496 HEALTH AND DISEASE 



of .space should be allowed for each individual, and a much larger space, 

 as might be expected, is required for the horse; but as a set-oft" to this 

 it may be remarked that it has been found that the horse can bear without 

 injury a larger percentage of CO, than man, and secondly, the construction 

 and arrangements of stables are more favourable to natural ventilation than 

 are the majority of human habitations. So if the space occupied by stables 

 be somewhat smaller than those built for man, the renewal of the air is 

 more effectually accomplished. 



The term " natural ventilation" has been suggested by Pettenkofer, 

 who has shown that, besides the interchange of air and gases that takes 

 place through open windows and dooi-s, or through the cracks and inter- 

 spaces of boards and ill-fitting frames of windows and doors when these 

 are closed, the walls of most dwelling-houses, when composed of brick or 

 sandstone and mortar, allow air to pass through them with much greater 

 facility than is usually believed. Thus it is found by experiment that 

 the natural pores of such walls allow of the passage of 2000 cubic feet of 

 air through — 



42 square feet of a free wall of sandstone. 



30 „ ,, „ quarried limestone. 



25 „ „ „ brick. 



19 „ ,, „ tufaceous limestone. 



Two circumstances are of great importance in regard to the activity 

 of natural ventilation — the relative temperature of the air within and 

 without, and the presence of moisture in the walls. Moisture, by filling 

 up the pores, greatly obstructs, if it does not altogether prevent, the passage 

 of air through walls. It is present in large quantities in new buildings, 

 in which, therefore, no natural ventilation takes place, and which are 

 proverbially unhealthy. 



Pettenkofer found that in a room 14 feet square and 14 feet high, with 

 a difference of temperature of 34° F. (66° F. inside, and 32° F. outside), 

 the contents changed once in an hour; with a good fire in the stove it 

 increased 25 per cent, and even when all apparent apertures and crevices 

 were closed it only diminished 28 per cent. ' A stable built of mud is 

 capable of permitting the entrance of a considerable volume of fresh air 

 by natural ventilation, and can thus afford shelter to more animals than 

 one built of sandstone. The activity with which the exchange of air is 

 secured by natural and wholesome ventilation in a stable does not depend 

 upon its cubic capacity, but upon the extent and nature of its ventilating 

 walls and appliances, and hence a small stable built of porous material 

 may secure better ventilation than a large one, partly because for each 



