Microorganisms in Relation to Man 549 



the air for hours, and finally become attached to dust particles 

 with which they are distributed. They invade nose, mouth, 

 throat, and perhaps reach the lungs. Experiments have proved 

 that tubercle germs expelled by a consumptive are sometimes 

 thrown two yards distant and remain in the air several hours. 

 Disease germs such as these when attached to dust particles 

 may live and be virulent for months. These facts suggest the 

 need of knowing what invisible living things may be mingled 

 and distributed with dust, and what may be their significance 

 in relation to man. 



II. MOLDS 



273. How to discover what dust carries. It is compara- 

 tively simple to devise means for detecting some of the living 

 things that are distributed broadcast with the dust in the air. 

 The following experiments are to be considered primarily as 

 suggestive ; others may be devised by the student either to 

 supplement them or to be used as substitutes. 



Exercise : Growing molds. Boil until thoroughly cooked several 

 medium-sized potatoes. Sterilize several glass tumblers and saucers 

 by dry heat in an oven with a temperature of approximately 150 C. 

 for an hour, or in water brought to boil and kept boiling for several 

 minutes. When the potatoes are cool, cut into two equal sections 

 with a knife sterilized in a flame or in boiling water. Place one sec- 

 tion on a saucer and cover immediately with a sterilized glass. Expose 

 the other sections to the air for ten to fifteen minutes, and then seal 

 under the sterilized glasses. Label the former No. 1, and the latter 

 No. 2. Take some dust from the sweepings of a room and put upon 

 the cut surface of another section, and label this No. 3. Expose a 

 fourth section to the air when a class is moving, or in a crowded as- 

 sembly and label this No. 4. See that glasses fit closely on saucers 

 and prevent evaporation of moisture from their contents. 



Place the " cultures," as these are called, in a dark part of the 

 room. The temperature should be kept at about 75 F. to 80 F., to 

 insure quick results. Perhaps another part of the building may offer 

 better advantages than the room in which experiments are made. 

 When the cultures have been properly put where they are to remain, 

 a small quantity of lukewarm water that has been boiled may be 

 poured into each saucer. 



