II. IMPURE AIR.--The dangers of impure air, water, and food depend largely upon the fact that through these media may be introduced into the body particles of organic matter, living or dead, which tend to produce disease in the recipient. The parasites are types of this mode of disease-production, and these blend with the contagia of the specific diseases in such a way that it is not easy to draw the distinction in all cases. There are also certain poisonous gases and inorganic compounds which may occasionally be present in air or water to such an extent as to produce disease; but as a rule the gaseous impurities of the air are offensive to the smell rather than dangerous, as will be seen when we come to consider the effluvium nuisances. The subject of ventilation, for the purpose of procuring an adequate supply of pure air, is one of so much importance, and one upon which the physician is so liable to be called for practical advice, that it seems proper to state briefly the general principles which should govern investigations into, or recommendations upon, this subject. The impurities of air which are to be disposed of by ventilation are for the most part derived from the human body, chiefly from respiration, and these only will be considered here. In some cases it is necessary to make special provision for the products of combustion from gas, etc., but as a rule this is rather for the purpose of regulation of temperature than anything else. The impurities of air due to the presence of human beings consist mainly of carbonic acid, ammonia, sulphuretted hydrogen, and sulphide of ammonium, and of various organic compounds, mostly in the form of minute particles of organic matter of uncertain structure, but extremely prone to decomposition. It is usual to estimate the degree of impurity by the amount of carbonic acid present, and this leads many persons to suppose that the carbonic acid is in itself the chief and most dangerous impurity. This gas is, however, not perceptible to the senses, {178} nor is it injurious to health, unless present in much greater proportion than that in which it will be found in the most crowded habitations or assembly-rooms. Its importance in questions of ventilation depends upon the fact that its increase in a room beyond the amount present in the outer air may usually be taken to be in direct proportion to the amount of the really dangerous and offensive impurities present, and that the amount of carbonic acid can be ascertained by chemical tests with comparative ease and rapidity; which is not the case with regard to the organic matter. The carbonic acid is therefore taken as the measure of the impurity, although it is not itself the impurity of which we are most anxious to be free.