The nexT, point to be considered is the means to be adopted for continually changing the air, bu as to ,cep it in its natural purity. We have already shown that this change must amount to at least 2,000 cubic ft. per head per hour for persons iu health; and sometimes double that amount, or more, for sick persons. 'the general principles of ventilation having been treated in the article, on WARMING AND VENTILATION, we Shall confine ourselves here to a few supplementary observations. In whatever way the fresh air is supplied there are several essential conditions to be observed, of which the following, as stated by Dr. Parkes, deserve special notice: (1) "The entering air must itself be pure. It must be warmed if too cold, and cooled if too warm. (2) Its move ment should be imperceptible, otherwiseAt will cause the sensation of draught, and will chill. The rate at which the movement becomes imperceptible is 1+ ft. per second; or 1.86 m. per hour; 2 and 2+ ft. per second, or 1.4 and 1.7 m. per hour, are imperceptible to some persons; 3 ft. per second, or 2 m. per hour, is perceptible to most; 3+ ft. is perceived by all persons. Any greater speed -than this will give the sensation of draught, especially if the entering air be of a different temperature, or moist. (3) It must be well diffused all through the room, so that in every part a movement snail be going on—in other words, the distribution must be perfect. (4) The outgoing air must be removed so immediately that there shall be no risk of it person breathing again either his own expired air or that of any other person."—Op. cit., p. 103. The action of the wind is a powerful ventilating agent. It it can pass freely through a room with open doors and winc:4s it changes -the air to an extent that can be effected in no other way. The most serious objection to winds as ventilating agents by perflation is the uncertainty of their movement, and 'the difficulty of its regulation. When the velocity reaches 4 m. it is found unpleasant by most people, and is, therefore, either excluded, or only admitted through small openings. when it fails to become properly distributed. For the various ways in which the perfiative power of the wind .has been emplevd in systems of venti lation, we must refer to Ritchie's Treatise on Ventilation, 1862; 'I onilinson's Treatise on Warming and Ventilation; and to the chapter on that subject by Dr. Parkes, who gives a diagram illustrating the mode in which Dr. Arnott has most successfully ventilated the Vield Lane ragged schools.. In the ventilation of ships the wind is always used, the air • being directed between decks and into the hold by means of wind-sails or tubes with cowls turning toward the wind. A description of Dr. Edmond's plan of ventilation, which is now commonly used in emigrant-ships, and is being adopted in the royal navy; • js given in a recent article in The. Lancet, on "The Medical Aspects of the Abyssinian Expedition." "In all cases," says Dr. Parkes, in which the air of a room—as in a basement story or in the hold of a ship, perhaps—is likely to be colder than the external air, and where artificial meals of ventilation cannot be employed, the wind should be taken advantage of its motive agent." In artificial ventilation by a fan or screw it is a question which of the two methods should be employed—the' method of extraction, in which the air is drawn out of a building or room; or the method of propulsion, in which air is driven in, so as to force out the air already in the room. Both plans have advo cates of authority. The advantages of the method of propulsion are its certainty and the ease with whic:t the amount may be altered. The stream of air can be taken front any direction, and can be washed, cooled, or warmed at pleasure. The fan or wheel commonly used in propulsion is essentially that proposed by Desaguliers in 1734. The following is the way in which it is applied to one of the largest rooms in this country— St. George's hall, at Liverpool. The air is taken from the basement; is washed by being drawn through a thin film of water thrown up by a fountain; is passed (in cold weather) into vessels for the purpose of warming it, in which it can be moistened by a. steam-jet; if the difference of the dry end wet bulb be more than five degrees, and is then pro pelled along the channels which distribute it to the ball. In summer it is cooled in the conduits by the evaporation of water. This system is employed with success in various hospitals, asylums, etc., in France and America; and during the Crimean war Mr. Brunel introduced into the hospital of Renkioi a wheel of Desaguliers' at the entrance of each ward of 50 beds, which was worked by hand, and could throw 1000 cubic ft. of air is o the ward every minute.
For information regarding the best means of keeping the air of rooms at the most fitting temperature •.ve must refer to the article WARMING. The degree of artificial warmth that should be given to the air varies according to circumstances. Healthy adults, who are wel: fed and clothed, usually find any temperature from 50° to GO° com fortable; while children and aged persons require a temperature of 65° to 70°. In hos pitals the prsper temperature is usually supposed to be about 60°; but in those diseases in which there is preternatural heat, except possibly in scarlatina, a lower temperature —as from 50° to 45°, or even 40°—is inure expedient. In most febrile cases, in the acute
stage, cold air moving over the body is very efficacious as a cooling agent.
'The next sanitary element to be considered is WATER. The daily quantity of water for healthy and sick persons is the'first point for consideration. Water is required by healthy persons (1) for drinking. A man weighing 10 stone will take on an average from 70 to 90 oz. of water in 24 hours, of which 30 or 43 oz. are taken imperceptibly in the solid food, while the remaining N) or GO oz. are taken in a liquid form. But the amount varies extremely. The usual allowance on board ship for both drinking and coning is 8 pints per adult daily. (2) For cleansing the person, clothes, and habitations. Dr. Parkes, estimates 4 gallons per head daily as the smallest amount; and if perfect cleanliness is to be secured, and baths* are taken, at least 16 gallons per head are required. (3) For sewage an additional 9 gallons must be added. The amount for a water-closet varies with its construction. At Netley hospital, to which Dr. Parkes is physician, Jecaings's closets are used, which require 10 gallons per head daily.
It may be of importance to many of our readers to know that a horse drinks from 8 to 12 gallons daily, and ought to have 3 or 4 more for grooming purposes; a cow or small ox drinks from 6 to 8 gallons; and•tt sheep or from 2 quarts to 1 gallon.
The different sources of water—rain-water, rivers, and springs; the chief impurities in these waters; the methods of detecting them; and the modes of purifying bad water, are so fully described in the article WATER-SUPPLY, that we have scarcely anything to add on these points. The organic matters in different waters used for drinking purposes require, however, a few additional remarks, on account of their extreme importance in a hygienic point of viev'. To the remarks on this subject in pp. 100-101 of vol. ix., we may add that their amount varies from 0.3 per gallon to as much as 12 or even 30 grains per gallon, the purest waters in this respect being thoseifromgranitic; or clay-slate, or chalk districts. The most common organic matter is derived from the vegetable .king dom, and consists of Mullin and ulmin, and of acids derived from humus; all which substances are non-nitrogenous, although the acids combidereadily'with ammonia. This form of organic matter is far less dangerous than that which has an animal origin, and contains nitrogen. This organic matter is usually derived from the contents of cess pools or sewers percolating into springs. Its exact composition is not known. Fecal and Wary matters doubtless contribute to the composition of this matter; and in addi tion, decomposed flesh, as the refuse of butchers' shops and slaughter-houses—substances from tripe-manufactories and gut-spinners, from size, horn, and isinglass maufactories, etc., often contribute to the organic matter of well and spring water. See Parkes op. cit., p. 12. Most of these substances, in clecomporing, produce both nitrous and nitric acid and ammonia; and the nitrites and nitrates thas formed unfortunately not only do not communicate any bad taste or smell to the water, but actually tend in many cases to render it especially palatable. The use of water of this kind is liable to produce diarrhea and choleraic symptoms.
The characters of good drinking-water—as laid down after much discussion by various sanitary congresses—are summed up by Dr. Parkes as follows: "It must be transparent, Colorless, without odor, and tasteless; it should be well aerated (as it then appears to be more eas.ly absorbed), cool, and pleasant to drink; it must have no deposit; vegetables should he readily cooked the total dissolved constituents must be Within a certain amount, which, with some limitation, may be represented by the following numbers: organic matter should not exceed 1.5 grains per gallon; carbonate of lime, 16 grains: sulphate of lime, 3 grains; carbonate and sulphate of magnesia, 3 grains; chloride of sodium, 10 grains; carbonate of soda, 20 grains; sulphate of soda, 6 grains; and iron, 0.5 of a grain." For details regarding the mode of examining water with the view of ascertaining its value for drinking purposes, we must refer to any of the leading works on practical and ana lytical chemistry, and an especial reference may be made to prof. Miller's recent memoir on portable water, and to Dr. Parkes'h section on the examination of water. To the sub stances named in the article on WATER-SUPPLY, as purifying water from organic matter, we may add the following: (1) Permanganate of potash, commonly known as Comly's fluid, which decomposes organic matter and ammonical compounds by rapid oxidation: A physician who has had long experience on board Australian emigrant-ships informs us that ha has often added a small quantity to the water which, when drawn from the casks, was almost undrinkable, with the effect of at once rendering it totally inoffensive. (On this subject, Condy's Air and Water, their Impurities and Purification, may be read with advantage.) (2) Strychnom potatorum, which is used in India to purify water; the nut being rubbed on the inside of the casks. (3) Certain vegetables containing tannin, as tea,$ kino. ,he holder rose (in Barbary). and bitter almonds (in Egypt).