ON THE STUDY OF PHYSICS. 289 



have a transparent solid mixed with air. At every 

 transition from solid to air, or from air to solid, a por- 

 tion of light is reflected, and this takes place so often 

 that the light is wholly intercepted. Thus from the 

 mixture of two transparent bodies we obtain an opaque 

 one. Now the case of the towel is precisely similar. 

 The tissue is composed of semi-transparent vegetable 

 fibres, with the interstices between them filled with 

 air ; repeated reflection takes place at the limiting 

 surfaces of air and fibre, 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 deport- 

 ment 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 enquiry 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 Lauras, 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 pheno- 

 menon to which his question refers, without first 

 making him acquainted with the radiation and con- 

 duction of heat. Take, for example, a blade of grass, 

 from which one of these .orient pearls is depending, 



