CRYPTOGAMS 307 



potato in (2), where the liquid was dropped? Are they more, or less 

 abundant than in (1)? Since cotton wool is entirely impervious to the 

 smallest microorganisms known, would you judge from this experiment 

 that bacteria can get into any place unless carried there by the air, or by 

 some other means ? 



EXPERIMENT 94. CAN BACTERIA BE CARRIED BY PURE AIR ? On a 

 warm (and preferably cloudy) day, put a slice of potato on a plate, and 

 leave uncovered in an unused room or closet, free from dust, and kept 

 carefully closed. Put another slice arranged in exactly the same way 

 in an open window on a dusty street, or in a room that is used and daily 

 swept and dusted. Do bacteria appear in the first plate ? In the second ? 

 Is air free from dust a good conveyor of bacteria ? 



EXPERIMENT 95. WHAT CONDITIONS ARE FAVORABLE TO BACTERIAL 

 GROWTH ? Strain some of your culture liquid into half a dozen small 

 bottles of the same size, filling each about half full. Put (1) in a dark, 

 cool place on ice, if the weather is warm ; (2) in a dark, warm place ; 

 (3) in a warm, well-lighted place ; into (4) put a drop of carbolic acid, form- 

 alin, corrosive sublimate, or boracic acid, and keep in a dark, warm place. 

 Keep (5) in boiling water for half an hour or more, and then place beside 

 (2). Keep (6) in a freezing mixture of salt and ice for several hours, then 

 place with (2) and (5). Examine all at intervals of from 12 to 24 hours. 

 In which bottles is the presence of bacteria indicated by cloudiness of the 

 contained liquid, or the formation of a surface film? In which do they 

 appear first ? In which most abundantly ? In which last, or not at all ? 

 What is the effect of light and darkness on their growth ? Of heat and 

 cold? Of disinfectants? Name the circumstances that tend to hinder 

 their growth, in the order of their efficacy. 



348. Microscopic study of bacteria. Put a drop of 

 hay infusion on a slide and examine with the highest power 

 of the microscope. You will see a multitude of very small 

 glistening bodies including different kinds of bacteria, a 

 majority of which are probably the hay bacillus, B. sub- 

 tilis, shown in Figs. 443, 444. Notice that some forms 

 move about freely, while others are non-motile. Which 

 kind are the more numerous ? The motion may be either me- 

 chanical, resembling that of the small dust particles we see 

 dancing about in the sunshine, or apparently voluntary, 

 and caused by the vibration of little whiplike cilia. Can 

 you distinguish the two kinds? Try to make out clearly 



