unconsciousness, paralysis of the heart, and, at high temperatures, convulsions. Small quantities cause headache, giddiness, and speedy insensibility. A mixture of carbon monoxid and dioxid seems to be more poisonous than the monoxid alone.

Hydrogen Sulphid.—This gas acts as a narcotic poison; I volume per 1000 volumes of air being fatal to dogs. The chronic effects produced by the inhalation of small quantities are depression, digestive disturbances, and anemia, with narcotic or convulsive symptoms in more acute and

severe cases.

Effects of Vitiated Air Generally.-Vitiation by Respiration and Perspiration.-It is impossible to separate the effects produced by these two forms of vitiation from one another, nor is it necessary, as they always coexist. In addition, the emanations from the alimentary tract are also a possible source of impurity. The effects produced by vitiated air of this character may be divided into two classes, those of extreme vitiation acting for a short time, and slighter vitiation extending over a long period of time.

Extreme vitiation for a short time, as produced by great overcrowding, may cause death from deficiency of oxygen and excess of carbon dioxid in the air breathed. It was supposed until recently that a large share of the effects produced by such an atmosphere was traceable to the organic matter present in expired air, but at present there is no convincing evidence that such is the case. On the other hand, recent experiments on animals indicate that deficiency of oxygen, excess of carbon dioxid, increased temperature, with high relative humidity of the air, are the principal factors in producing the effects. The increased temperature and high relative humidity of the air are probably most important factors in producing the effects through their operation upon the heatregulating functions of the body.

Slighter vitiation of the air when continued for some time causes anemia, weakness, and general depression of

the vital forces, nutrition being gravely interfered with. Pulmonary affections appear to be produced, either directly or indirectly, by causing a predisposition to them. Such conditions of the air are met with in confined workshops and in overcrowded schools. In many instances the effects of breathing impure air are complicated with the sedentariness of the occupation and its consequent interference with the normal functions of the body; poverty and insufficient nourishment being frequent concomitants. It is a matter of fact that headache, malaise, want of appetite and of energy, are caused by habitual breathing of impure air.

The prevalence of certain diseases of the lungs in those constantly breathing impure air indicates a causative. relation. The prevalence of phthisis among soldiers. and sailors was considered to be due to impure air by Parkes and many other competent observers. With the improvement of the ventilation of their quarters phthisis has diminished. Notwithstanding the indications, impure air is not directly the cause of phthisis; only indirectly, through the lowered vitality it produces and the facilities it affords for the transmission of the specific micro-organisms. The reduction in the number of cases of phthisis among soldiers and sailors resulting from improvement in the ventilation of their quarters is brought about through the increased vitality induced, and through the greater dilution of the impurities in the air-the tubercle bacilli-and, in consequence, the facilities for their transmission are less favorable. The prevalence of pneumonia also appears to bear some relation to the degree of purity of the respired air. Typhus fever spreads rapidly in crowded dwellings, but with free ventilation it is rendered less dangerous, if not entirely harmless. In all the specific zymotic diseases, but especially in typhus fever, light and oxygen are the greatest enemies to the disease.

Effects of Air Vitiated by Combustion.-Generally speaking, the gaseous products of combustion are diffused

so rapidly that there is no prejudicial effect on health; but in some instances, as in crowded work-rooms, where much gas is burned, and in hot, crowded places, such as theaters, the effects are obvious and not to be disregarded. The effects produced by such air when inhaled constantly are anemia, vital depression, headache, and sometimes gastric derangement when the exposure is only for a few hours.

Effects of Solid Impurities in Air.-The solid impurities in the air may produce irritation of the mucous membrane of the respiratory tract and lead to bronchitis and laryngitis. The effects of the solid impurities are most frequently seen in those following certain occupations, as in coal-mining, cotton-weaving, emery-grinding, polishing of metals, etc.

In the pottery trade the workmen are exposed to dust, and as a result emphysema is quite common, and is known as "potters' asthma." Grinding of steel, especially of the finer tools, is very dangerous unless wetgrinding is employed or proper ventilation is introduced. The makers of pearl buttons suffer from chronic bronchitis. In the textile industries the fine particles of wool, flax, and cotton floating in the air are injurious. The makers of matches are not infrequent sufferers from phosphorus-poisoning.

CHAPTER II.

VENTILATION.

By the term ventilation we understand the continuous introduction of pure air into a room or building, thoroughly mixing it with the contained air, and the simultaneous extraction of a like quantity of impure air. The ventilation of rooms and buildings is necessary in order to prevent the accumulation of the impurities of respiration, perspiration, and combustion.

Diffusion of Gases.-The principles employed in ordinary ventilation depend upon a property common to all gases-that of diffusion. Gases which have no chemical affinity for each other will mingle regardless of their relative weight or density, and form a perfectly uniform mixture. The time required for the diffusion of gases is inversely proportional to the density, and directly proportional to the square root of the absolute temperature.

Amount of Fresh Air Required.—Amount of Fresh Air Respired.-The quantity of air taken into the lungs by an adult person at each ordinary inspiration averages 500 cubic centimeters. Assuming that 17 inspirations are taken each minute, the total amount of air inspired in twenty-four hours is 12,240 liters. About 5 per cent. of the oxygen contained in the inhaled air is absorbed. If 17,000 liters of air are inhaled in twenty-four hours, 850 liters, or 1200 grams, of oxygen are absorbed. These figures are based on results obtained when a fair amount of exercise is taken. A man of average weight excretes 17 liters of carbon dioxid per hour in repose, 25.5 liters with gentle exertion, and 51 liters with hard work. Weight for weight, children give off about twice as much carbon dioxid as adults.

The Standard of Purity.-The air under ordinary conditions contains about 4 parts of carbon dioxid per 10,000 parts, and the standard of purity for the air of dwellings is not to exceed 6 parts in 10,000, thus allowing an excess of 2 parts per 10,000 as "respiratory impurity." The amount of fresh air required in order to maintain the standard of purity in the air of dwellings can be very readily determined, provided we know the velocity with which the air enters, the size of the openings, and the number of persons in the room.

If we take the proportion of carbon dioxid in the air as an index of the character of the ventilation, the method of calculating the amount of fresh air required to maintain the standard of purity is based on the following data: The amount of carbon dioxid exhaled per head per hour, and the ratio per 1000 of respiratory impurity. The calculation is made according to the formula where

e the amount of CO, expired, in liters, per head per hour,

r

d,

the ratio per 1000 of CO,—the permissible limit due to respiratory impurity, and

d the delivery of fresh air per hour, expressed in cubic meters. Example 1: Let e = 17 liters, the average amount for a mixed audience in repose, and r = 0.2 volume per 1000, then 17 ÷ 0.2 = 85 cubic meters or 85,000 liters of fresh air per head per hour.

Example 2: With gentle exertion an adult man excretes 25.5 liters of carbon dioxid per hour. Then the formula

e

25.5 d becomes

0.2

= 127.5 cubic meters or 127,500 liters per head per hour. It has been found that the amount of air required per hour, in liters, is as follows:

For muscular adults a larger amount of fresh air must be supplied than the average amounts given. A larger amount should also be supplied for the sick than for the healthy; an increase of one-fourth of the air-supply being