PHYSICAL FORCES 131 



odd quantity left over will remain unchanged, alongside 

 of the sulphide produced. 



There are certain substances which are termed " ele- 

 mentary " and chemical " elements" because they do not 

 seem capable of being resolved into other substances, 

 and some of these elements have an overpowering 

 attraction for each other. We have already noted that 

 which exists between iron and oxygen, which results in 

 the "rust" or "oxide of iron." Similarly, a metal, 

 calcium, and oxygen will rush together and produce a 

 rust, or oxide of calcium, which is lime. Oxygen and 

 another metal, termed silicon, will unite to produce a 

 rust known as oxide of silicon, which is flint, or silica. 

 Similarly one component of clay, namely alumina, is an 

 oxide of yet another metal aluminium. 



Not only elements, but various compound substances 

 (so called because capable of being resolved into others), 

 when placed in close proximity to certain other sub- 

 stances, will undergo a spontaneous transformation and, 

 as it were, exchange partners ; and this is often facilitated 

 by their being dissolved in water. Thus in the case of the 

 common effervescing powders before referred to, we may 

 have one powder consisting of carbonate of soda (which 

 can be resolved into carbonic acid and soda)* and the 

 other powder formed of the acid of lemons, or citric acid. 

 When these are simultaneously dissolved, the citric acid 

 will seize upon and unite with the soda, while the car- 

 bonic acid of the carbonate of soda is set free and escapes 

 in a gaseous form in the bubbles of the effervescence. 



It may be that a substance resolvable into two ele- 

 ments, may be robbed of one of its elements by a third 

 element placed in its vicinity, so producing a new and 



* See post, p. 140. 



