THE MECHANICS OF VENTILATION. 461 



HYGIENE. 



THE MECHANICS OF VENTILATION. 

 BY GEO. W. RAFTER, C. E. 



The necessity for change of air in inhabited spaces is rendered evident 

 by considering the sources of contamination ; tliey are, 



a. The production of carbonic acid by respiration. 



h. Increase of moisture from the same cause and exhalation from the 

 body. 



(\ Organic impurities from the bodily exhalations. 



d. Heat thrown off from the occupants and from the lights at night ; and 



e. The production of carbonic acid from the lights. 



The object of ventilation, therefore, is to remove foul air and substitute 

 fresh air in place of the foul air so removed. 



Ventilation, then, reduced to its simplest terms, is a matter merely of the 

 movement of bodies of air; and, since air in a substance possessing weighty 

 the whole question is one of mechanics, and is in the fullest sense susceptible 

 of numerical comjDutation. 



This being granted, further progress is comparatively rapid, consisting 

 (1) of the development of the laws of air in motion, and (2) in the applica- 

 tion of thoyo laws to the removal of foul air, and the consequent supplying 

 of fresh air in buildings and closed structures generally. 



Before entering upon the strictly mechanical portion of the subject, it 

 will be well to consider briefly (1) the nature and extent of the contamina- 

 tion rendering change of air in inhabited confined spaces necessary, and (2), 

 the collateral head of ventilation — warming. 



a. Production of carbonic acid by respiration. When air passes into the 

 lungs it undergoes a chemical change whereby a certain amount of carbonic 

 acid is produced. This Avhen expired adds to the impurity of the air, and. 

 were its production in respiration continued a sufficient time in any tight 

 space, the air would immediately become so impure as to produce death. It 

 has been found bj^ experiment that air in its normal condition contains in- 

 all parts of the world an amount of carbonic acid equal to from three tCNfo-ur- 

 volumes in 10,000. That some must exist in the air will be readily in^ferredii 

 by considering that respiration and combustion produce this gas. The- 

 amount, however, was only determined b}^ the researches of Regnault, who- 

 analysed air from many localities. When carbonic acid is the only impurity 

 from eight to ten volumes in 10,000 may be respired without serious incon- 

 venience, though six volumes in 10,000 is taken as the limit of good ventila- 

 tion for reasons to appear hereafter. An adult jDroduces 0.6 of a cubic foot 

 of carbonic acid per hour. 



b. Increase of moisture from respiration and bodily exhalations. The 

 amount of moisture present in the air at different times varies greatly. Ob- 



