CARE OF THE EGGS. 295 



that ventilation is an absolute necessity. But if air is 

 admitted to the egg chamber in currents, excessive evap- 

 oration is liable to result, and this is not all ; for there is 

 danger that some portions of the chamber will be cooled 

 faster than the others. It is hard to warm an apart- 

 ment, large or small, uniformly in every part at the 

 same level, even when the air is at rest, and still harder 

 when there are gusts and eddies of cold air. 



The plant and animal both need oxygen. The latter 

 while taking it in gives out carbon dioxide, a noxious 

 gas, the excessive accumulation of which in the air 

 around the animal would cause its death, though it is life 

 to the plant. This gas is heavier than air, hence it was 

 once believed that it would settle to the bottom of a room, 

 as water seeks the bottom when it is placed in the same 

 vessel with oil ; but this notion was exploded when the 

 law of the miscibility of gases was discovered. Through 

 this law, gravity is overcome by a stronger force, which 

 compels two gases to mix, and if one is much heavier 

 than the other, this mixing power is all the stronger. 

 Aside from any currents of air whatever in the air 

 chamber, the carbon dioxide exhaled from the eggs 

 becomes diffused through all the air in the chamber. 

 Then if no more of this gas should be produced, the 

 air and gas in the apartment would be in what is called 

 an equilibrium. Now suppose the air in the incubator 

 should contain a greater proportion of the poison than 

 the air outside does, and suppose it were possible to heat 

 and maintain the air outside, in the incubator cellar, on 

 a level with the machine, at absolutely the same degree 

 as on the inside of the latter, and a small door should be 

 opened between the air chamber and the cellar ; there 

 would, of course, be such a perfect balance of tempera- 

 ture within and without the egg chamber that there 

 would be no draft through this door. But now, although 

 the heat is in equilibrium between the inside and out- 



