HYGIENE OF RESPIRATION 179 



result from even one in a short time. (You will compre- 

 hend this better by referring to "geometrical progression " 

 in your arithmetic.) Dryness and cleanliness prevent their 

 growth ; cold retards their growth and division, hence the 

 utility of ice in preserving meat, milk, fruits, and other 

 foods in warm weather. 



307. Can you now arrange the names yeast, mold, and 

 bacterium in order of size ? Can you 



give the kind of material that each 

 uses for food ? Can you tell how each 

 multiplies, or reproduces its kind? 

 What is a bacillus ? For what is " mi- 

 crobe " another name ? 



308. Use of the Small Organisms. 

 Molds are nature's tools for destroying 



hard and durable tissue like, logs, bone, FIG. 154. Bacilli; the 

 and hides, which otherwise would not fame S, peci ? ma s ni ' 



fiedtoTwo Sizes. 



decay but would fill the earth, leaving 

 no room for living creatures. Ringworm is caused by 

 the growth of one form of mold within the skin. It does 

 not grow in a sound and clean skin. Falling of the 

 hair is often accompanied by the growth of mold in the 

 hair follicles, but it must be preceded by overheating and 

 starving of the cells of the scalp by a tight hat, too much 

 brain work, or other cause (Fig. 152). Yeast prevents 

 the calamity of all plant food accumulating in the form 

 of starch and sugar. Bacteria destroy the dead and excreted 

 matter of vegetables arid animals so that it melts away and 

 is stored in the soil, ready to be utilized as plant food 

 again. Mold, yeast, and bacteria are great blessings, for 

 they are indispensable friends of all other living beings 

 (Fig. I55> 



309. Their Work in the Body. They do not always wait 

 for the dead or dying material to be excreted, or separated 

 from the animal or plant, before attacking and destroying 



