The work that may be done with bacteria is limitless but, to advance very far, we shall need facilities for obtaining what are known as “pure cultures.” Let us make the term clear. Suppose we take milk, water, butter, anything in fact upon which bacteria will grow and examine them carefully. If we have the requisite knowledge and recognise what we see, we shall find not one kind of bacterium but a number of different bacteria. Now by certain manipulations, which need not be described here, all the different kinds of bacteria may be sorted out, so that we have colonies consisting of one kind of bacterium only and such a colony is known as a “pure culture.” In practice, bacteriologists do not use the rough and ready methods that we used in dealing with the Hay Bacillus. They prepare pure cultures and cultivate the bacteria on various substances, differing markedly from those on which they originally lived. For example, a jelly-like substance, mainly composed of beef broth and gelatine is one of the favourite substances on which to grow bacteria, milk is also used in some cases and also slices of potato. All this may seem to have little to do with the microscope, but indeed the bacteriologist relies as much on the behaviour of his pure cultures, growing on gelatine, etc., as on their appearance under the microscope. Some bacteria will not grow on the surface of the gelatine but only in the body of the substance, where air cannot reach them; others cause the gelatine to become liquid; others give off a characteristic smell or impart a well-marked colour to their food material; a few even cause the gelatine to become luminous. These easily seen characters are quite as typical of the various bacteria which bring them about as are the microscopic characters; in fact it sometimes happens that only by a combination of the two is it possible to be certain of what one has obtained. CHAPTER XII MEDICAL WORK WITH THE MICROSCOPE To the medical man, a microscope is all important. In the first place it is absolutely necessary for him to have an accurate knowledge of the human body; he must be able to recognise healthy blood, he must know the varied cells composing muscle and bone, etc. Then again the medical aspect of bacteriology is all important; we have devoted a separate chapter to bacteria and there we warned our readers not to look upon all bacteria as harmful, nevertheless, the harmful ones are all important in medical work. Of recent years a knowledge of the structure and habits of certain insects has become an important branch of medicine for many of these creatures carry diseases from one patient to another. This branch of medicine is more important in the tropics than in this country, for most of the really harmful insects live in warm countries.