2o6 THE POPULAR SCIENCE MONTHLY. 



We change and pollute the air within our houses in two ways : 

 1. By admixture of siihstances which were not in the air when it 

 came to us ; and, 2. By changing its normal composition. Both 

 are unavoidable, but there are limits, which must not be overstepped. 

 The impurities may be in the nature of gases, or dust. We often be- 

 come aware of them by our senses, by sight, by taste, mostly by smell. 

 The last sense is exceedingly sensitive for many substances ; for in- 

 stance, traces of ethereal oils. Nothing is more wonderful than its 

 acuteness with some savages and animals. If we consider the minute- 

 ness of the substances left by hunted game on the soil, which it 

 scarcely touches in its flight, and how the dog detects them even a 

 long time after, we cannot sufficiently admire such a performance of 

 the sense of smell. Other substances make themselves known in other 

 ways, sometimes by some physiological effect. Oxide of carbon, for 

 instance a gas which is generated from burning charcoal is not 

 perceived by any of our senses, but if it is present in air to the extent 

 of a half per cent, only, it destroys human life after a while. A few 

 grains of veratria, rubbed down into a powder, will set all the persons 

 in the room sneezing. Other substances, as the products of distilla- 

 tion of fats, or the smoke of wood, irritate the membranes of the eyes. 

 Other vapors and kinds of dust act on the taste ; for instance, aloe- 

 powder. 



We rightly consider all air, which acts on our senses or our feel- 

 ings differently from air in the ojjen, to be polluted. 



The second way in which we render the air impure on its journey 

 through our houses is that of altering the quantities of its compo- 

 nents. We deprive it of oxygen by our respiration, by the burning 

 of lights and fires ; we increase its carbonic acid and its water by the 

 activity of our lungs and skin, and by numerous proceedings of the 

 household. 



All tliese pollutions and alterations are partly avoidable, partly 

 unavoidable. Among the latter are those by our lungs and skin, be- 

 cause we cannot live without producing them. To the former belongs 

 everything that from want of cleanliness, careless treatment of waste 

 and refuse, passes into the air-current, the utilization of which ought 

 to be the privilege of our skin and lungs. It is an inexcusable waste 

 of ventilation, if it is directed against avoidable pollutions of the air, 

 besides its being generally of not much use for this purpose. If I had 

 a nuisance in my room, I should be a fool if I kept it there and trusted 

 to stronger ventilation. The rational way is to do away with the pol- 

 lutions, not to keep them and to fight them by ventilation. Without 

 strict cleanliness in a house or public institution, all contrivances for 

 ventilation will not do much good ; the proper domain of ventilation 

 begins where cleanliness, by rapid removal or careful shutting up of 

 air-polluting substances, has done its best. It is only against the de- 

 terioration of the air by respiration and perspiration, whicli is beyond 



