114 Francis B. Sumner 
mean for 12 determinations made during a period of four months 
being 39 per cent. This is somewhat less than the mean relative 
humidity at Phoenix, Arizona, for the year 1906, as stated by the 
U. S. Weather Bureau, that point having the dryest atmosphere, 
with a single exception, of any place in the United States for which 
records are given. In general, the humidity of the warm room 
varied inversely as the temperature, since no compensation was 
made by the evaporation of water; but the degree of saturation 
of the outside air must have been a factor. ‘The humidity of the 
cold room varied from about 49 per cent to about go per cent, the 
mean of nine determinations being 67 per cent. ‘This figure is a 
trifle less than that given for the mean humidity of Philadelphia 
during the year 1906. 
The physiological results of such differences in humidity must 
be far reaching. The quality of the air inspired must affect the 
processes of respiration, and the rate of evaporation from the body 
surface must differ widely in the two cases. ‘This last was shown 
by the eagerness for water displayed by the warm-room individu- 
als. To what degree the results which I offer have been brought 
about by differences in temperature and to what degree by differ- 
ences in humidity it is impossible to state. In these experiments 
it has not been practicable to control the humidity, independently 
of the temperature, and thus it has been impossible to decide this 
question definitely. The subject will be referred to later. 
Disposition of Stock. ‘Iwenty broods of young mice, aggregat- 
ing 135 individuals, were employed for this experiment. | In order 
to insure, so far as possible, a division into two lots of a similar 
hereditary endowment, one-half the individuals of each litter were 
subjected to the high temperature, the other half to the low. For 
this purpose, exchanges were made between the offspring of differ- 
ent mothers according to the following plan: One-half of brood 
A, plus one-half of brood B, were consigned to the care of the A 
mother in the cold room, the other half of each brood being con- 
signed to the care of the B mother in the warm room, and so on 
through the series. This disposition of the young was made 
while the latter were 2 to 4 days old (in a few exceptional cases as 
much as 11 days). “The members of each litter were marked at the 
