146 
THE ART OF REARING 
conspiracy against their health and life, and their 
resisting it, and living through it, shews them to 
be of great strength of constitution. Let us now 
mention the means of purifying the internal air of 
the laboratory, and of neutralizing and destroy- 
leaves have exhaled mephitic air, whilst in the sun they ex- 
haled vital air. 
I do not think it can be necessary to make the exact calcu- 
lation of tlie deterioration of the air caused by more or less 
vegetable substance, when exposed to darkness. The two 
extremes I have just proved by the experiment, are sufficient 
to give an idea of it. 
Besides, light volatilizes any watery vapour with which 
it is in contact ; there is therefore no doubt, that all circum- 
stances equal, the air of a well-lighted laboratory will be drier 
than that of a laboratory which is dark. 
Many think that light is injurious to silk-worms. It is 
certain that in their native climate it does not injure them, 
although they are exposed to it by various circumstances ; 
however, there is here no question of exposing them to the sun, 
but only of rendering their habitations as light as our own. 
I have always observed, that on the side on which the light 
shone directly on the hurdles, the silk-worms were more nu- 
merous and stronger than in those places where the edge of 
the wicker hurdle intercepted the light, and formed a shade, 
which is my reason for having very low edges to the wicker 
trays : any body may make this observation. I have even seen 
the sun shining full on the worms, without their seeming an- 
noyed by it. If the rays had been too hot, and shone too long 
on them, they might have suffered ; but this could not occur, 
nor does it affect the question : as I do not propose exposing 
the silk-worms to the sun, but only desire to shew that the 
air is more vitiated, and that there is more damp in a dark la- 
boratory than in a light one. 
