COMPOSITION OF A DROP OF DEW. 39 



which limit this tendency to unite among the 

 elements in a very simple and remarkable 

 manner. A drop of dew supplies us with an 

 excellent illustration of the operation of these 

 controlling laws, by which we may hope to 

 render their action readily intelligible. This 

 drop of dew consists of two gases, oxygen and 

 hydrogen, which are chemically united into 

 one substance the water. Having obtained 

 the same gases by chemical means, let us mix 

 them together ; if we then set fire to the mix- 

 ture there follows a great explosion, and we 

 find the jar in which the gases were contained 

 no longer filled with air, while drops of water 

 bedew its sides.* Let the reader now ask 

 himself, Why is this? Why are we quite sure 

 that on mixing these gases thus together, and 

 firing them, we shall produce water? Why 

 not something else ? The reason is, that although 

 these gases have a powerful tendency to unite 

 together, there are certain laws which compel 

 them to unite in a certain manner, and to 

 produce, so long as they do so, a certain 

 result. If these laws did not exist, it would 

 be impossible for us to tell what would be 

 produced when we mixed and lighted the 



* This experiment should only be performed with small 

 quantities of the gases, and the jar should be thick and 

 strong, and covered, all but the mouth, with a coarse cloth. . 



