170 
THE ART OF REARING 
temperature at the degree fixed, it must be as 
nearly kept as may be possible, by using all means 
of cooling the air, but never excluding it from 
free circulation in the laboratory. 
If, in airing the laboratory, too great a body of 
cold air should be allowed to enter, it will harden 
the silk-worms a little ; and there is nothing to be 
done, but when the air is changed, to light the 
stove and grate fire, and again bring up the tem- 
perature to 69°, leaving the ventilators open. 
The skin of the silk-worms will soon resume its 
softness, and this check will have done them no 
injury. 
The silk-worms, to the touch, always appear 
cooler than the atmosphere of the laboratory ; 
their transpiration is the cause of this. Although 
it is insensible, it is very abundant ; for as they 
void no liquid except when they rise, transpiration 
is the only means they have of evacuating the 
moisture they swallow with the leaves. 
Our own skin is never cooler than when it is in 
a state of moisture, exposed to the impression of 
the air ; evaporation and cold augment in propor- 
tion to the action of the air, although the tempera- 
ture may be rather warm. 
It would be dangerous not to keep the air of 
the laboratory in motion ; because it could not 
otherwise carry off and absorb the quantity of 
