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from the lungs and skin. In fact, a consider- | of excessive contamination by gases, dust, and able quantity of carbonic acid gas is breathed smoke. Dr. Angus Smith made 339 analyses by workers in certain manufactories, e.g., of the air of mines. Of these, 38 had the soda-water, &c., without injury, although in normal amount of oxygen. The mean of the large quantities, and undiluted, it is rapidly 38 normal specimens was 20.94 oxygen. The fatal; while comparatively speaking small mean of 31 normal specimens in which the quantities in theatres, assembly-rooms, and carbonic acid was estimated was 083. The other places, where human beings are crowded mean of the whole 393 specimens was 20-26 together, have a very depressing effect, be- per cent. oxygen; carbonic acid, 785. The cause there are other impurities in the air. highest oxygen found was 21'04 per cent.; the lowest was 18-27 per cent. The least carbonic acid found was 02 per cent.; the greatest number for carbonic acid was 2.73 per cent. The analyses were divided into three groups-those that showed the air normal, or nearly so; those that were decidedly impure; and those that were exceedingly impure :

Ozone.-This is generally considered to be an allotropic form of oxygen. Three atoms of oxygen are condensed into one molecule, as is represented by the formula Og. It exists in variable quantity in the air, and probably is of some importance. For full details and tests, see Ozone.

The Air of Towns has generally traces of sulphuric, hydrochloric, sulphuretted, and other acids, derived from combustion and different manufactories, besides a considerable quantity of suspended particles of carbon-dust, derived from traffic and emanations from human beings. The air, even of small towns, has more organic matter than country-places (see RAIN-WATER, ANALYSIS OF), which is easily shown by estimating the ammonia and albuminoid ammonia in air. The carbonic acid is of course

increased. The oxygen is decreased, but only to a small amount. For example, the mean of the 22 analyses by Dr. A. Smith of the worst places in Perth gave 20-938; while on the seashore and the heath the mean of several analyses gave 20-999. Odorous particles of all kinds are more common in towns.

The Air of the Country and Open Places varies a little, according to elevation, vegetation, whether populated or not, &c. But the general result is that the oxygen is greater, and the carbonic acid less, than in towns, while the air is free from the acid emanations and carbon so copiously poured out from towns. Of all places, heaths and mountains, as would be expected, possess the best and purest air. Dr. Angus Smith's analysis of mountainous districts in Scotland gave a mean of 20.94 oxygen, while the carbonic acid of the same mountains, taken, however, at a different time, gave 0331. Dr. Pietra-Santa observes that the air of hills or mountains, at the height of 2300 feet, is lighter than common air, contains in equal volume a smaller proportion of oxygen, and is impregnated with a more considerable amount of aqueous vapour; it also contains a good deal of ozone. He considers such a climate peculiarly soothing to persons suffering from certain maladies, such as chest diseases, &c.

The Air of Mines.-The greatest variety of atmosphere occurs in mines, the quality of the air ranging from that of fair purity to that

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of the volume of air passing through the lungs. It will be readily understood, after these figures, the importance of pure air, and how minute differences in composition are really of great importance, since the lungs act, as it were, as immense strainers or filters, and catch the floating particles, while they rapidly absorb deleterious gases. The amount of air required by each person in a room is no less than 2100 feet per hour. When the ventilation does not allow of this constant change, it smells stuffy, the furniture becomes coated with a film of organic matter unless constantly cleaned, and the carbonic acid becomes increased to more than its normal amount.

The effect of constantly breathing impure air necessarily varies as to its state of pollu

tion and other circumstances. When the impurity is moderate, the first effect is headache, lassitude, and a general paleness of the face and skin, owing to a diminution of the red globules of the blood. If the food is insufficient, other evils, such as scrofula and consumption, are very common. For instance, Dr. Guy showed the great mortality from consumption in those trades in which workmen pursued their calling in hot, close, gas-lit rooms, in comparison to those who passed most of their time in the open air.

If the air is vitiated to a large extent, it is quickly fatal, not alone probably from the carbonic acid exhaled, but from the exhalation from the skin and lungs. In the Black Hole of Calcutta, as well as in the case of the Austrian prison after the battle of Austerlitz,

d d e

Fig. 2.

where 260 out of 300 prisoners died rapidly, the symptoms were rather those of bloodpoisoning than anything else. There was great fever, restlessness, and eruptions and boils appeared among the survivors. The effect of impure air is not alone seen on man, but also on animals. Cows, horses, and sheep, if penned up in close stables or outhouses, show a great mortality from phthisis and other diseases.

The effect of dust in air, affecting the workmen employed in various arts, will be considered under DUST.

Analysis of the Air.-For health purposes much information may be obtained on the composition of the air from chemical examination of the rainfall of the different parts of a district, for the rain washes down the impuri

ties in the air to the ground as it descends. See RAIN.

The ordinary analysis of air embraces the estimation of the following constituents: oxygen, nitrogen, carbonic acid, aqueous vapour, and ammonia.

Aqueous Vapour, Determination of. - To determine the water, an aspirator must be used. They are easily made, and not expensive. The above is a diagram of the arrangement generally adopted (fig. 2). a is an aspirator made of galvanised iron or sheet zinc. It holds from 50 to 100 litres. A known volume of air by this means is drawn through the tubes marked b, c, d, e, which may be filled with pumice stone, moistened with strong sulphuric acid; but if the carbonic acid is to be estimated as well, b and c are

filled with moist hydrate of lime (potash used to be employed, but hydrate of lime is to be preferred, as the potash absorbs oxygen as well), and d and e as above. Each of the tubes is accurately weighed previous to connecting them with the apparatus. It is obvious that each of the tubes must be connected by perfectly air-tight joints. They are usually coated with sealing-wax. The gain of weight in d, e gives the water, in b and c, the carbonic acid.

Carbonic Acid.-For the exact determination of the carbonic acid the following method, known as Pettenkofer's, is better. It may be shortly defined as follows: Baryta-water of definite strength is prepared and accurately standardised by a standard solution of oxalic

acid.

A portion of this baryta-water is then made to act upon a definite quantity of air. It will absorb the whole of the carbonic acid in that air. In consequence, the alkalinity of the liquid will be diminished; it will take less of the oxalic acid solution than before, which shows so much less caustic baryta, and from which the carbonic acid absorbed may be easily calculated.

The Actual Analysis.-Two kinds of barytawater may be used, the one containing 7 grm. to the litre, the other three times that strength. 1 c.c. of the stronger = 3 mgrins. of carbonic acid, 1 c.c. of the weaker, 1 mgrm. The barytawater is best kept in the bottle represented in fig. 3.

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The bottle a contains the baryta-water. It has an accurately-fitting double-perforated caoutchouc stopper. The left-hand tube is connected with tube b, containing pumicestone, moistened with potash, while the righthand one is a siphon. When required for use, the stopcock ƒ is opened, and suction applied by a glass tube to F. The siphon is thus filled, and the stopcock closed. If a pipette is required to be filled, its nozzle is inserted at F, the stopcock compressed, and the fluid immediately rises into the pipette. The air entering the bottle as the fluid decreases in a is of course thoroughly deprived of its carbonic acid by the tubes at b.

The first thing to be done is to standardise the baryta solution by a solution of oxalic acid, containing 2 8636 grammes of crystallised oxalic acid to the litre. (See ACID, OXALIC.) Thirty c.c. of baryta solution are run into a small flask, and the oxalic acid run in from a Mohr's burette with float, the vanishing-point of the alkaline reaction being ascertained by delicate turmeric paper. As soon as a drop placed on turmeric paper does not give a brown ring the end is attained.

The actual analysis is performed by filling a bottle of known capacity, with the aid of a pair of bellows, with the air to be analysed, then distributing over its sides 45 c.c. of the

baryta-water, it is left for half an hour. The turbid water is poured into a cylinder, closed securely, and allowed to deposit; then take out 30 c.c., by a pipette, of the clear fluid, run in the solution of oxalic acid, multiply the volume used by 1.5, and deduct the produce from the c.c. of oxalic acid used for 45 c. c. of the fresh baryta-water. A different method to this has been suggested by Dr. Angus Smith-viz., to measure the carbonic anhydride by the turbidities of the barytawater; in fact, a colorimetric test, as it were. Lastly, Mr. Wanklyn has suggested the following method, which is probably the simplest of all:-

A solution of carbonate of soda is first made as follows: 4-47 grammes of gently-ignited carbonate of soda are dissolved in one litre of water, giving a solution of such a strength that one cubic centimetre contains exactly one cubic centimetre of carbonic acid (=1.97 milligrammes of CO2), a large quantity barytawater (strength about 0.1 per cent.) is prepared.

If now 100 cubic centimetres of clear barytawater be treated with one cubic centimetre of carbonate of soda just described, a certain degree of turbidity is produced. If two cubic centimetres of the solution be taken, another degree of turbidity is produced, and so on. If then a bottle capable of holding 2000 cubic centimetres of air, together with 100 cubic centimetres of baryta-water, be filled with

the sample of air to be tested, there will be a certain depth of turbidity produced in shaking up.

Having got the air to expand itself on 100 cubic centimetres of baryta-water, the degree is to be found by comparison with another 100 cubic centimetres of baryta-water in which a like turbidity has been induced by means of the standard solution of carbonate. Every cubic centimetre of soda solution counts for a cubic centimetre of carbonic acid in two litres of air. A consumption of one cubic centimetre will correspond to 0.05 volumes of carbonic acid per cent. Good air should accordingly not take more than one cubic centimetre of the soda solution: air which takes two cubic centimetres being already bad.

In order practically to execute this determination of carbonic acid, the following apparatus is required: Several bottles capable of holding 2-210 cubic centimetres, and well stoppered (failing bottles of exactly the right capacity, Winchester quart bottles will answer); a small pair of bellows; several colourless glass cylinders marked at 100 cubic centimetres capacity. The nesslerising cylinders will answer for this purpose. A graduated pipette or burette to deliver tenths of a cubic centimetre of solution; the standard solution of carbonate of soda and the baryta-water, which may be of moderate strength.

The testing is managed thus: Winchester quart bottles having been marked clean, are rinsed with distilled water, and allowed to drain a little. They are then closed with their stoppers, and are ready for use. The operator having provided himself with two or three of these bottles, and a small pair of bellows, enters the room, the air of which is to be tested. The stopper is then removed from one of the bottles, and some air of the room blown through with the bellows, and then the stopper is replaced, and the bottle carried away to be tested.

The testing is done by pouring into the bottle 100 cubic centimetres of clear baryta-water, shaking up for two or three minutes, and then pouring out into a cylinder of colourless glass, and observing the

depth of turbidity in various lights, and against various backgrounds. The turbidity is to be exactly imitated by means of the standard solution of carbonate of soda. In order to imitate the turbidity produced by a Winchester quart full of good air, only one cubic centimetre of this solution of carbonate of soda is required.

If two cubic centimetres, or more than two are

required, the air is bad, and the ventilation is de

fective. In place of the first cubic centimetre of solution of carbonate of soda, the carbonic acid naturally present in a Winchester quart of good average air may be used, and a little practice and intelligence will suggest the necessary precautions.

For rough everyday work the process of Angus Smith is extremely useful. It depends upon the fact that the amount of carbonic acid in a given quantity of air will not produce a precipitate in a certain given quantity of lime or baryta water, unless the carbonic acid is in excess. The following is one of his tables. Columns 1 and 2 give the rates of carbonic acid in the quantity of air which will produce no precipitate in half an ounce of lime-water. Column 3 is the same as column 2; but 14-16 c.c. (half an ounce) is added to give the corresponding size of bottle, and column 4 gives the size of the bottle in ounces :

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was that of explosion. Bunsen's eudiometers were used, five or six of them at once, and exploded by a large battery and Ruhmkoff's coil; he preferred it to Liebig's method given below, as more expeditious, and perhaps more accurate. The following are the principles of the former method (Miller's Chemistry, vol. ii. p. 53): "By means of the eudiometer various gaseous mixtures may be analysed with great exactness. Many different forms of this instrument are in use. One of the most convenient is Hoffmann's. It consists of a stout siphon tube (fig. 4). Into the sides of the tube, near the sealed end, two platinum wires, a, b, are fixed for the purpose of transmitting an electric spark through the cavity of the tube. The sealed limb is accurately graduated to tenths of a cubic centimetre, or other

Fig. 4.

suitable divisions. Suppose it is desired to ascertain the proportion of oxygen in atmospheric air: The instrument is first filled with mercury, after which a small quantity of air is introduced; the bulk of this air is accurately measured, taking care that the liquid metal stands at the same level in both tubes, which is easily effected by adding mercury, or by drawing off the mercury, if needed, through the caoutchouc tube which is fixed upon the small inlet tube, just above the bend, and which is closed by means of a screw tap c. A quantity of pure hydrogen, about equal in balk to the air, is next introduced, and the bulk of the mixture is again accurately measured. The open extremity of the tube is now closed with a cork, below which a column

of atmospheric air is safely included. This portion of air acts as a spring which gradually checks the explosive force, when the combination is effected by passing a spark across the tube by means of the platinum wires. The mixture is then exploded by the electric spark. The remaining gas now occupies a smaller volume, owing to the condensation of the steam which has been formed. Mercury is, therefore, again poured into the open limb, until it stands at the same level in both tubes, and the volume of the gas is measured a third time. One-third of the reduction of bulk experienced by the gas will represent the entire volume of oxygen which the mixture contained."

Liebig's method is as follows. It is based upon the fact that an alkaline solution of pyrogallic acid absorbs oxygen:

1. A strong measuring tube holding 30 c.c., and divided into or c.c. is filled to with the air intended for analysis. The remaining part of the tube is filled with mercury, and the tube is inverted over that fluid in a tall cylinder widened at the top.

2. The volume of air confined is measured-a quantity of solution of potassa of 14 sp. gr. (1 part of dry hydrate of potassa to two parts of water), amounting to from to

of the volume of the air, is then introduced into the measuring tube by means of a pipette with the point bent upwards (fig. 5), and spread over the entire inner surface of the tube by shaking the latter. When no further diminution of volume takes place, the decrease is read off. The carbonic acid is thus removed,

3. A solution of pyrogallic acid, containing 1 gramme of the acid in 5 or 6 c.c. of water, is introduced into the same measuring tube by means of another pipette similar to the above. The mixed fluid (the pyrogallic acid and solution of potassa) is spread over the inner surface of the tube by shaking the latter, and, when no further diminution of volume is observed, the residuary nitrogen is measured.

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Fig. 5.

4. The solution of pyrogallic acid, mixing with the solution of potassa, of course dilutes it, causing thus an error from the diminution of its tension; but this error is so trifling that it has no appreciable influence upon the results. It may, besides, be readily corrected by introducing into the tube, after the absorption of the oxygen, a small piece of hydrate of potassa, corresponding to the amount of water in the solution of the pyrogallic acid.

5. There is another slight error on account of a portion of the fluid adhering to the inner surface of the tube, so that the volume of the gas is never read off with absolute accuracy.

It is unnecessary to add that the usual corrections for temperature, pressure, &c., must be made.

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