DISEASES OF CATTLE 59 



rising of warm air is called convection, and this convectional move- 

 ment of air is one of the forces, if we may so speak, that operate in 

 ventilating buildings. In an occupied room or stable, the heat from 

 the bodies of the occupants warms the air and produces the upward 

 tendency. If this tendency is encouraged so far as to allow the rising 

 air to escape altogether, and if at the same time provision is made for 

 the cooler fresh air to find its way into the room or stable, we have 

 ventilation. Whether or not this convectional movement is a suffi- 

 cient force to give satisfactory ventilation, is a question to be consid- 

 ered later. The second of these natural aids to ventilation is Dif- 

 fusion. 



Diffusion. The air that we take into the lungs may contain but 

 little water-vapor; and, if pure, it contains a very small percentage 

 of carbonic acid gas. The latter is a chemical compound of carbon 

 and oxygen. It exists in the free atmosphere in very small quanti- 

 ties the average amount of which has been estimated at 3^ vol- 

 umes of carbonic acid gas in 10,000 volumes of air. On the other 

 hand, the breath expired from the lungs is saturated with moisture, 

 and contains between 4 per cent and 5 per cent of carbonic acid gas ; 

 or, to state it in the same form as above, in 10,000 volumes of expired 

 air there are about 430 volumes of carbonic acid gas. The problem 

 of ventilation is to prevent these two products moisture and car- 

 bonic acid gas from accumulating in excess within the occupied 

 spaces. Aqueous vapor, at the same temperature and pressure, is 

 lighter than air ; carbonic acid gas is considerably heavier. It might 

 be supposed, therefore, that when these products are emitted from 

 the lungs, the aqueous vapor would raise to the ceiling, and the car- 

 bonic acid gas settle to the floor. There is a tendency to this move- 

 ment ; but, at the same time a process goes on which is equally as ef- 

 fective as the force of gravity. This process is known as diffusion. 

 A simple illustration of diffusion may be seen by putting a few drops 

 of milk into a glass of clear water. Soon the milk is seen to diffuse 

 through the water, giving a uniform whitish shade to the whole. The 

 same thing goes on with the aqueous vapor and carbonic acid gas. 

 Instead of separating completely according to density, from the air of 

 the room, these products diffuse throughout the whole room ; so that 

 wherever the foul air opening is placed, it will find almost uniform 

 proportions of the products that ventilation is required to remove. 

 If any difference, however, exists in the distribution of these gases, 

 the excess of carbonic acid will be found at the floor; while that of 

 water-vapor will be found at the ceiling. 



Another aspect of diffusion is the movement of gases through 

 porous walls. Suppose that the air of a room becomes overcharged 

 with carbonic acid gas, and at the same time robbed of its oxygen ; 

 then the carbonic acid will diffuse outward through the plaster of the 

 walls, and through the brick or between the clapboards; and the oxy- 

 gen from the outside will in the same manner diffuse into the room. 

 So that a natural ventilation proceeds at all times, even if the room 

 appears perfectly air tight. This natural ventilation is, of course, 



