There is such an extraordinary amount of misconception concerning bacteria that, it will be time well spent if we attempt to clear up all misunderstanding at the start. Bacteria, often called microbes or germs, are looked upon with considerable awe by most people, who associate them in some vague way with disease. There is no denying that many bacteria are responsible for certain diseases; many more are perfectly harmless and a goodly number are exceedingly useful. To enumerate all the bacterial activities would require a large book but briefly, apart from the disease-causing bacteria, they enter into the manufacture of cheese and butter, of wine and vinegar; they are essential to brewing and tanning; they act as scavengers over the face of the earth, breaking up a mass of decaying animal and vegetable matter into simple chemical substances which can then be used again as food for plants; some of them also can take a gas called nitrogen, from the air, and pass it on to green plants. What are these active little creatures? The question is a natural one. They are merely very minute, one-celled plants. Each one possesses a firm cell-wall, filled with living matter; in an earlier chapter, we described the one called protean animalcule and, although it was composed of but a single cell it had no definite wall. This is one of the essential differences between plants and animals, both of them are made up of one or more, maybe millions of cells, but each plant cell is surrounded by a well-defined wall, animal cells have no such walls. The exact position of the bacteria in the plant world is still open to doubt. Most scientists place them amongst the fungi; for, with very few exceptions they possess no chlorophyll. One or two of them, however, do possess a green colouring matter which, if not chlorophyll is very near to it; on this account other scientists are of the opinion that they are related to the seaweeds. It is a matter, however, that does not concern us very deeply, for our purpose it is sufficient to know that they are plants. When they were discovered, nearly three hundred and twenty-five years ago, they were looked upon as minute animals and it is curious that the belief has survived this long period of time in the popular mind. Long before the activities of bacteria were connected with various phenomena, such as infectious diseases, souring of milk, etc., it was thought that these changes were brought about by chemical action. Like many of the early theories, this one contained a half truth, for a great many of the changes brought about by bacteria are really due to chemical action initiated by the organisms. In other words, the bacteria set free certain substances which actually cause the changes to take place.