Buildings and Yards. 



543 



little noticed except when extremely cold weather causes the incoming 

 air to condense the moisture of the stable into frost where it enters. 

 Every one has noticed the beneficial influence of a heavy snowfall in main- 

 taining the temperature of house or barn, because it fills the cracks and 

 crevices on window sills and about doors. 



Since the exchange of air is concerned with the subject of ventilation, 

 its eflfect in maintaining temperature will be discussed in connection with 

 the next topic. 



///. To secure pure air, zvith proper degree of humidity 

 While the composition of air is not constant, it may be stated ap- 

 proximately as follows : nitrogen, 78.49 per cent ; oxygen, 20.63 per cent ; 

 water, 0.84 per cent ; carbonic acid gas, 0.04 per cent. Usually there are 

 also present slight traces of ammonia and other substances. If the weight 

 of a given volume of air at freezing 

 point under one atmosphere of press- 

 ure is one pound, an equal volume of 

 nitrogen will weigh 0.9714 lb. ; of oxy- 

 gen, 1. 1057 lb.; watery vapor, 0.6225 

 lb.; and carbonic acid gas, 1.5291 lb. 

 It may be asked why these gases do 

 not settle with the heaviest gas at the 

 bottom and the lightest at the top. 

 The reply is that it is the nature of 



gases when brought together to mix 



Fig. 322. — Why has the upper light 

 gone out first in this tight box? In 

 a few moments all will go out. 



rapidly into a homogeneous mass. 



It is said that a horse draws into 

 his lungs 45 cubic feet of air per hour 

 and exhales 6.5 cubic feet of carbonic acid. It does not follow from 

 this fact that 45 feet of air per hour is sufficient for a horse, because the 

 large amount of carbonic acid in the exhaled air vitiates a considerable 

 amount of the surrounding air. 



Air containing an excess of carbonic acid gas, is considered unfit to 

 breathe, not so much because of any injury done by the carbonic acid 

 gas, as because the carbonic acid gas is considered a measure of injurious 

 impurities of an organic nature, and because it may replace (or take the 

 place of) some of the life-giving oxygen. In fact, except in so far as 

 the carbonic acid gas seryes to dilute the oxygen, it is believed to be en- 

 tirely harmless. 



Animals not only exhale carbonic acid gas but they also excrete 

 through the lungs and skin a considerable amount of water in the form 

 of vapor. Two well known facts are pertinent in this connection: (i) as 



