minutes. Now take the tray out of the water, cool the bottles with cold water and ice as quickly as possible, and keep them at this low temperature till used." Place one bottle of Pasteurized milk (No. 3) beside the bottle in the room temperature, and the other (No. 4) in the refrigerator beside bottle No. 2.

1. At the end of three days shake the two bottles kept at the room temperature and open them. Smell or taste of the milk in each. State your observations and conclusions.

2. In a similar manner, test the two bottles that have been kept on ice for a week. State your observations and conclusions.

3. Why are milk, meat, and other foods of the kind put into the refrigerator, especially in summer time? Does this kill the bacteria? How do you know?

4. Why are meats cooked, milk Pasteurized, and fruits boiled before they can be kept for any length of time?

C. The effect of lack of moisture. -Expose for ten minutes two Petri dishes of nutrient agar in a dusty room or corridor (as in A above). Place the two dishes (No. 11 and No. 12 side by side in a warm room (over 90°). Cover dish No. 11 and leave dish No. 12 uncovered.

1. Describe the similarity and the difference in the treatment of dishes 11 and 12.

2. How is the agar mixture affected by removing the
cover?

3. In which dish do colonies of bacteria develop?
4. What do you conclude, therefore, as to the necessity
of moisture for the growth of bacteria?

5. Why is hay dried before it is put into the barn?
Name some foods used by man that are kept for
a long time after being dried.

6. As a conclusion from these experiments (in A, B and C) state what conditions you have found favorable for the growth of bacteria.

7. State also what conditions you have found that hin der the growth of bacteria.

D. The effect of antiseptics. Prepare a pure culture of bacteria in dish No. 13 in the following manner. Heat a dissecting needle on a piece of platinum wire in a hot flame to kill all the germs upon it. When it cools, touch a colony of bacteria in a Petri dish with the needle-point or wire; carefully raise the cover of dish No. 13 and make several scratches in the agar (the date of the experiment or the number of the room may be scratched in this way). In a similar way prepare dish No. 14 and then pour over the surface some peroxid of hydrogen or other antiseptic solution. When the dishes have been treated as described above, put them in a warm, dark place for several days.

1. Describe the preparation of dishes 13 and 14.
2. In which of the two dishes do you find no colonies of
bacteria at the end of several days?

3. Peroxid of hydrogen is employed in treating wounds.
What reason have you for thinking bacteria would
be killed by this treatment?

III. BACTERIA AS THE FRIENDS OF MAN

18. Relation of bacteria to soil fertility. Having discussed somewhat the structure and functions of bacteria, we are now to consider the great importance of these microscopic organisms to human welfare. In the first place, were it not for their never ending activity, all life upon the earth would soon cease to exist. Let us see why this is so. When animals or plants die, their bodies fall upon the ground, and had not these lifeless masses been taken care of, the whole surface of the earth would long since have been covered with a vast number of unburied organisms. All this dead material, however, as we have seen, is food for the countless

bacteria; they cause it to decay, and thus decompose it into simpler chemical compounds that soak into the earth and may then be used in the nutrition of the higher plants. And since plants are constantly taking

from the soil the food materials that they need, this soil would tend to become less and less fertile were it not for the work of the bacteria that cause decomposition. This is the reason why rotting manure adds to the fertility of soil.

Again, it has been proved that certain kinds of bacteria directly increase the amount of nitrogen compounds that are so essential for plant growth. It has long been known that corn and other crops will grow better in soil that has just borne a crop of peas, beans, clover, or other members of the pea family. Within recent years an explanation of this fact has been found. When the roots of these pod bearing plants are examined, small swellings are seen (Fig. 9). These contain multitudes of bacteria that are able to take the free nitrogen from the air, where it exists in such abundance, and store it away in the form of nitrates, which are very important mineral matters needed by all crops. Since these bacteria can be put into soils that do not have them, it may be possible in the near future to restore much of the fertility that has been lost (Fig. 10).

FIG. 9. - Roots of horse bean, with tubercles.

19. Relation of bacteria to the flavors of food. Again, many of the flavors of food are due to the action of bacteria. The flesh of animals, for instance, that have just been killed, is often tough and tasteless. If allowed to stand, however, these meats become tender and acquire their distinctive flavors by the decomposing action of bacteria. A similar action takes place when butter or cheese ripens, and the dairy industry has been perfected to such a degree that bacteria of

FIG. 10. Bacteria from root tubercles.

certain kinds have been

proved to give rise to definite flavors, and these bacteria may be produced in pure cultures for the dairymen.

20. Bacteria in the industries. Without the help of bacteria the preparation of linen, jute, and hemp would be impossible. All these valuable products are plant

fibers which are connected with woody materials so closely that they cannot be separated without first subjecting the stems of flax, hemp, and jute to a process of decay in large tanks of water. Moisture and warmth induce the rapid growth of germs, and the resulting decay loosens the tough fibers so that they may be separated from the useless parts of the plant. The change of alcohol into vinegar is also caused by bacteria. Formerly in the preparation of indigo other forms of bacteria were allimportant, but at the present time indigo is largely made artificially.

IV. BACTERIA AS THE FOES OF MAN

21. Injurious effects of bacteria. Most of the common bacteria are either harmless or distinctly beneficial to mankind (18-20). The experiments we tried with milk (17, B), however, show that this kind of food soon sours unless it is kept in a very cold place. Every housekeeper knows also that meat and many other kinds of food quickly spoil if they are not cooked or otherwise preserved. In a following section we shall consider some of the methods that are used to prevent this decaying action of bacteria.

Unfortunately, too, there are certain germs1 that find favorable conditions for growth in living animal tissue, and by their growth cause certain diseases, some of which are tuberculosis, diphtheria, and typhoid fever. In later sections we shall learn that these disease-producing bacteria are all too common in dust, water, and foods; but we shall likewise see that scientists are fast learning effective methods of preventing the ravages of these disease-producing bacteria, which are called by Dr. Prudden "Man's Invisible Foes." 2

22. Methods of food preservation. We saw in (17, A and C) that bacteria thrive whenever they can get plenty of food and moisture, and whenever the temperature is favorable for their growth. We also learned that, whenever any one of these necessary conditions is wanting, bacteria cease to carry on their functions. If, then, we wish to

1 Disease-producing bacteria are commonly spoken of as germs or microbes.

2 In general it is unwise and unnecessary that boys and girls should be taught much regarding the symptoms and effects of disease; but since so much may be done to prevent these diseases that we have mentioned and others that afflict mankind, it is essential that the young should learn something of the deadly work of some of the germs which are all too common.