ALLOTROPISM AND ISOMERISM. 



The two substances by their union become 

 hot enough to heat the contents of the inner 

 vessel. 



More remarkable still is the effect of pouring 

 water upon baryta or barium monoxide, a sub- 

 stance of very similar constitution to that of 

 quicklime. If a glass vessel, not liable to crack, 

 say a beaker of about four inches diameter, 

 be filled with lumps of baryta which have been 

 kept quite free from moisture, and water be 

 poured into the intervening spaces, the heat 

 developed by the union will soon make it 

 boil ; and presently through the water and 

 glass some of the pieces can be seen to be at a 

 bright red heat ; so intense is the fire produced 

 by the application of cold water. 



Similar heat has been seen developed in 

 enthusiasts when cold water is thrown on their 

 favourite schemes. 



3. Paradoxes of Substance and Form. 



The very few people who nowadays have 

 learned no chemistry are much astonished on 

 learning what widely different forms the same 

 element can take. 



It is strange enough at first, though we soon 

 get used to it, to find how completely an ele- 

 ment can have its nature changed by combina- 

 tion with other elements. The oxygen which 

 we breathe as a gas in the air forms, in combina- 



207 



