PREFACE. 



NATURE is a great conjurer. With many of 

 her tricks we are so familiar that they do not 

 astonish us ; but on giving them a little con- 

 sideration we often find that they are really 

 very puzzling performances, and we become as 

 eager to learn the explanation as children at a 

 Christmas conjuring party to hear the per- 

 former's account of how he does it, or children 

 of an older growth to learn from the political 

 prestidigitator at the head of the opposite 

 party how he reconciles his action with his 

 principles. 



Equally interesting puzzles occur in the 

 course of experimental work in the laboratory ; 

 but in this book the choice will be limited, so 

 far as is reasonably possible, to such experi- 

 ments as can be readily performed, with very 

 simple apparatus, at home ; and the form of 



