 with question 5. 



