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occupied by more than one person. Probably less than 5 per cent. of the population ventilate their homes as much as the demands of health require.

The fresh air problem is, therefore, one of personal hygiene, combined with public provision of the means whereby a pure atmosphere may be obtained, namely, the prevention of black smoke and other air pollution nuisances, and the provision of open spaces in towns.

Our individual moral responsibilities with regard to the air we breathe are great. Our first duty is not to befoul the air more than we can help, to keep all about us clean and pure, inside our houses and rooms free from dust, and outside not to allow the accumulation of refuse. Our next duty is to see that a proper supply of fresh air is admitted to our dwellings, and especially our bedrooms and workshops, where two-thirds of our lives are passed, all too frequently in a smelling, frowsy atmosphere. Both employers and employed should remember that there can be no doubt of the economy of giving an ample supply of fresh air, for more and better work will be done, it will be performed more cheerfully, and it will be more healthy and comfortable for all concerned.

CHAPTER VI.

THE CAUSE AND PREVENTION OF INFECTIVE

DISEASES.

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When man learnt how to protect himself from the wild beasts he made the first step in civilisation. To-day man is learning how to defend himself from the microbes-it is a step of equal importance. A day will come when in Berlin, in London, in Paris, man will not die of diphtheria, of typhoid, of scarlet fever, of cholera, of tuberculosis, any more than he dies in these cities to-day from the venom of snakes or the tooth of wolves."-FRANKLAND'S Pasteur, p. 129.

WE

E have already seen that it was a linen draper in Holland who, in 1632, first demonstrated the existence of microbes. These germs or bacteria were shown by primitive microscopes to occur in water, and other fluids, but they were not thought to be anything more than accidental. It was Plenciz of Vienna who, in the middle of the eighteenth century, first conceived the idea that they were almost universal in nature and probably the cause of decomposition, and it was just a century later that, by a series of important discoveries, it was established beyond dispute that some of these micro-organisms were able to bring about fermentation, putrefaction, and even disease.

An Italian named Spallanzani, the famous French chemist Pasteur, and John Tyndall of England, were three of the workers who first proved this to be true. They showed that the atmosphere contained unseen germs of life which, when they gain access to organic infusions, set up fermentation. Quickly following in their footsteps came Davaine, Pollender, and others, who found in the blood of animals suffering from certain diseases particular forms of micro-organism, which when introduced into other animals set up the original diseases again. The wonderful history of the growth of man's knowledge of these invisible forms of life and the enormous part they play in human life and death, may be summarised in three periods by saying that, in the seventeenth century, these living moving organisms were first detected; in the eighteenth they were found to be the cause of fermentation; and in the nineteenth the producers of disease. During the last twenty or thirty years immense strides in our knowledge have been made, and we are now able to say that anthrax, tuberculosis, leprosy, plague, cholera, typhoid fever, diphtheria, tetanus (lock jaw) and a few other less important diseases are certainly due to infection with bacteria, and only a little less certain are we of the cause of scarlet fever, pneumonia, and small pox. As far as we know disease breeds true, and these diseases are not contracted except by infection with the particular germs which cause them.

The Germ Theory of Disease.

Now in the production of infective disease, that is to say, of those diseases which are produced by particular bacteria, there are two factors to consider. First, there is the infecting micro-organism, and

secondly, there is the person infected. We may think of the first as the seed and the second as the soil. And we must now turn to give some thought to each. We are as yet far from a complete knowledge of bacteria, and so it is not possible at present to say many certain things about them. We know, however, that they are among the lowest and simplest forms of life upon the earth, that they are so small that they are quite invisible to the naked eye, and that they exist almost everywhere. We know too, that they are more vegetable than animal, that they are first cousins to the Moulds on the one hand and the Yeasts on the other, and that they possess powers of movement. They increase by dividing, or by producing spores even smaller than themselves. There are three chief forms, namely, the round form (micrococcus), the rod-like form (bacillus), and the thread-like form (spirillum). Above these, as a little higher form of life, are a group of micro-organisms which have branches and something in the nature of seeds. Most bacteria are "harmless" that is to say, they do not cause disease. They are all, however, concerned in some way in the economy of Nature, living either on dead organic matter (saprophytes), or at the expense of some other organism (parasites). As a general rule, and speaking relatively, the saprophytes contribute most to the benefit of man, and the parasites do the reverse. The more we learn about bacteria the more we recognise that they are agents of good or ill no longer to be ignored, and that the evolution of biology, as represented in these unseen hosts, is coming to occupy a new place in the minds of men, as part of the general social evolution which is taking place. For micro-organisms play a great

part in man's life. They occur in our drinking water, in our milk supply, in the air we breathe. They ripen cream, and flavour butter and cheese. They purify sewage, and remove waste products from the land. They break down organic matter, and also make it available for other forms of life. They are the active agents in a dozen industrial fermentations. They produce disease and are the main cause of death on this planet.

There is an interesting question concerning bacteria and disease which will occur to the reader. How do bacteria produce disease? For a long time scientific men believed that these micro-organisms, so to speak, "devoured the tissues" of the human body, attacking it in the same sort of way as larger parasites. But we now know that bacterial action is not of that character. It works in a two-fold manner. In the first place, the bacteria themselves may cause disease, their presence in vast numbers setting up inflammation of the tissues affected. In the second place bacteria act by producing poisonous bodies, or toxins, in the body of the infected person. Between the period of entrance (that is infection) and the appearance of the symptoms, we have an "incubation period," during which time the germs are multiplying and producing sufficient toxin to result in symptoms of disease. For example, a man drinks some typhoid polluted water. For ten or fourteen days the bacilli are making headway in his body without his being aware of it. At the end of that incubation period the toxins begin to produce results and we get the signs of disease. Some microbes spread rapidly through the body, producing toxins wherever they go in the blood stream or the lymph. Others remain at the point of

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