66 THE NATURE AND TREATMENT OP 



few hours only, a contaminated atmosphere, such, for example, 

 as pervades swill milk est-ablishments, and filthy barns, where 

 the principles of both ventilation and cleanliness are almost en- 

 tirely disregarded. 



The object of ventilation is to secure a current of pure- air 

 at all times, by which means the vitiated is replaced, and a 

 constant supply of the pure material is kept up. This neutralizes 

 the morbid effects which follow the re-respiration of a pent-up 

 atmosphere. 



Pure air is that immense mass of elastic fluid which sur- 

 rounds the globe we inhabit. It is capable of varying its di- 

 mensions under the influence of heat and cold ; heat expands 

 or rarities it, and cold contracts or condenses it. It is trans- 

 parent, so as to be invisible, except by the blue color it reflects 

 when in large mass, as seen in the regions of space. It is 

 without odor, except that of electricity, which it very frequently 

 manifests, and, like other gases, it is capable of holding in so- 

 lution, or receiving an admixture of vapor. For example, it 

 absorbs water in considerable quantities, without decrease in its 

 own specific gravity ; it is supposed, however, that the air sus- 

 pends vapor, not really by absorption, but by separating its par- 

 ticles, and thus preventing their condensation. 



Pure air almost always contains vapor; for the rain and 

 dew, which attract our attention, as well as the clouds and fogs, 

 all deposit moisture, and even when the atmosphere appears 

 perfectly transparent, water can be extracted from it. The 

 usual process of demonstration is as follows : Take one ounce 

 of sulphuric acid (which has an extraordinary affinity for water, 

 more than any other acid), and expose it in a shallow vessel, 

 subject to the free action of pure air, and in a short time it is 

 converted into a dilute acid ; its weif^rht havin^j increased in the 

 ratio of one hundred per cent. The vapor or water, thus 

 absorbed, can again be separated by distillation, showing con- 

 clusively that watery vapor is always present in pure air. 

 Everybody knows that iron and steel, when exposed to the atmos- 

 phere, will corrode ; that lime will slake ; salt and sugar will 

 imbibe moisture ; consequently the theory needs no further 



