ON" THE! STUD 7 OF PHYSICS. 221 
In the case of snow, powdered quartz, or salt, we have a 
transparent solid mixed with air. At every transition 
from solid to air, or from air to solid, a portion of light is 
reflected, and this takes place so often that the light is 
wholly intercepted. Thus from the mixture of two trans- 
parent bodies we obtain an opaque one. Now the case of 
the towel is precisely similar. The tissue is composed of 
semi-transparent vegetable fibers, with the interstices be- 
tween them filled with air; repeated reflection takes place 
at the limiting surfaces of air and fiber, and hence the 
towel becomes opaque like snow or salt. But if we fill the 
interstices with water, we diminish the reflection; a portion 
of the light is transmitted, and the darkness of the towel is 
due to its increased transparency. Thus the deportment of 
various minerals, such as hydrophane and tabasheer, the 
transparency of tracing paper used by engineers, and many 
other considerations of the highest scientific interest, are 
involved in the simple inquiry of this unsuspecting little 
boy. 
Again, take the question regarding the rising or falling 
of the dew a question long agitated, and finally set at rest 
by the beautiful researches of Wells. I do not think that 
any boy of average intelligeiiee will be satisfied with the 
simple answer that the dew falls. He will wish to learn 
how you know that it falls, and, if acquainted with the 
notions of the middle ages, he may refer to the opinion of 
Father Laurns, that a goose egg filled in the morning with 
dew and exposed to the sun, will rise like a balloon a 
swan's egg being better for the experiment than a goose 
egg. It is impossible to give the boy a clear notion of the 
beautiful phenomenon to which his question refers, with- 
out first making him acquainted with the radiation and 
conduction of heat. Take, for example, a blade of grass, 
from which one of these orient pearls is depending. During 
the day the grass, and the earth beneath it, possess a 
certain amount of warmth imparted by the sun; during a 
serene night, heat is radiated from the surface of the grass 
into space, and to supply the loss, there is a flow of heat 
from the earth to the blade. Thus the blade loses heat by 
radiation, and gains heat by conduction. Now, in the 
case before us, the power of radiation is great, whereas the 
power of conduction is small; the consequence is that the 
blade loses more than it gains, and hence becomes more 
