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CHAPTER V.

THE GENERAL PRINCIPLES OF VENTILATION.

(32) The Mechanism by which the Air of Rooms is made Impure. To understand the mechanism of air contamination of dwellings, it is, in the first place, essential to have some knowledge of the laws of gaseous diffusion, and also of the changes which take place in respiration.

Diffusion of Gases.

Carbon dioxide is 1 times as heavy as air. If a jar of it be placed in a still atmosphere, and the stopper of the jar removed, the gas will remain for a short period in the jar, but ultimately an analysis of the air in the jar and that of the room will be identical. Similarly, hydrogen is very much lighter than air, and a flask of the gas held mouth downwards will for a short period retain the gas, but not for long. It soon diffuses through the atmo

sphere.

Diffusion obeys a well-ascertained law, which is, that the diffusibility of two gases varies in the inverse ratio of the square roots of their densities. If we wish to know how rapidly a gas will diffuse in air, we only want to know its specific gravity in relation to air. For instance, the specific gravity of hydrogen is 06925, that of air 1000; but taking hydrogen as 1, the specific gravity of air 1444; that is, air is 1444 times as heavy as hydrogen, and the ratio of the square roots of their densities is 、 +44 to 1, or as 38 is to 1. If two equal jars, the one containing hydrogen, and the other air, be placed in a room, and the

whole of the hydrogen diffuse in 1 hour, the air of the jar will diffuse in 3-8 hours; so, again, as the inverse square roots of the densities of hydrogen and carbon dioxide are as 3.8 is to 8, which is nearly in the relation of 1 to 5, it will require 5 times the time for an equal quantity of the heavier gas to diffuse-that is, if it takes 1 hour for a litre of hydrogen to diffuse, it will take 5 hours for a litre of carbon dioxide to diffuse; or, if put into an apparatus called a diffusiometer, for every 38 c.c. of hydrogen which diffuses, 8 c.c. CO2 will take its place.

This diffusion of gases takes place through porous substances, such as plaster, walls, and very thin animal membranes; but if the porous material has very fine pores, and is of considerable thickness, the rates of diffusion are different, for under such circumstances the pores act as so many capillary tubes, and the law governing the transit of gases through capillary tubes is different to the above.

In the respiration of animals, the air on entering the lungs comes in contact with the capillary blood-vessels of the lungs, the walls of which are very thin, and diffusion takes place. If the blood were simply an ordinary organic liquid, the gases of the air would be absorbed according to their solubility at the particular temperature and atmospheric pressure at the time of inspiration; but the blood has peculiar properties. The venous blood contains in the red corpuscles a substance called hæmoglobin, which forms a well-defined, although loosely bound compound with oxygen. The venous blood, on arriving at the lungs with its myriads of hæmoglobin-containing corpuscles, greedily absorbs oxygen, oxyhæmoglobin being formed. The blood now becomes bright (arterial) red, and carries the oxygen to all the tissues. The living tissues soon rob the oxyhæmoglobin of its oxygen, reducing it to hæmoglobin. Upon this happening, the blood becomes of a dark (venous) colour, nor does it recover its bright hue until the lungs are again entered. The red corpuscles may be likened to legions of little boats, whose office it is to float with the circulatory tides to the lungs, for the purpose of each taking in a tiny cargo of oxygen. After discharging it, they return for more. While the corpuscles are thus busy in collecting and dispensing oxygen, there is at the same time a transpiration of carbon dioxide, for there is so far a true combustion in the tissues-that is, there is develop

ment of heat; oxygen unites with carbon, and carbon dioxide is formed. This absorption of oxygen and expiration of carbon dioxide is quite independent of atmospheric pressure. On the other hand, whether less or more nitrogen is absorbed is intimately connected with the height of the barometer. If the barometer sinks, nitrogen escapes from the body until equilibrium is restored; if the barometer rises, then a certain absorption of nitrogen appears to take place.

There is also a considerable expiration of aqueous vapour.

(33) Composition of Expired Air.

Expired air then is deficient in oxygen, is rich in carbon dioxide and in moisture. It also contains very often organic volatile bodies, such as ether and gases derived from food or drink consumed, and it is not unfrequently somewhat ammoniacal. A persons' breath smells of ether after taking alcohol, the expired air will blacken silver after eating eggs, and eructations of marsh gas are known to occur in some forms of indigestion. The healthiest person's breath also contains other forms of organic matter, the exact nature of which is obscure.1 The most recent researches show that the breath is singularly free from micro-organisms, the lungs acting as a very efficient filter.

The content of carbon dioxide in expired air varies according to body weight, to age, and other circumstances, such as activity or rest. It may be taken generally as 5 per cent. of the volume of the expired air in the case of an average-sized man doing ordinary work. Speck, in 41 experiments made on his own breath under different conditions, found a maximum of 5:43 per cent. and a minimum of 3 33 per cent., while the oxygen also varied from 15.01 to 17.21 per cent.

Pettenkofer and Voit made some very elaborate researches on the amount expired of oxygen, moisture, and carbon dioxide, of two healthy persons, during the 24 hours, under very different conditions of food and labour.

1 R. Wurtz (Comptes Rend., cvi. 213-214) drew the air expired from the lungs through a 1 per cent, solution of oxalic acid. He found ammonia to be the chief product; but besides this, isolated a base the hydrochloride of which had a peculiar odour, and formed crystallizable soluble salts with the chlorides of gold and platinum.

The following are mean numbers of some of Voit and Pettenkofer's researches :

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The facts given with regard to the respiratory contamination of may be summarized as follows:

An adult man at rest consumes in 24 hours about 816 grms. of oxygen, part of which is retained in the body; part is expired in the form of CO2. The oxygen retained amounts to from 120 to 200 grms., or 439 litres. The carbon dioxide will be about 870 grms., or 439 litres. He will also separate nearly a litre of water. At work these quantities will be increased. In ordinary not very laborious occupations, he will consume 861 grms. of oxygen, and will exhale 996 grms. (504 litres) of CO2 and 1,464 grms. of water.

A man will therefore, under ordinary conditions, in the 24 hours, spoil about 3,000 litres (1059 cubic feet) of air, and if undergoing any exertion 3,700 litres (1307 cubic feet) or more. It is not far from the truth, and easy to remember that adults spoil every hour about 5 cubic feet of air (140 litres)—that is, render 5 cubic feet of air absolutely irrespirable.

(34) The Contamination of Air by Animals.

It is of practical importance in a public health point of view. to also study changes in the air produced by the respiration of cattle, horses, and domestic animals, the more especially because there are many dwellings in which the atmosphere is common to beast and man; for instance, in the case of stablemen with rooms over a stable, in which the separation is merely porous lath and plaster and the floor boarding.

There is one peculiarity with regard to the contamination of

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