Then there are many people who are indifferent about ventilation. They dislike trouble; they can suffer much, but they can do nothing. They will bear all the evils, all the loss, and all the vexations of a bad stable, rather than make any effort to improve it. If an offer were made to ventilate their stables, without cost and without trouble, they would permit it to be done. When advised, for the sake of their horses, to get the stables properly aired, one will reply, " Ah, it is very true what you say, but you may see the thing can not be done!"

Stables are often constructed in such a manner that it is very difficult to ventilate them. The process may be both troublesome and expensive; there ought to be some good reason for suffering the one and incurring the other. Opposition has been excited by magnifying the evils of a close stable; but, divested of all exaggeration, it will be seen that they are not insignificant.

The Object of Ventilation is to procure a constant supply of air in sufficient purity to meet the demands of the animal economy. Sufficient purity is not perfect purity. Neither the horse nor any other animal requires air absolutely pure. In towns and in stables there is no such thing; and that is proof strong enough that it is not essential.

The Composition of Pure Air has been repeatedly ascertained by chemical research. The atmosphere consists of two simple gases. According to Lavoisier, 100 measures of pure air contain 73 of nitrogen and 27 of oxygen. [Accord ing to later authorities, within a fraction of 21 of oxygen and 79 of nitrogen, and about 1/2500 of carbonic acid.] It has been proved that a breathing animal consumes the oxygen, and that death ensues when the supply falls below the demand. When a small animal is enclosed in an air-tight vessel, it soon dies. The air suffers no apparent diminution in bulk, yet it undergoes a change in composition. The oxygen is consumed, or a large portion of it is consumed, and its place is occupied by another gas, termed carbonic acid, which is given out from the lungs. This kind of air is rather heavier than that of which the atmosphere is composed. In certain situations it mingles with the air in the proportion of about 1 to 100. When an animal is completely immersed in it, he dies immediately. Some contend that carbonic acid is poisonous; others that it destroys life merely by excluding the common air, without which no breathing animal can live. The carbonic acid is an evacuation; it exists in the system, but it must not accumulate there.

It must be thrown? out almost as rapidly as it is formed. As it is evacuated, it contaminates the external air with which it mingles. Hence, in the neighborhood of all animals, the air is more or less impure.

The Use of Air, in the animal economy, is to purify the blood. This fluid is in a state of constant change. As it circulates through the various parts of the body, it performs functions innumerable; these operations change its composition, and render it unfit to repeat them unless it be duly renovated. In the lungs the air and the blood come in contact, and both are changed. The air loses a certain portion of oxygen and acquires carbon. It becomes of a brighter red; from a dark purple hue it is changed to bright scarlet. The process is briefly described by the word purification. But it must be remembered that, besides parting with some noxious ingredient, the blood is altered in some other way probably by the addition of oxygen, and certainty by the agency of oxygen. If the air be destitute of this constituent, or if it do not contain a certain quantity, the blood can not undergo the change by which it maintains life.

The Composition of Impure Air is not always the same. By impurity is here meant any alteration which renders the air less fit for breathing. The impurity varies according to the quantity, the number, and the kind of foreign matters which mingle with the air, and according to the degree in which one of its constituents is deficient in quantity. Air may be bad, merely because it is deprived of part of its oxygen. It is probable, indeed it is certain, that in particular situations the air does not contain its full proportion of oxygen, and that the animals who breathe it do not experience any serious inconvenience. Though there is not the usual quantity there is sufficient. When the air contains so little oxygen that it can not meet the demand of those animals by whom it is breathed, it may very well be called bad. It has power to do mischief; the animal suffers, not from the presence of a pernicious agent, but from the absence of that which enables the blood to perform its functions. The air, however, may be rendered actively injurious or poisonous, by the addition of foreign ingredients. These are of various kinds, many of which can not be discovered by the chymist.

They are known to exist only from their effects upon the health of the living animal.

The Impure Air of a Close Stable is deficient in oxygen, and mingled with carbonic acid, ammoniacal gas, and some other matters. The deficiency of oxygen in stables has never been proved by actual experiment. But there can be no doubt but it occurs wherever the air is confined around a breathing animal. Repeated investigations have shown a deficiency in theatres, hospitals, churches, and other places crowded by human beings. A French chymist analyzed the air of a large theatre, that of the Tuileries, before and after the play. He found it of the usual composition, 100 parts containing 27 of oxygen and 73 of nitrogen, before the performance; at the conclusion, there were 76½ of nitrogen, 2½ of carbonic acid, and only 21 of oxygen. There is every reason to believe that the air of a close stable is deficient in oxygen to a much greater extent. Stables are often as closely packed as a theatre; the animals are much larger, the building much lower, containing less air in proportion to the demand closer, and closed for a longer time, than the habitations of man, and the deficiency of oxygen must be so much the greater.