The explosion of gunpowder, nitroglycerine, dynamite and other compounds of saltpetre also dissipates free nitrogen in the air, though saltpetre itself is a good food for plants.

So while hostile armies of human beings meet on the field of battle and waste nitrogen by blowing one another to pieces with gunpowder in the service of Mammon, the more humane and thrifty armies of bacteria run to the field to take care of the dead and gather up again the valuable nitrogen squandered by men, and the poor farmers spend their meager earnings buying nitrogenous fertilizers to take its place.

Thus our food cycle is complete. Beginning with the nitrates in the soil, the food matter circulates from soil to plant, from plant to animal, from animal to bacterium, and from bacterium through other bacteria back to the soil, and so on again in a never ceasing circulation.

Disease germs play a comparatively unimportant part among the bacteria, and might be stamped out altogether were men only half as alive to their own best interests as are the bacteria themselves. Nevertheless I have shown them in the cycle in the upper left hand corner as one of the agents of the death of animals. The germs in the circle showing flagella are the bacilli of typhoid fever. Those in the photograph at this corner were found in the blood of a yellow-fever patient. * Disease germs secrete, during their growth, poisons which have been separated and studied, some of them being the most virulent of which we have any knowledge. These poisons produce the violent symptoms of the disease.

The bacteria shown in the circle under the typhoid bacilli are the kind which produce pleasant flavors •in butter and are used commercially for the ripening of cream, but they closely resemble many species active with the products of plant life. The other three corner photographs were chosen simply because they represent germs having forms similar to those in our circle, the two lower coming from potatoes in different stages of decomposition.

Even in the first formation of the soil we are dependent upon bacteria. Soil is partly produced by the weathering or crumbling of rocks into powder, which is partly a physical action, but which appears also to be partly due to acid secretions of bacteria which soften the rocks so that the physical agencies work upon them more rapidly. The soil also contains certain sulphates, phosphates and silicates which are deposited by the aid of bacterial action. But the more important element of soil fertility lies in its nitrogen compounds which are supplied partly as already explained by bacteria acting together or with legumes, and partly by the action of bacteria on fertilizers applied to the soil. The manure is not suitable as plant food until its highly complex compounds of nitrogen are reduced by bacteria to its simpler forms. This is done with the aid of oxygen. Science is teaching us how to avoid waste of sewage material as fertilizers, by aiding the bacteria through special chemicals in their work of restoring nitrates and salts of ammonia to the soil. These chemicals reduce to useful salts the free ammonia, nitrogen and nitrites the bacteria produce and thus aid in preventing their dissipation in the air. Hence it is that the problem of the sanitary engineer, so far as it concerns the fertilization of the soil, appears to resolve itself into a proper handling of bacteria.

In the early periods of the world's history and in thinly populated districts, waste matters could be simply spread over the ground with comparatively little danger to the health. Air and bacteria had plenty of room in which to do their work unaided by art. Where it was spread about favorite trees or over the lawn intermittently and in modest quantity the result was entirely satisfactory, so much so indeed, that this method is the one modified only by being carried out on a larger scale, which modern science is leading us back to, as the best for entire cities. But as people began to crowd together in towns and cities, and land surface became more and more limited, refuse began to be buried underground in cesspools, and the dangers coming from putrefaction developed and increased with the growth of the communities until the need of scientific treatment was realized and the sanitary engineer and plumber were born. Unfortunately the leaching cesspool with all its noisome horrors is still the favorite monstrosity of many towns and villages, their most approved location being within friendly reach of the family well. The ground between becoming more and more charged with filth until the "old oaken bucket" draws disease and the doctor as well as sparkling water into the house. Col. Waring gives these two rules as the cardinal principles of modern sewerage. "Organic wastes must be discharged at the sewer outlet in their fresh condition - before putrefaction has set in;" and (2) "They must be reduced to a state of complete oxidation without the intervention of dangerous or offensive decomposition."

Sewage consists of all the water-borne refuse of a community, its most important elements from the sanitarian's standpoint consisting of the wastes of the kitchen and pantry sinks and water closets. These matters once in the sewers become after a very short time substantially the same in appearance, odor and composition, and must receive substantially the same care, precautions and treatment. Both consist mainly of organic food matter, only in one case it has been eaten and digested and in the other case it has not. It averages in proper sewerage systems about one part in a thousand of water, and the object of sewage purification is to detach and render harmless that one part of organic material as economically as possible and leave the effluent water pure. The contents of water closets, may, it is true, have the distinction of containing at times certain specific disease germs, but these germs can only live for a time and may be found in any household waste water. The Liernur system of sewerage is based on separating water closet from other wastes by means of a separate pipe system, but nothing is gained by attempting this. Infected refuse should, if possible, be burned or all pathogenic germs in it destroyed by disinfection in the sick room, where they are produced, before being discharged into other sewage. But the main thing which the science of biology has taught us as sanitary engineers, is that the bacteria of putrefaction are to be everywhere avoided as producing offensive products injurious to the health both of man and animals on land and of fish in the sea, and that the bacteria of quick decomposition and nitrification are to be cultivated as reducing sewage to useful materials absolutely without danger or offense of any kind. It is found that the latter bacteria and the environment of fresh air, in which they flourish, are the enemies of those of putrefaction and disease and that they will destroy them when arrayed against them in sufficient force.

A consideration of the different methods of sewage disposal which are the result of modern sanitary science will form the subject of another lecture, our purpose for the present merely being to show why it is essential to avoid everywhere seats of putrefaction, cesspools and fermenting chambers of every description large or small and hurry the sewage away as rapidly as possible in order that it may be delivered in a fresh state to places where the friendly bacteria may dispose of it to the best advantage.

It may seem, at first thought, a trifling matter to insist upon the abandonment of such small cesspools as pan closet receivers, grease traps, pot traps and small unscoured dead ends of every kind in our sewerage and plumbing systems, but when we remember that in large cities and towns these little cesspools must be multiplied by thousands or hundreds of thousands, we see that the aggregate of putrefaction generated in them all when emptied into the sewers, gives to our enemies, the dangerous class of bacteria; a very considerable equipment for harm, and enables them to resist far longer the friendly battalions of our allies.