20 



SMITH'S INTERMEDIATE CHEMISTRY 



mercury liberated, almost exactly 8 parts of oxygen by weight 

 are set free. Using the numbers commonly employed in chemis- 

 try, which represent the same proportion by weight: 



Mercuric oxide (216.6) 



Mercury (200.6) -f 



Oxygen (16). 



We find also that mercury and oxygen can be 

 made to combine to form mercuric oxide, and 

 the proportions by weight required are the same. 

 Moreover, every sample of mercuric oxide, whether 

 made by combination, or in any of the other pos- 

 sible ways, always contains this proportion of the 

 two elements. We have already seen that the 

 oxides of lead and tin contain fixed proportions 

 (p. 9) of the metal and oxygen and that ferrous 

 sulphide has a constant composition by weight. 

 The same principle is found to apply to all 

 chemical compounds, and is stated in the law of 

 definite or constant proportions : In every sample 

 of any compound substance, formed or decom- 

 posed, the proportion by weight of the constituent 

 elements is always the same. 



Certain elements have recently been shown to 

 exist in two or more forms (isotopes), which would, 

 if separable, give compounds possessing the same 

 specific properties yet differing in composition. Detailed discussion 

 of this point must be deferred to a later chapter (pp. 550-552). 



Hro. 

 TIT. 



Fia. 12 



Conservation of Mass. The most painstaking chemical 

 work seems to show that, if all the substances concerned in a 

 chemical change are weighed before and after the change, there is 

 no evidence of any alteration in the quantity of matter. The two 

 weights, representing the sums of the constituents and of the 

 products, respectively, are, indeed, never absolutely identical, but 



