ELEMENTS. COMPOUNDS. LAWS 139 



the iron in the same condition as before mixing. When a portion of 

 the mixture of iron and sulphur is heated in a covered crucible, it is 

 found that a new substance is obtained which possesses neither the 

 properties of iron nor of sulphur. It is not attracted by a magnet and 

 it does not burn when heated. Nor can the iron or sulphur be recovered 

 from it by simple means. A distinction must be made between such a 

 mixture and the new substance which results on heating it. 



A complex substance, or compound, may be denned as a new substance 

 formed by the union of two or more elements, and from each of which it 

 differs more or less completely in properties. 



Compounds are homogeneous, i.e. each particle of them, no matter 

 how small, is the same in composition and properties as every other 

 particle. Another property of compounds is dealt with in Section 64. 



64. LAW OF CONSTANT COMPOSITION. 



In the course of experimental work, in which the substances used 

 have been weighed, it has been noticed that frequently each member 

 of the class obtained the same result or a result differing very little 

 from the mean class result. This is evidently more than a coincidence. 

 The following results were obtained : 



Washing soda contains 63 per cent, water and 37 per cent, anhydrous 



Blue vitriol contains 36 per cent, water and ,54 per cent, anhydrous 

 vitriol. 



Magnesium oxide contains 60 per cent, metal and 40 per cent, oxygen. 



More accurate work than can be carried out in class has shown 

 that these and other pure substances always contain their constituents 

 in exactly the same definite proportions by weight. When facts so 

 invariable as these are observed they are stated as a law. It is important 

 to bear in mind that a Law in Science is simply a statement made as 

 a result of observations, which have never been found to be contra- 

 dicted, and it is believed to be true for other substances although they 

 may not have been fully examined. 



Law of definite proportions, or constant composition. " The proportion 

 by weight of each element in a compound is fixed and constant." 



65. LAW OF INDESTRUCTIBILITY OF MATTER. 



i. The burning of phosphorus in an enclosed volume of air. Take 

 a 500 c.c. round- bottomed flask and in it place a small piece of phosphorus 

 which has been carefully dried on a piece of filter paper, dose the flask 



