Seeing the great anxiety for pure air, and anxious also to understand the subject, I have attempted to answer some of the questions relating to it; and, first, what is the composition of pure air? I came to the conclusion, from numerous experiments, that the purest air contains 20.99 of oxygen, that air ought not to contain less than 20.98, and that very probably 21 is still better. I am inclined to believe that we are capable of feeling these minute differences, if not by our ordinary senses, at least by our chemical senses, so to speak, that is, by the continued influence of chemical action in ourselves. At the same time, these differences are probably caused by the presence of intruding gases and vapours, carbonic acid especially being known to us.

The power of carbonic acid must be great when, as was found, 2 per cent. in air puts candles out, not, we must remember, from the fact that there is too much oxygen removed, but because carbonic acid is present; this we know by burning candles in air having still less oxygen than that in which the candles spoken of went out, but being free from carbonic acid.

The amount of this gas presented to us in nature is small, from 300 to 350 in a million, or 0.03 to 0035 per cent. Some curious apparent anomalies occur when we examine the matter carefully, and places contain only 0.03 which we might expect to contain more. In districts quite separated from the influence of vegetation over all the north of Scotland, the sea shores and the hills, it is between 0.0320 and 0.0340 almost without exception. The numbers about London, that is in the parks, are even smaller. The amount on the Alps is greater, if we can implicitly trust the analyses of Saussure, and I believe we can, old though they be. Perhaps we are to conclude from this that these small differences are of no consequence if the air is otherwise pure. I do not feel quite sure of that, but am very certain that small differences are of great moment in ordinary circumstances, probably because they are the accompaniments of other impurities.

When we come to towns such as Manchester, we find the carbonic acid rising up to 403 in a million. In foggy days it is up to 734 at times, and at the middens 774. At ordinary times, in workshops, it rises up to 3,000, whilst in mines it goes even to 20,000 in a million.

In towns we have sufficient evidence that it is not the carbonic acid purely that does harm, even if it does any, in open streets. But the increase in close and unclean places is to be remarked. On enquiring in some of these places there was generally found sulphuretted hydrogen, in others unpleasant odours; but wherever it was unpleasant, or much shut in, there an increase of carbonic acid was found.

The next object was to find, by actual experiment, if an increase of carbonic acid was actually hurtful. It was found by a number of experiments that, in close air the pulse falls as regularly as the carbonic acid increased, and, at the same time, the inspirations

increased. In some cases the pulse rose for a little, but in every case there was a disturbance of the circulation and inspiration. It was desired to know if this disturbance was owing to the presence of carbonic acid or only to the presence of the organic emanations. The experiments were tried with pure carbonic acid, and similar results obtained. The diminution of the number of beats of the pulse was less remarkable than the great feebleness which occurred in every case tried. Experiments made with large amounts of carbonic acid do not show its influence clearly or fairly.

We cannot doubt that carbonic acid is an agent of great influence, and that it is to be avoided. I may add, that these experiments went much against my previous reasonings, and they lead to a demand for greater purity of air for reasons clearer and more substantial than the vague sensations that are usually our guides.

Like many other questions, that of the air becomes more complicated the more it is examined; and it is well for us to be exceedingly careful at every step.

I believe it is important to ascertain the amount of carbonic acid, but it is no less important for us to ascertain also, in many cases, the amount of oxygen.

Besides this, we cannot forget that the gases of putrefaction are also to be found mixed with organic exhalations. Some of these gases and vapours are readily decomposed, and the amount ascertained by permanganates. On examining the subject of organic matter, it seemed to me clear that we are annoyed by its presence exactly in proportion to the warmth of the atmosphere. One of the chief reasons, I believe, is simply this-the matter is volatile, and being so, it fills the air exactly in proportion to the warmth applied. In cool weather, the exhalations of the person are readily condensed, whereas, the presence of one individual for an hour in a room of a size frequently used, is sufficient in warm weather to cause unpleasant closeness. The same room would be sufficient for many in cold weather.

Two main evils, then, annoy us-carbonic acid, and exhalations from the person caused by warmth. If we can bear 1,000 of carbonic acid in 1,000,000 of air at the temperature of 50 degrees F., we may be quite unable to bear it at the temperature of 70 degrees. More than that, I believe, that when the proportion of carbonic acid is extremely low, we may still have such warmth that the exhalations will be volatile and the air extremely oppressive. Why do these exhalations annoy us? They probably act like anesthetic substances preventing oxidation and, therefore, diminishing first the clearness of the mind, and then the strength of the body. We require to remove them from us when it is well known that no carbonic acid, in any injurious quantity is present. This is one reason why we desire the air to be changed more frequently in warm weather; it is not only because there is a desire for a lower temperature, there is a desire for air perfectly freed from exhalations. It is for a similar reason that men can live in narrow huts made of ice without any

attempt at cleanliness. The limit of endurance is probably measured, in such cases, by the carbonic acid only, and not by the organic matter, which ceases at that temperature to be volatile. A low temperature diminishes the power of air to support the combustion of a candle. I do not know if a man can endure more of the pure carbonic at low temperatures or at high, and the subject is one which requires experiment rather than speculation, but he can endure inferior ventilation.

We may readily make tables of the amount of air required for one, two, or any number of individuals in any given space, so as to keep the air at any required state of purity. In future we shall be able, no doubt, to be more exact, and to vary these tables with the temperature. It is extremely probable, however, that temperature is a subject on which men never can agree, from their differences of constitutions, ages, and habits.

This effect of temperature on organic matter makes it clear why we instinctively diminish the amount of ventilation in cold weather without fear of hurt. It is true that we obtain more carbonic acid in the air, and so far lose purity, but we, at the same time, know that the organic matter is not proportionately increased if we keep only a moderate temperature. The proper proportion of ventilation is more difficult to attain when warming is to be effected at the same time. Great errors are made on both sides.

We find two extremes. Most persons warm without regarding purity of air. Many persons of the advanced class ventilate without due regard to warmth.

The proportion will require investigation, because we must consider many conditions, especially moisture, concerning which we know too little. There is another point which I think of value to remember. The temperature of 55 degrees, Fahrenheit, has been called temperate on our thermometer, and on inquiry into the putrefaction of organic substances, I always found 54 degrees, or nearly so, to be the temperature where putrefaction ceased to be very perceptible. Practically I could collect no gases under that temperature. For the present, then, I am inclined to believe that we must have less of the organic exhalations at and under that point. Below that we must ventilate chiefly to remove carbonic acid, and of that we can measure exactly the amount. Above that temperature we have another enemy besides carbonic acid, and ventilation must increase. More air must be removed than the presence of carbonic acid alone demands, the proportion rising with the temperature. To ascertain this proportion for the average man, is a problem which may be solved without difficulty, but not without a good deal of work.

I do not doubt that the senses of most persons may be educated so as to be a sufficient measure of ventilation, but it is also true that many are not so educated, and the peculiar condition in which others work renders the education extremely difficult. If a person works in a factory which has always some odour of oil in the atmosphere, it is difficult to measure the degree of impurity caused by the organic

exhalations of human beings, and still more so the increase of carbonic acid, which, although affecting the functions of life, is not, in small amounts, directly perceptible to the senses. Still more difficult is it for a miner, who is removed from any chance of comparing the air he breathes below with that which he breathes under the open sky. It is, therefore, of great importance that a measure should be had.

I do not attempt here to speak of all the impurities of air. The evil arising from gases which come from putrefaction, such as sulphuretted hydrogen and carbonic acid, may be measured sufficiently by measuring carbonic acid only; that is, if we remove the one we remove the other. The sulphurous acid of the air is also a great evil—an evil affecting those large towns which burn much coal to an extent which renders life in them less cheerful because vegetation around is blighted, and the aspect of everything is gloomy. The existence of oxydizable matter in the air may be measured by permanganate of potash, as I long ago showed, but the process of proving it is not one which can be made so simple and easy to an inexperienced person.

Carbonic acid is the gas well known as a great agent of mischief; it is found on all ordinary occasions where air is rendered impure. It is found where we breathe; it is found when matters decay; and it accompanies, perhaps in all cases, sulphuretted hydrogen, whilst it also comes with the sulphur of the coals, although not always in the same proportion. It is certainly no measure of the peculiar exhalations of the skin which render so very much ventilation necessary in warm weather, nor is it the measure of those exhalations which bring plague and specific diseases. It is, however, a fair measure of those gases which come from decomposition, and, generally speaking, want of ventilation.

On considering these things, two questions, above others, seem to demand an answer. The first is this. How shall we know when the air is unwholesome, or measure its impurity? The second is, how shall that impurity be prevented? The second question admits of as many answers as there are varieties of houses and of temperaments, but perhaps some day an answer will be found which will, in a general way, be applicable to all.

I shall speak now only of the first. For the purpose of showing if the ventilation corresponds to the number of persons present, it is enough that the carbonic acid should be estimated.

If we fill a 6-ounce bottle with the air to be examined, and add to it one half-ounce of a clear solution of caustic baryta, put on the stopper and shake, we obtain no precipitate if the amount of carbonic acid is not greater than 0.0400, or 400 in a million. Now we must consider ventilation very good if no cloudiness occurs. Nay, we may go further, and say that 600 in a million is not too much in a household; we may then be satisfied, even if there is a cloudiness in the 6-ounce bottle, and use a smaller one, say a bottle of 4 1-10 ounces. There are even cases where 1,000 in a million may be

I think there are conditions, say in a mine, where even 2,500 in a million may be thankfully received, and then we must have a bottle of only 1.36-ounce dimensions. These numbers are obtained by simple inverse proportion, 0.06 is to 0.04 as 5.42 to 3.61; to 3.61 we add 0.5, or half-an-ounce, and so obtain 41, the size of the bottle.

This plan goes on the supposition that the smallest cloudiness is a proof of the amount of carbonic acid obtained, but solutions are not always so clear, and I sometimes prefer the following method

A precipitate of carbonate of baryta caused in half-an-ounce of baryta water, by 0.2515 cub. cent. of carbonic acid, or by nearly three times that amount in lime water, is most easily remembered. If the carbonic acid in air is sought, the air is made to act on the baryta water until the desired precipitate is obtained. In other words, we use the smallest measure of air which will produce the precipitate. For this reason the name minimetric is adopted. The method may also be used for hydrochloric acid, sulphuric and sulphurous acids, sulphuretted hydrogen, &c., and probably has been used frequently without being brought forward as a method for

accurate use.

Two ways of using this method of analysis were described. The first was by the use of a finger pump, an elastic ball with two valves, connected with a bottle containing baryta water. On pressing the ball the air in it is driven out, and on removing the pressure, air is drawn in through the liquid. The air and liquid are then shaken together, and the operation repeated until the precipitate is obtained. For ordinary use a ball to contain two ounces is found most suitable. The bottle should have the same capacity as the ball and the space required for the liquid, so that the air contained in the bottle at the commencement of the experiment may count as one ballful. A table is constructed like the following, but of course it must be adapted to the size of the ball :