Numerous diseases arise from the breathing of impure air. Air may be vitiated by respiration or by the products of combustion; it may contain irritating mechanical particles or noxious gases and vapours; it may act as the carrier of specific germs, and this especially in the absence of free ventilation and sunlight, which are potent factors in the removal and destruction of such germs. Respiration vitiates air by depriving it of oxygen, and charging it with carbonic acid, water-vapour, and certain littleunderstood putrescible organic matters arising as exhalations from the animal body and given off mainly- from the lungs and skin. The last-named impurities are admittedly the main injurious agent in air fouled by respiration, and are the e of the close, unpleasant smell in ill-ventilated or overcrowded living-rooms; the amount of carbonic acid present is some sort of index of the proportion of this organic matter in the air. The combustion of fuel, coal-gas. and other illuminants, vitiates air by abstracting oxygen, adding carbonic acid and water-vapour, and - especially where combustion is imperfect - other and more poisonous gases, such as carbon monoxide One of the main advantages of electricity as a domestic illuminant is the absence of this vitiation of the atmosphere. The effect of breathing air polluted in these ways may be acute asphyxia where the pollution is excessive, as in the historical instance of the Black Hole of Calcutta. The common result of persistent lack of proper ventilation is an undefined condition of genera] ill-health characterized by headache, languor, and anaemia, in which the system has a much-lowe resistance against the specific agents of disease - tubercular disease in particular.

The effect of the irritating dusts produced in certain trades, such as steel-grinding, tin-mining, and various textile industries, shows itself in a largely-increased mortality from lung-diseases. Improved air-space and ventilation, with the introduction of appliances for the mitigation of the dust, have done much to diminish this heavy mortality.

Chemical-works and other manufactories often give rise to offensive effluvia, but the objection to these is usually an aesthetic one; it is nevertheless some-times the case that health is impaired in this way, though it is doubtful whether any specific disease can be produced. Where typhoid fever has occurred in connection with sewage-farms, it has probably not been air-borne.

From a domestic point of view, one of the chief air-pollutions to be guarded ■gainst, is that due to the escape of coal-gas. In extreme cases the result may be fatal, - probably from poisoning with carbon monoxide, - and even small leakages of coal-gas, if continued for some time, may lead to headache, and general deterioration of health, and in some cases to sore throats.

The different infectious diseases vary in the degree to which they are liable to be conveyed by air. In some the poison is readily oxidized and destroyed. It is an old saying that, if the window be opened, typhus goes out. Typhus fever is a disease the poison of which is easily destroyed by fresh air. Smallpox, on the other hand, may be carried through air for considerable distances, and experience has shown that the aggregation of small-pox patients in special hospitals in towns is on this account a source of measurable danger to those living in the vicinity The germs of other zymotic diseases, such as scarlet fever, diphtheria, measles, and influenza, may also be carrried by air. In the ease with which fresh air can destroy the poison, these are intermediate between typhus and small-pox. The danger of conveyance of these diseases by air is probably slight, except in the immediate neighbourhood of the sick person.

Consumption is a disease with which impure air has very definite relations. As already mentioned, under conditions of overcrowding and bad ventilation a state of ill-health is established in which liability to the disease is much increased. But this is not all. The tubercle bacillus, which is the direct infecting agent, is coughed up in enormous numbers in the expectoration of consumptive patients. Nuttall calculated that in a moderately-advanced case from one-and-a-half to four billions of bacilli were expectorated daily. Where, as is commonly the case, no pains are taken to collect and destroy the expectoration, it is liable to be scattered about, dried, and disseminated as dust; and known that the Willi retain their vitality in the dried condition for many months. It follows that the dust of dirty and ill-ventilated houses in which consumptive patients have lived may produce the disease when inhaled by a susceptible person, and such houses may become positive centres of infection.

should added that direct sunlight and fresh air are important agents in the disinfection of such premise Direct sunlight was found by Koch to kill tubercle bacilli within a few hours, and bright diffused daylight in a few days. Similar facts are known with regard to other resistant microbes, and they enforce a very salutary lesson in sanitary house-construction.

The air of seuers has always had the reputation of producing disease to a marked extent, and. indeed, the evidence that sewer-gas escaping into dwelling-houses has various injurious effects is so strong as to admit of little dispute. Its precise mode of action is, however, somewhat obscure. The composition of sewer-air varies widely with the perfection of the ventilation and the nature of the sewage. In well-ventilated main sewers the air shows on analysis a moderate excess of carbonic acid, with traces of marsh-gas and sulphuretted hydrogen, and a little ammonia. It is air which can be breathed without injury for considerable periods, and it is well known that the men employed in sewers do not suffer much, if at all, in health as a consequence of their avocation. The air of unventilated sewers, however, may become irre-Mpirable and even explosive, and cases of acute sewer-gas poisoning, characterized by vomiting, purging, headache, and prostration, sometimes occur, and occasionally end fatally. The air in house-drains also, which is that immediately escaping into houses, is often dangerous, as the ventilation of small drains is always difficult, and too frequently the means adopted are utterly inadequate.

What is the deleterious factor in sewer-air to which these symptoms, and the more insidious effects produced by its escape into houses, are to be attributed ? The discovery that many diseases are caused by bacteria made it natural to assign the ill effects of sewer-air to its action in conveying the specific germs of disease. But, in spite of the plausibility of this suggestion, actual experiment is in opposition to it Miquel, Carnelly and Haldane, and, more recently, Laws, have shown that the number of bacteria in sewer-air is actually less than in fresh air at the same time, and very considerably less than in the air of houses. Moreover, it was shown by Laws that the micro-organisms present in the air of London sewers were related to, and presumably deri from, those of the fresh air, and bore no sort of relation to those which are present in the sewage. There exists no proof that, under ordinary circumstances, sewage gives up bacteria to the air in contact with it; and there is reason to believe that it does not do so under any circumstances, except where an extreme amount of splashing is produced. Where sewers are blocked and ventilation inadequate, there the number of bacteria in the sewer-air has been found to be exceptionally low. It would hence appear that some other must be found for the deleterious effect- of sewer-air. It can hardly be supposed that, when largely diluted after escaping into a house, it can owe its evil properties to its gross chemical composition. Something more subtle than this must be at work, and it has been suggested that the organie matter present in it may contain an obscure chemical poison, possibly of an alkaloidal nature. There is, however, no proof of this.

Although no definite explanation can be offered as to the way in which sewer-air produces disease, the observed facts show beyond reasonable doubt that its escape into houses constitutes a serious source of danger to those dwelling therein. The diseases which in particular have been ascribed to tin condition are diarrhoea, typhoid fever, diphtheria and other forms of sore throat, pneumonia, erysipelas, puerperal fever, etc. Most, if not all, of these are specific diseases due to definite and well-known microbes, and, as has just been stated, there is no evidence of the carriage of such microbes by sewer-air. It cannot, however, be doubted that the constant inhalation of even diluted sewer-air, - which in concentrated form has been known to cause acute mephitic poisoning, - must so lower the resisting powers as seriously to predispose to these specific diseases, and to render them much more severe when they occur. It must also be borne in mind that where the sewers are faulty, and sewer-air can escape into houses, there the subsoil and ground-air are liable to pollution, with possible contamination of the water-supply. Where one sanitary defect exists others are often present at the same time, so that the causation of disease may in such cases be complex.

Where water is stored in a house, special care must be taken to avoid contamination of the cistern by sewage-emanations. For this reason it essential that cisterns serving water-closets should he separate from that from which drinking-water is taken, and that waste-pipes from cisterns should have no connection whatever with soil-pipes or other possible sources of effluvia. Food is commonly stored in the basements of houses, where the liability to contamination by exhalations from drains and ground-air is greater than elsewhere. Organic matters such as food readily absorb emanations of this sort, and have been shown to undergo putrefactive changes much more rapidly when exposed to contamination by sewer-air. This may constitute a very real source of clanger. Milk in particular should never be stored where there is the slightest possibility of its contamination. It forms so excellent a breeding-ground for bacteria that their access to it is peculiarly harmful, and this especially as it is so largely used for the food of young children. The methods devised to obviate such risks will be discussed in full detail in ensuing chapters.