568 HYGIENE &c. By respiration and combustion air becomes vitiated, the oxygen diminishing and the carbonic acid and organic and suspended matter increasing. Within certain limits the amount of carbonic acid is in itself immaterial to health, but it is important as a measure of the amount of organic matter, which is really the dangerous impurity. Air vitiated by respiration is also much more dangerous than when the carbonic acid is partly the result of combustion. It is now pretty generally admitted that air cannot be con sidered as really good and fit for respiration in which the respiratory impurity reckoned as carbonic acid much ex ceeds two parts in 10,000 by volume. On the other hand, if air can be kept down to this point, the condition may be looked upon as satisfactory. The amount of impurity given off by living beings varies of course with size, weight, age, sex, and work, but it may be allowed that under ordi nary circumstances it amounts to about six cubic feet of carbonic acid per head in ten hours during repose. This requires an hourly supply per head of 3000 cubic feet of fresh air for its dilution, arid this amount should be largely increased during work or in sickness (see HOSPITAL). The diseases which have been shown to arise from the effects of vitiated air are widely prevalent, including such as con sumption and other forms of scrofulous disorders, bronchitis and pneumonia, sore throat, &c. Crowded and ill-venti lated places also tend to increase the virulence and the rapidity of spread of the various communicable diseases. 5. Cleansing, including the removal of slops, excreta, &c., forms one of the most important and also most difficult of questions. The main principle is that all should be immediately and effectually removed from the house and its neighbourhood, and that there should be no possibility of reflux of foul air from drain or cesspool. The system of water-carriage is certainly the cleanest and most convenient, especially among large communities, but other systems find advocates. In villages and isolated houses the earth system and other dry methods have many advantages. The question of the disposal of sewage is a very large one, into which it is impossible to enter here. Hitherto all or almost all the material has been wasted by being poured into rivers or the sea, the streams being thus polluted and the shores rendered offensive. The object to be aimed at is to utilize a product of undoubted fertilizing influence, without endangering the health of the community. The diseases to be apprehended from imperfect methods of sewage removal are enteric fever, cholera, diphtheria, sore throat, and an aggravation of most other diseases, especially those of an eruptive character. Ashpits ought to be especially attended to, their neglect being attended with much danger. 6. Water-supply, although included under the head of food and beverages, merits special consideration, so important is its relation to health, both directly as a drink and indi rectly with reference to its many other uses. It is required for drinking, cooking, the cleansing of person, clothes, and dwelling, and the flushing of closets, sewers, and drains. The hygienic requirements are that water should be good in quality and sufficient in quantity. Good water should be clear, colourless, quite free from suspended matter, of a good lustre, and should have a pleasant sparkling taste, the latter qualities being due to the carbonic acid and atmo spheric air dissolved in it. In its chemical composition it ought to be as free as possible from organic matter. The evidence in favour of communication of disease by means of drinking water is now very extensive, and we may cite diarrhoea, dysentery, ague, enteric fever, and cholera as among the diseases which may be conveyed through this channel. Numerous parasites also find their way into the human body by this means. Hard water is objectionable for cooking and washing, nor can it be recommended for drinking, although some insist upon a certain amount of hardness being essential. In the case of water being impure, boiling, distilling, or filtering may be resorted to. The two former are the most efficacious, but the last has advantages of convenience if properly carried out. Char coal filters, if properly cleansed, or renewed sufficiently often, are useful, but it is better to have a material that purifies without risking any deterioration of the water itself. Such filters as the spongy iron and the carferal effect this. All niters, however, require the medium to be cleansed or renewed periodically. In a house the chief dangers are from dirty cisterns and from pipes being connected with drains and closets. All supply should be on the constant system, and no pipe supplying a closet should be resorted to for drinking purposes. All overflow pipes should deliver in the open air. The quantity of water required per head may be stated at a minimum of 12 to 16 gallons per diem where there is no general system of drainage, and about 25 gallons with drainage. In towns more than this is neces sary, and from 30 to 50 gallons are desirable. In sickness generally double the amount is necessary that is required in health. The source of the water ought to be pure, springs, deep wells, and upland surface water being the best. Shallow wells and rivers to which sewage gains access are most to be avoided. 7. For food and beverages the reader is referred to the article DIETETICS. 8. Work and Exercise. The kinds of work performed by man are of course very various, but they may be reduced more or less to a uniform standard, which is usually reckoned as so many tons (or pounds) raised through one foot, or, tersely, as foot-tons or ton-feet. A fair day s work is generally taken at 300 foot-tons, a laborious day s work at 450, and the maximum to be expected, except under very special conditions, at 600. For this work a certain time should be allowed, as the strain increases (almost in a geometrical ratio) with the velocity. Usually speaking 50 foot-tons an hour is a fair amount, and this ratio is equal to a walk of three miles for an average man. The amount for mental work has not been accurately calculated, but it may be safely assumed that a man of sedentary occupation ought to take exercise of a physical kind varying from 50 to 100 foot-tons per diem. In all cases his food ought to be proportioned to his work, for it is now recognized that man is a machine, whose work depends upon the energy derived from the food he eats. 9. Clothing and Personal Cleanliness. Clothing should fulfil the functions of preserving warmth in cold weather, providing covering without being too oppressive in hot weather, keeping out wet in wet weather, and yet allowing sufficient transpiration for health. At the same time it ought to admit of frequent change and cleansing. Dr Parkes has pointed out that it is probably due in some measure to cleaner habits with reference to clothing that the diminution of typhus fever should have been so marked in recent times. Personal cleanliness is also a matter of great importance, a daily general bath being advisable for every one. For animals as well as human beings it has been shown that cleanliness is conducive to improved appetite and general health. Filth is one of the prime factors in the production and propagation of most of the devastating plagues known to mankind. 10. Prevention of Disease.- This is a large question, on which we can only briefly touch. Much depends upon our knowledge of aetiology or the remote causes of disease. The best rule for preventing disease is to follow out care fully the principles of general hygiene, laid down with reference to pure air, pure water, proper food, cleanliness, &c. Some diseases may be more specially provided against, such as paroxysmal fevers by the use of quinine, and small
pox by vaccination, but for the great majority of diseases
