Such a result obtained in a particular case, is evidently capable of general application.

Cooling by steam vapour therefore acts inefficaciously upon the organism; it simply cools the area in which it acts, without refreshing the individuals contained in that area.

It will hereafter be seen, in the description of the kind of apparatus which have been tried, that the system em-ployed at the Institute has proved the truth of this view. This system cools the fresh air by evaporation, but also by water drawn from a well at 12° C.; the lowering of the temperature of the interior atmosphere varies from 4° to 6° C. Although the conditions of this refrigeration are more favourable than if obtained by evaporation alone, experience has shown that it produces no benefit, and even rather occasions a certain amount of discomfort.

A great number of contrivances have been put up upon this principle, but in general the experimentalists have neglected to take note of the tests to which they have been put; these would have most certainly been valuable to them. Therefore the proposed engines, with the exception of some few, are defective; and even those which have received more general approval do not comply with the economical conditions forming in this case, as in others, one of the best guarantees of success. D'Arcet has shown that to be in a proper healthy state every cub. metre of air should not contain more than 7 grm. of water, agreeing in this with the physicians.

It would be running into another extreme to dry the air beyond a certain point. It is necessary that there should be a certain quantity of water in the air, but not too much. In the course of his interesting researches into ventilation, General Morin was struck with the stress laid by the English experts, who have greatly occupied themselves with the question, upon the advantages in point of health, upon arrangements giving the heated or unheated air introduced into inhabited places a great amount of hygroscopicity. Thus, in summer the following plan is often adopted in London, well deserving attention. Immediately beneath the hall to be ventilated is another chamber, into which the outer air penetrates by several large bays; before this falls a canvas curtain to arrest the sooty particles which are floating about in the atmosphere. In front of the curtain, a tap is fitted to a tube pierced with a number of capillary holes, regulating the fall of a regular water spray or dust mixing with the atmosphere imperceptibly, so that it falls to the ground without wetting it to any extent. This arrangement is intended to augment the hygroscopicity of the atmosphere, and should have upon it an influence similar to that produced by steam.

The English engineers who have adopted this method explain its advantage in an original way. According to them, the vaporisation of the water-spray thus traversed by the affluent air is accompanied, like the dew, the rain storms, and in accordance with the experiments of De Saus-sure and De Pouillet, by the development of a certain amount of electricity, which modifies the air in a salutary way by producing ozone in it. Thus, there would be a method of purifying the air, both simple and efficacious, especially in the summer. It is at present beyond doubt that the dispersion and dissolution in the air of a certain quantity of water in the form of spray, as it is used in some bath establishments, sen-sibly modifies the electric condition of the air; this element containing active oxygen, has in it a high degree of the property of destroying, by combustion, certain miasmata, certain bodies in a state of putrefaction. In consequence it is only necessary to prove the existence of this beneficent gas in the atmosphere which permeates the kind of fog formed by water in the form of dust, to couclude that the evaporisation of this water, beside the accretion of hygro-scopicity and the lowering of the temperature produced by it, should have an influence upon the animal economy and the cleansing of human habitations, deserving of the attention of inventors.

If such be the case, there ought to be, in proper arrangements of contrivances founded upon this principle, some solution of the problem of air cooling. To this we are further led by the observation of very palpable facts. In proportion as greater heights are attained in the atmosphere, so the presence of ozone is recognised; therefore there is no other reason to be assigned for its absence in the atmosphere, even at a very small degree of elevation, than the presence of these noxious matters so abundantly disengaged at the surface of the soil. The part which ozone has to play in the great circle of the phenomena of nature is no doubt in those superior regions to cleanse and purify the atmospheric air from the deleterious compounds produced by the decomposition going on at the surface in vegetation and life, introducing themselves in a continuous manner into it, and to restore them to the condition of the fixed elements - nitrogen, oxygen, carbonic acid, and water. Ozono-metrical observations which have been made at different times and places, have constantly established a coincidence between the ozonisation of the atmosphere and various epidemics.

This ozonisation is considerable during epidemics affecting the throat and chest, when affections of the lungs are common; and the contrary may take place, it may even arrive at a zero point during cholera, malaria, and the prevalence of paludine fevers.

Much was expected of the apparatus designed by Duvoir for the ventilation of the halls of the Institute. Despite all his experience, the intelligent constructor took a wrong course; but it must be admitted that his apparatus was not without merit. A wind shaft was placed on the roof above the hall, the conduct house containing a vertical cylinder, 13 ft. high, full of water, similar to heating pipes; 104 equally vertical tubes were enclosed in this kind of casing of a diameter of 1 1/2 in., and a stream of water was kept continually flowing through these tubes. The fresh air passing by these tubes became moist and cooled. The windows being closed, the movement became evident by the chimney draught; it continued strongly even when doors and windows were opened. The fresh air was sucked down rapidly through the wind shaft, and the apparatus began to work independently of the chimney. The action was, therefore, similar to that of a chimney draught; the air being cooled instead of heated, the draught being downward instead of upwards. Having acquired a certain degree of rapidity owing to the pressure consequent upon its refrigeration, the fresh air reached the rooms in horizontal layers; heated by the persons assembled, it rose and escaped by open windows placed at a uniform height.