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 exy lizable 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.

Carbenie acid is the gas well known as a great agent of mischief; it is found on all crdinary 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 nough that the carbenie seid 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 topper 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 farther, 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 allowed, and a 266 or 2 6-10-ounce bottle may be used.

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 :

TABLE FOR FINGER PUMP commonly used, the ball marked 2 oz., but only about two-thirds of this actually used :

The number of ballfuls required to produce the precipitate being counted, on referring to the table the percentage of carbonic acid in the air is at once seen.

An easy method of finding the precipitate for comparison is by shaking half-an-ounce (14·17 cub. cents.) of baryta water in a bottle with 23 ounces of Manchester air or nearly 30 ounces of London air (elsewhere according to residence).

Experiments made with this apparatus show it to be extremely delicate. The carbonic acid in the air of a room can be estimated in a few minutes. For very bad air smaller balls are used, but in all cases only half-an-ounce of baryta water.

The second way of using the minimetric method was partly described in the report on the air of mines, and long tables were there given both for baryta and lime water. It is more particularly adapted for use in houses and workshops.

Suppose, if we desire to know if the air contains more than 004 per cent, of carbonie seid, we fill a bottle containing 5.422 ounces with air, by pumping or otherwise, and shake in it half an ounce of baryta water. If there is any precipitate at all the amount of carbonic acid in the air is above 0-04 per cent. This would indicate that the air is less pure than outside.

If we allow 0-06 per cent. of carbonic acid in a room, we take s bottle of the size of 3:281 + ounce 3·781 ounces (or 107-19 cub, cents), and if, after a trial as above, we find a precipitate, however small, or even a slight milkiness, the air is deteriorated beyond 0-06. This relates to dwelling houses.

If for workshops we allow per cent. (0-25) a bottle of 0788 4 | ounce = 1:288 ounces (or 36-51 cub. cent.) is sufficient. If per cent, is allowed (0-50) a bottle of 0·394 x ounce =0·894 onnors (or 23:34 cub, cents.) is enough.

Both of these two last sizes of bottles are small enough to be

carried in the waistcoat pocket. The addition of the ounce is for the space occupied by the baryta water.

Lime water, being very common, will probably be often substituted for baryta water. The experiment is exactly the same, only larger bottles are required.

0.06 per cent. carbonic acid in the air gives us precipitate or milkiness when ounce of lime water is shaken in a bottle of air containing 10567 ounces (=299.57 cub. cents). 0.25 per cent., 2.916 ounces (=82.67 cub. cents). 0.50 per cent. 1708 ounces (=48.42 cub. cents.).

By this simple method the greatest refinement may be attained. Of course care must be taken to have the baryta or lime water perfectly clear to begin with.

The author said he had not used these methods for purely scientific investigations.

HEALTH OF TOWNS.

The Danger of Deterioration of Race from the too rapid increase of Great Cities. By JOHN EDWARD MORGAN, M.A., M.D. Oxon. Physician to the Salford Hospital, Hon. Sec. Manchester and Sulford Sanitary Association.

TH

HOSE who have studied the distribution of the population of Great Britain, during the last fifty years, cannot fail to have observed on the part of the labouring classes an increasing tendency to mass together, and either to form new centres of industry, or to swell the population of those already existing. The desire of raising themselves in the world, keeps constantly urging vast numbers of the dwellers in rural districts to emigrate to the nearest town; where wages are higher, and the conditions of life apparently more attractive. To this cause, more than any other, is to be attributed that extraordinary development of every branch of our manufacturing industry, which induces even the most distant nations to look to this country as to the great mart which, however varied their wants may be, can best supply them. In this national prosperity, in successful competition with numerous rivals, and in the acquisition of immense wealth, we see much that is calculated to gratify our vanity as a people. Yet at the same time arises a momentous question—“ May not nations, like individuals, curtail their day of power in the world's history, by overtaxing the physical and mental energies at their disposal, thus prematurely consuming that national life-blood on which permanent greatness mainly depends?"

Among the many important questions connected with this widespread migratory tendency, there is one, which, to sanitary reformers,