BACTERIA IN AIR AND WATER That bacteria have existed from very early periods is clear from their presence in fossils; the researches of B. Renault and P. van Tieghem have shown that large numbers of bacteria existed in Carboniferous and Devonian times. To-day they are universally present in still ponds and ditches, in running streams and rivers, in the sea, especially in drains, rubbish and manure heaps, in the soil and wherever organic matter is allowed to stand for a short time. Any liquid (blood, urine, milk, beer, etc.) containing or ganic matter, or any solid food-stuff (meat, bread, potatoes, etc.,) allowed to stand exposed to the air soon swarms with bacteria if sufficient moisture is present and the temperature is not ab normally cold. Though they occur all the world over in the air their distribution is by no means equal in all parts. As might be conjectured they are most numerous in the air of towns, but even here the number of living germs is not so great as one would ex pect considering the enormous number of dust particles. Every mote in the air doubtless carries its quantum of bacteria, but the conditions of rapid drying and the exposure to ultra-violet light are responsible for the death of the majority of organisms that are raised by the wind from the earth's surface. The air of the country is relatively pure as com pared with that of the towns and in forest areas the presence of bacteria in the atmosphere is usually hard to demonstrate, the leaves of trees seeming to act as efficient bacterial filters.
The atmosphere of the ocean be comes more and more free from organisms the farther one goes - - - - - - out from the coast. Notwithstanding the large number of bacteria in the atmosphere the breath of individuals is practically germ free. This means that the micro-organisms are retained in the nose, mouth and throat ; the number thus retained must be very great. It is obvious then that other things being equal the danger from infection is greatest among town dwellers and those whose life is spent in ill-ventilated factories and least among sailors and dwellers on the mountain-side. The danger of infection from breathing disease germs is, however, not great in proportion to the numbers of such germs which are respired, since it can be shown that of the disease germs that fall upon an exposed plate of culture media, though many remain viable, only a few of these retain their virulent property. It can, however, be shown that the or ganisms of the common cold can be received on a plate of media held at a distance of seven feet from a person who sneezes openly. The explosive force of coughing and even speaking and whisper ing carries disease germs to some considerable distance.