them in fact as particles of degraded bioplasm. This theory is not in favor, or rather is not fashionable, at the present time. One objection to its validity is constituted by the fact that particles of living animal matter die very rapidly after they have escaped from the body, whereas many contagious germs preserve their vitality and capacity for evil for a very long time.
Another theory which was promulgated some twenty years ago was to the effect that the contagious particles are of the nature of those low vegetable organisms which are termed fungi. This view gains support from the manner in which these bodies increase in number when planted in a suitable soil, and the power which they possess of decomposing many organic substances. The fact, already referred to, that several diseases of the skin and hair in men and animals are undoubtedly due to fungi, also tends to recommend this theory. Recent experiments, however, have shown that these organisms, capable as some of them are of growth and development on the surface of the body, do not possess the power of growth and reproduction within the body, and it is therefore unlikely that they should be the causes of disease in which the system is charged with poisonous materials.
A third theory is one which is extremely popular at the present day, advocated as it is by investigators of the highest repute. It is almost needless to say that I refer to the view which credits certain minute organisms, termed bacteria, with the power of causing the infectious diseases—that is, with being in themselves the poisonous agents. So firm is the hold that this view has obtained that "disease-germs" and "bacteria" are used as though they were synonymous terms. It is, moreover, probable that more experiments have been made with reference to bacteria than on any other subject whatever.
The term "bacterium" signifies a rod, and many of these organisms are minute, rod-shaped bodies. They or their germs are very widely diffused throughout nature; they swarm in the air and in water, especially if containing organic matter, and are likewise found in great numbers within the bodies of men and animals. Any one who possesses a microscope with a magnifying power of five hundred diameters can readily examine a very common form of bacterium. It is only necessary to take a glass of ordinary water from a spring or river, and to leave it in a room exposed for some days to the air. A thin coating, looking like a deposit of fine dust, is formed on the surface of the water; this dust consists of myriads of bacteria, which are readily seen when a drop of the water is examined. The bacteria are found to be in several stages of transformation: some are in long, jointed rods, others represent one or more detached portions of these rods, and others appear as extremely minute, rounded particles. The rods are capable of movement, and they are seen to wriggle through the fluid like small eels or snakes. The minute, rounded particles are the spores, which eventually become rod-shaped bodies.
