Fig. 29 shows the cycle of life and the part played therein by the different kinds of bacteria.

At the bottom of the circle is mother earth. It contains some of the principal ingredients which form the food of plants, and above all compounds of nitrogen called nitrates. Carbon dioxid and water are also required by plants and these are obtained partly from air. Other compounds in the soil which plants use are salts of potassium, phosphorus and some other elements, but these are here omitted for simplicity as of less importance in this connection.

The roots of the plants shown on the earth at the left take up the nitrates from the soil, and their leaves absorb the gases from the air, and with the energy furnished by the sun's rays build these simple compounds into more complex ones shown in the circle above the plants as the second step. They are sugar, starches and fats, forming the complex food required by the animal kingdom, indicated by the figures of a man and a horse at the top of the cycle. Some parts of these foods are at once decomposed by the animals and given back to the air from their lungs and pores in the form of carbonic acid and water, and we have shown these in our picture by dotted lines returning at once again to the plants at the bottom. But this quick return does not occur with the nitrogenous foods. These require further treatment and must continue around the circle of changes, forever repeated in nature's great laboratory. Animals build these nitrogenous foods into new albumens, reducing part directly into urea, which is excreted. But a plant can neither feed directly either upon the nitrogen compounds stored in the bodies of the animals nor upon those which are thrown off by them during life. It cannot feed upon the flesh, fat, bones nor excreta which constitute the products of animal life as shown in our picture at the right of the animals. They must be reduced to simpler forms, and the third step in the food cycle is taken by the decomposition bacteria. The animals have died, as we see, and the bacteria are already engaged in their work as scavengers. Our drawing shows, at each step, the particular form of bacterium actually found at work at that stage, and copied from life, as revealed by the microscope, and in the four outer corners we have reproduced photographs of the four principal types of bacteria most interesting to us in this connection as sanitary engineers. You see, therefore, thrown upon the screen, individual members of this valuable community as they actually appeared while engaged in their great work of food preparation and sanitation. Only they are here magnified by the electric rays to several hundred thousand times their real dimensions. They are our great little co-workers, the faithful engineer's assistants, the patient, industrious, eager, non-complaining plumber's helpers. We may take a lesson from these helpers in their unceasing, ever-cheerful toil.

*I am indebted to Prof. Conn for the idea of representing Nature's food cycle graphically, but the special design is my own.

"Ever at toil," they bring "to lovliness All ancient wrath and wreck."

These organisms exist in the air, soil and water, always on the alert for any organic substance requiring their presence, and no sooner is it provided, instantly, as if by magic they appear and begin to break it up for plant food! A portion of these cleavage products takes the form of carbonic acid gas and water, which are dissipated in the air and return at once, as shown by the dotted lines, to the plant. But the other portion, containing nitrogen, is broken up into ammonia, N H3, or into compounds called nitrites. But these are too simple for plant food. The chemical destruction by this particular detachment of helpers has too thorough, and another gang of workmen must be employed to complete the process begun by the first before the food is quite ready for the plants.

Fig. 29. The Cycle of Life.

Fig. 29. The Cycle of Life.

Bacteria.

We come now to the species of microorganisms which, for us sanitarians, is perhaps the most interesting and important of all - the bacteria of nitrification. They exist everywhere, especially in fertile soil, and they are so very valuable and commendable that I have induced a few of them to sit for their photographs for this occasion. They appear at the right of the circle and are nearly spherical in shape. These bacteria, by further oxidizing the nitrites and ammonia, prepare the food finally for the plants, and form the last link in the chain that binds the animal to the vegetable kingdom, for the nitrates and nitric acid which they form are left in the soil where they can be seized upon by the roots of the plants and then begin again their journey around the food cycle.

Thus the nitrifying bacteria perform the great final function of waste disposal and sewage purification by both removing the matters in decomposition without offense or danger and providing the means for new life at the same time. They differ therefore entirely from the bacteria of putrefaction, which create products not only dangerous and offensive but incapable of supporting new life.

In our food cycle some of the processes of decomposition throw off a portion of the nitrogenous matter into the air in the form of free nitrogen, and this is out of the reach of plants, because plants do not possess the power alone of extracting free nitrogen from the air. Hence unless the plants can obtain some external aid in reaching this nitrogen it will be gradually exhausted by dissipation in the atmosphere, and life upon the earth become gradually extinct. But here again we have found, within the last few years, that we can again count upon our busy friends, the bacteria, only in this case there are two distinct species operating in concert in our service. They act in at least two different ways in reclaiming from the atmosphere more or less of this dissipated nitrogen; first by cooperating with each other, one kind turning the ammonia into nitrous acid, and the other kind the nitrous acid produced by the first into nitric acid, and second, by cooperating with some of the higher plants belonging to the great family of legumes, including the pea, bean and clover plants. The bacteria which thus combine with these legumes in collecting the nitrogen are shown in the lower right hand corner of our circle, and the legumes themselves are seen growing in the earth below them. The bacteria form in colonies or nodules on the roots of the legumes as shown, and here in this humble position, perform unseen the marvelous and all-important function we have described.