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be made as there are jars used for collecting samples of the air from different parts of the same room or building. The air to be examined is forced into the jars by means of a pair of bellows, and, by a suitable arrangement of tubing connected with the bellows, it can be pumped into them from any part of the room. In the case of small occupied spaces, such as prisoncells, when it is not desirable to disturb the ventilation by opening the door, the contained air can in this way be pumped into the jars through any opening, such as the inspection hole in the door,-care being taken that the tubing and its connection with the bellows are perfectly air-tight, and that the bellows-valve acts efficiently.

Instead of bellows, a bellows-pump may be used, but in either case the nozzle should be long enough to reach the bottom of the jar. Dr. Angus Smith prefers using the bellows-pump, exhausting the air in the jar, and thus ensuring that its place shall be taken by the air to be examined. Pettenkofer and Dr. Parkes, on the other hand, pump the air into the jar; but either method answers very well, provided that care be taken that the jar is really filled with the air to be examined.

After the jar has been filled, 60 cubic centimetres of lime water are introduced, and the mouth of the jar closed by a tight-fitting india-rubber cap. If tubing has been used to convey the air from a distant part of the room, or from a small inhabited place without entering it, it is necessary that this part of the experiment should be performed rapidly, in order to prevent escape by diffusion, and therefore the measured quantity of lime water should be ready to be poured into the jar whenever the nozzle of the bellows is with

drawn. The jar is then well shaken, so that the lime water is made to wash the contained air thoroughly, and afterwards is left to stand for a period of not less than six or eight hours, and not more than twenty-four. 60 cubic centimetres are introduced, in order that 30 may be taken out for analysis. So much of the lime water adheres to the sides of the jar, that the whole amount introduced cannot be poured out; and hence, if a repetition of the experiment is necessary, another jar must be used.

In making the analysis, 30 cubic centimetres of the lime water which has been employed are poured into a mixing jar, and its causticity determined as above described by the test solution. Then 30 cubic centimetres are taken from the jar, and the causticity also determined. The causticity of the lime water is found to vary from 34 to 41, according to its strength; in other words, from 34 to 41 cubic centimetres of the oxalic acid solution will be required for neutralisation, while the causticity of the lime water in the jar will be lessened in proportion to the amount of carbonic acid in the contained air. The difference between the first and second operations is doubled, to account for the 30 cubic centimetres left in the jar, and the product gives the amount of lime which has combined with the carbonic acid. The amount of the latter, as already observed, is obtained by converting weight into measure according to the atomic weights, and in one sum the factor is found to be 39521. The capacity of the jar being known, and a deduction of 60 cubic centimetres made for the space occupied by the lime water, the amount of carbonic acid becomes a question of simple proportion. Thus, to take an example-Suppose the

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causticity of 30 cubic centimetres of the lime water is 39.5, and the causticity of the lime water in the jar is 33.5; suppose also that the capacity of the jar is 5060 cubic centimetres; then, to find the ratio of carbonic acid per 1000 volumes, that is per 1000 cubic centimetres, the problem is as follows:(506060): 1000:: [(39.5-33.5) × 2 × 39521]: X therefore X = 6 × 79042 948 carbonic acid per 1000

volumes.

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It will thus be seen that the calculations may be simplified by adopting the following rule:-Multiply the difference between the causticity of the lime water, before and after it has been placed in the jar, by 790, and divide this sum by the number of centimetre cubes contained by the jar, minus 60. The result will be the ratio of carbonic acid per 1000 volumes.

But a certain correction must be made for temperature, according as it is above or below the standard of 62° Fahr. As the co-efficient of expansion of air is 0020361 for every degree Fahr., the rule for correction may be stated with sufficient accuracy thus :-For every 5° above 62° add 1 per cent to the amount of carbonic acid calculated as above, and deduct the same percentage for every 5° below 62°.

If the place of observation is much above the sealevel, a correction must also be made for the difference of atmospheric pressure. The standard barometric pressure being 30, the formula for this correction is as follows:

30

(observed height of barometer): capacity of jar : Z. The result expressed by Z is substituted for the actual capacity of the jar in the calculation for carbonic acid.

Amongst various popular tests for the estimation of the carbonic acid in air vitiated by respiration, the following, proposed by Dr. Angus Smith, is worthy of notice, because it does not require skilled manipulation, nor is it hampered with troublesome measurements or calculations. The method is based 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 water, unless the carbonic acid is in excess. This will be better understood by comparing the different columns in the subjoined table, which is taken from Dr. Smith's work on Air and Rain :—

EASIEST PROPOSED HOUSEHOLD METHOD.

TABLE. To be used when the point of observation is "No precipitate." Half an ounce of lime water containing 0195 gramme line.

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Columns 1 and 2 give the ratio of carbonic acid in the quantity of air which will produce no precipitate in half an ounce lime water: column 3 is the same as column 2, with the addition of 14.16 cubic centimetres, or half an ounce, to give the corresponding size of bottle and column 4 gives the size of bottle in ounces avoirdupois. It will thus be seen that different-sized bottles containing half an ounce of lime water will indicate approximately the ratio of carbonic acid in the air contained in them, by giving no precipitate when the bottle is well shaken. Thus, if a bottle of 10 oz. is used, and there is no precipitate, it will indicate that the ratio of carbonic acid does not exceed '06 per cent; or if one of 8 oz. is used, and there is also no precipitate, it will indicate that the ratio does not exceed 08, and so on. Dr. Smith says that "the lime water may be prepared of the same constant strength so closely that we may neglect the difference. Burnt lime is slaked with water and dissolved by shaking. It is then kept in a bottle to stand till it is clear. The bottle or bottles used should be wide-mouthed, so that they can be readily cleaned and dried, and the air to be examined may be made to enter them by inhaling the air contained in them through a glass or caoutchouc tube, care being taken not to breathe into the bottle."

As a practical application of this method, which can be practised by any one, Dr. Smith proposes the following rule" Let us keep our rooms so that the air gives no precipitate when a 10 oz. bottleful is shaken with half an ounce of clear lime water."

2. Organic Impurities.-To obtain an approximate estimate of the organic impurities, the air may be drawn through, or washed, in a very dilute solution of potas

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