2O Elementary Biology. 



kind. It has been already stated, that there are about seventy 

 such elements at present known (footnote, p. 9). A com- 

 pound has also been defined as composed of two or more 

 elements. A distinction must be carefully drawn between 

 a compound and a mixture. In a compound each element, 

 or constituent, loses its own peculiar characters, whilst in a 

 mixture each constituent retains its own characters. Thus, 

 for example, hydrogen is a colourless, tasteless, odourless 

 gas, extremely light and inflammable, and possessed also of 

 certain other chemical peculiarities that do not require to 

 be specialised here. Oxygen is also a colourless, odourless, 

 tasteless gas, sixteen times as heavy as hydrogen, not inflam- 

 mable, but a supporter of combustion. If certain definite 

 quantities of these two gases be mixed in a vessel, it is found 

 that the hydrogen in the vessel still retains all the properties 

 of hydrogen, and the oxygen all the properties of oxygen. 

 If, however, a light be applied to the mixture, an explosion 

 immediately takes place, both gases disappear as such, and 

 in their place is left a compound gas, water vapour, which on 

 analysis is found to contain exactly the same amount of 

 hydrogen and oxygen which went to form the mixture, but 

 which differs entirely in properties from either of its consti- 

 tuents. The force which tends to bring about this union is 

 known as the force of chemical affinity, and is one of the 

 most important sources of energy known to us. It may be 

 defined in the following terms : the force which tends to 

 make certain chemical elements or compounds unite with 

 certain other elements or compounds in definite proportions, 

 so as to lead to the formation of a new substance, or sub- 

 stances, with different properties from those of the indivi- 

 dual constituents. 



The original position of separation of the constituents of 

 a compound, previous to combination, is energy of position, 

 or potential energy. The supply of an equal amount of 

 energy to that given off at decomposition is necessary to 

 effect recombination. 



