per 1000 volumes in the former, and only 720 in the latter.

With a small cubic space it is impossible to obtain uniform diffusion of the contained air, if a large amount of fresh air is supplied, because between inlet and outlet a direct current is established, and a considerable quantity of air passes right through without being utilised. Again, it is evident that if the ventilation is impeded or becomes arrested, impurities will collect with far greater rapidity in a small than in a large space, and this of itself is a great argument in favour of the adoption of an ample cubic space as a basis. Dr. de Chaumont, in his remarks on this point, writes :"Let us suppose two occupied spaces, one of 500 and the other 1000 feet, ventilated so that the ratio of carbonic acid is 06 per cent, and that from some cause or other the ventilation is arrested in both, the condition will then be as follows:

With ordinary means of ventilation (artificial excluded), both Dr. Parkes and Dr. de Chaumont maintain that the cubic space for a healthy adult ought at least to be 1000 feet. It is true this is very much in excess of what is generally obtained. In the crowded dwellings of the poorer classes it seldom exceeds 200 to 250 cubic feet: but then the disastrous effects de

clare themselves but too clearly in the increased rate of mortality. In metropolitan lodging-houses the allowance per head is as low as 240 cubic feet; and in the Dublin registered lodging-houses it is 300. The Barrack Commissioners, on the other hand, recommended a minimum space of 600 cubic feet for soldiers, insisting at the same time that the air should be renewed at least twice every hour. "The only safe principle." they said, "in dealing with the subject is to have a large margin for contingencies; and the question really is, not whether 600 cubic feet per man be too much, but whether 600 cubic feet per man be enough for all the purposes of warming, ventilation, and comfort." Experiments that have since been made, and particularly those conducted by Dr. de Chaumont, prove most incontestibly that even this comparatively large allowance is inadequate for these purposes; but it was as much as could be obtained at the time, without putting the country to enormous expense. The Commissioners themselves observe :- It has been said that the question of cubic space is simply a question of ventilation, but it is rather a question as to the possibility of ventilation. The more beds or encumbrances you have in a room, with a limited cubic space, the more obstruction you have to ventilation; the fewer the beds the more easy is it to ventilate the rooms. There are fewer nooks and corners, fewer surfaces opposed to the movement of the air, and less stagnation. We have been in rooms, both in barracks and hospitals, in which the atmosphere was positively offensive with the doors and windows open."

For further remarks on cubic space in hospitals, see Chapter on Hospitals.

In summing up this part of the subject, the following may be accepted as the standard conditions necessary for the requirements of perfect health :

1. That the limit of maximum impurity of air vitiated by respiration ought not to exceed 6 carbonic acid per 1000 volumes.

2. That to ensure the maintenance of this standard under ordinary circumstances, 3000 cubic feet of pure air must be supplied per head per hour.

3. That for this purpose, and with ordinary means of ventilation, a space of at least 1000 cubic feet should be allowed per head in buildings permanently occupied.

It may be objected that these conditions aim at too high a standard, and that in general they are seldom met with; but it must be remembered, as Dr. de Chaumont has so well pointed out, that they are based on a firm foundation of facts, and that though it may not be possible to prove in all cases that bad effects result from a neglect of them, it does not follow that such bad effects may not have been produced. In a country like this, with a climate so variable, the cubic space allowance is a most important element in any scheme of ventilation. It should be ample enough to permit of a sufficient supply of fresh air without creating injurious draughts, and yet not too large to interfere with the maintenance of a sufficient and equable temperature during cold weather. Where artificial ventilation is provided, and when the fresh air can be heated before entering, it may be as low as 400 cubic feet, but even then the ventilating arrangements must be much more perfect than they usually are. In the case of healthy adults, such as soldiers and prisoners, the standard allowance may also be considerably lessened, if care be

taken that the free entrance of fresh air at all hours and in sufficient quantity shall not be interfered with. Unfortunately the question of cubic space is a question of large outlay, and hence the desire to economise tends to curtail the minimum not within safe limits, but within limits that will not be attended with glaring injurious effects.

In advocating these conditions, however, it is but right to state that the numerous experiments and weighty opinions of Dr. Angus Smith are somewhat at variance with them. In the first place, Dr. Smith's experiments only gave 4 cubic feet of carbonic acid per hour, which would reduce the requisite amount of fresh air supply per hour to 2000 cubic feet; and, in the second place, Dr. Smith maintains that uniform diffusion of the contained air is the exception and not the rule, and in fact that it does not occur at all. With regard to the first of these points, the discrepancy between Dr. Smith's results and those of other physiologists may be reconciled on the ground that his trials were admittedly not made on large men; but with regard to the second, there still exists considerable divergence of opinion. If by uniform diffusion throughout an occupied space is meant the exact uniformity of the chemical composition of the air in every part, then it must be conceded that Dr. Smith is strictly correct; for so long as fresh air is entering and foul air issuing from a room, there will not only be a difference between the composition of the air in the immediate proximity of the inlets and outlets, but there will also be a difference in various parts caused by the currents, however imperceptible these may be. In small occupied spaces, such as prison cells, provided with adequate means for

artificial ventilation, the amount of fresh air required to keep the carbonic acid from exceeding 6 per 1000 volumes must obviously be much less than the amount required per head in a large room, because uniform diffusion is impossible, there being a constant movement of the air from inlet towards outlet. But in a large space the case is different, even though the cubic space per head be not greater than that of the prison cell. The entering currents and the currents produced by inequalities of temperature are, in this instance, much more numerous, and produce a much greater mixing of the air, while the impurities given off by respiration have greater scope to be affected by the laws of gaseous diffusion. For all practical purposes, therefore, the condition of uniform diffusion, as applying to a room occupied by several persons, may be accepted as sufficiently accurate; and this being so, the standard amount of fresh air to be delivered per head per hour must, as already stated, be 3000 cubic feet. Indeed, the whole of the controversy between Dr. Angus Smith, on the one hand, and Drs. Parkes and de Chaumont, on the other, regarding this point, seems to be based on a misunderstanding; each party estimates the requirements of ventilation for a single individual, but under different conditions—the former taking it for granted that the space is occupied by one, the latter that it is occupied by several.

SECTION III.-NATURAL VENTILATION.

Natural ventilation is carried on by the agency of natural forces, such as gaseous diffusion and movements of air produced by inequalities of temperature.

1. Diffusion. The force of gaseous diffusion, upon