466 NATURE STUDY AND LIFE 



and we will place upon each of the glasses a bit of compressed yeast 

 the size of a pin head. Ask each to divide his yeast, first in halves, 

 then one-half in halves again, and so on until he has a particle that 

 he can just see. Let the pupils now plant these just visible particles 

 in their vials. They may then cork them and observe the growth 

 that takes place from day to day. If a piece of rubber dam is 

 stretched over the top of one of the bottles containing fruit juice 

 and tied tightly, the gases produced by the growth of the yeast will 

 puff up the rubber and thus help to show that something is going on 

 inside. The liquid will soon become turbid, full of bubbles, and at 

 last a mass of white substance will settle to the bottom. This is 

 composed of yeast plants, but may be many thousand times the 

 amount with which we started. The liquid will have lost its sweet 

 taste and will smell and taste of alcohol, or possibly of vinegar. The 

 particle on the starch paste will gradually overgrow the whole drop, 

 changing it to a whitish mass of yeast plants. 



a b 



Fig. 194. Forms of Bacteria 



a, grippe ; /', bubonic plague ; c, diphtlieria ; d, tuberculosis ; e, typhoid fever ; 



/, spiral types. 



Bacteria. — The smaller a living particle is, the more 

 powerful may it become. This is because the smaller a 

 cell is, the more surface it has in proportion to its bulk 

 for the absorption of food. Bacteria are the smallest liv- 

 ing things we know and, in many ways, the most power- 

 ful. Different forms of bacteria are shown in Fig. 194. 

 Some are spherical and so minute that it would take 

 125,000 of them placed side by side to measure an inch. 

 Others are rod shaped, but so short that 1500 placed end 

 to end would make a line only across the head of a pin. 

 Many of the elongated forms are bent into commas or 



