ON THE STUDY 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 

 .it 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 intelligence 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 Laurus, 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 



