218 The Princijjhs of Ventilation. 



Our interest, however, is more immediately with processes of ventila- 

 tion actually practiced in our own time, and Avith the fundamental 

 principles of the science of ventilation. 



I. Standard of Purity. 

 Pure air, as understood by sanitary engineers, and as the object 

 sought by ventilation, is a mixture of gases having a close approxima- 

 tion to the following composition: 



Nitrogen... 77.517 



Oxygen ^0.990 



Water... 1.460 



Carbonic acid 0.033 



100.000 



II. Impurities. 



Many impurities are encountered in various places and in varying 

 amounts. In-doors are the organic particles exhaled from the lungs 

 and thrown off from the skin, and in bed-rooms from chamber uten- 

 sils; excess of carbonic acid from respiration, and gas or lamp flames ; 

 carbonic oxide from too hot or leaky stoves and furnaces ; hydro-car- 

 bons from the kitchen; sewer gases from plumbing fixtures and 

 cellars, with their accompanying hosts of living germs; dust, con- 

 sisting of organic material from carpets, upholstering and clothing, 

 and mineral matter from wall papers. Out-of-doors are the fumes 

 from manufactories; miasmata from low-lying regions, alternately 

 flooded and left bare, and from depressions filled with stagnant water; 

 dust from the streets ; ammoniacal fumes from stable dung-heaps, with 

 their myriads of germs; exhalations from cess-basins ; and odors from 

 privies. 



Of course these out-of-door impurities cannot be removed by venti- 

 lation, but, in designing any system, they have to be taken into 

 account so far as they affect the given locality. 



III. Eequirements oe Ventilatii?"g Apparatus. 

 1. The amount of carbonic acid must not exceed, say, six volumes 

 per 10,000 of air, sinqe a greater proportion produces various unpleas- 

 ant symptoms, as headache, lassitude and drowsiness, besides the more 

 important effect of lowering the general tone of the body. In this 

 connection it must be noted that carbonic acid, per se, is not a poison ; 

 its injurious effects are due to its replacing oxygen in respiration. In 

 addition it has been slK)wn that when carbonic acid has been produced 



