At the Ontario Station, the average amounts of carbon dioxide under a large number of hens was .32 of one per cent., or about ten times that of fresh air, or one-sixth of that which the man breathed so happily in the respiratory calorimeter. With incubators, every conceivable scheme was tried to change the amount of carbon dioxide. In some, sour milk was placed which, in fermenting, gives off the gas in question. Others were supplied with buttermilk, presumably to familiarize the chickens with this article so they would recognize it in the fattening rations. In other machines, lamp fumes were run in, and to still others, pure carbon dioxide was supplied. The percentage of the gas present varied in the machines from .06 to .58 of one per cent. The results, of course, vary as any run of hatches would. The detailed discussion of the hatches and their relation to the amount of carbon dioxide as given in Bulletin 160 of the Ontario Station, would be unfortunately confusing to the novice, but would make amusing reading for the old poultryman. Speaking of a comparison of two hatches, the writer, on page 53 of the bulletin says, "The increase in vitality of chicks from the combination of the carbon dioxide and moisture over moisture only, amounting, as it does, to 4.5 per cent. of the eggs set, seems directly due to the higher carbon dioxide content." I cannot refrain from suggesting that if my reader has two incubators, he might set up a Chinese prayer machine in front of one and see if he cannot in like manner demonstrate the efficacy of Heavenly supplications in the hatching of chickens. The practical bearing of the subject of ventilation in the small incubator is almost wholly one of evaporation. The majority of such machines are probably too much ventilated. In a large and properly constructed hatchery, such as is discussed in the last section of this chapter, the entire composition of the air, as well as its movement, is entirely under control. Nothing has yet been brought to light that indicates any particular attention need be given to the composition of such air save in regard to its moisture content, but as the control of this factor renders it necessary that the air be in a closed circuit, and not open to all out-doors, it will be very easy to subject the air to further changes such as the increasing oxygen, if such can be demonstrated to be desirable. Turning Eggs.