38 SMITH'S INTERMEDIATE CHEMISTRY 



The proportion of copper to oxygen, so far as this one measure* 

 ment goes, is therefore 85 : 21. 



The results of quantitative experiments are often recorded in 

 the form of parts in one hundred. To find the percentage of each 

 constituent, we observe that the proportion of copper is 85 : 85 -f- 

 21, or T 8 ^ of the whole. That of the oxygen is T y^ of the whole. 

 Thus the percentages are: 



Copper, 106 : 85 :: 100 : x. z> 80.2. 

 Oxygen, 106 : 21 :: 100 : x f . x f = 19.8. 



Naturally, the mean of the results of a number of more carefully 

 managed experiments will be nearer the true proportion. The 

 percentages at present accepted as most accurate are 79.9 and 20.1. 



In the case of mercuric oxide, we may decompose a known 

 weight of the oxide (p. 16), collect the mercury and weigh it, and 

 ascertain the oxygen by difference. 



The names of the constituent elements in a compound, together 

 with the proportion by weight in which they are present, are called 

 the composition of the substance. Thus, the composition of 

 cupric oxide is copper : oxygen :: 79.9 : 20.1. This is the per- 

 centage composition, but other numbers expressing the same pro- 

 portion (such as 63.57 : 16) will serve the purpose. 



All experiments involving measurement, such as those used in 

 determining composition, are called quantitative experiments. 



Another Quantitative Experiment. The following will 

 show how the combining proportions may be measured when the 

 product is a gas, the weight of which must be ascertained. Sul- 

 phur burns in oxygen to form sulphur dioxide. A known weight 

 of sulphur is placed in a porcelain boat in a hard-glass combustion 

 tube (Fig. 22). The U-shaped tube to the right contains a solu- 

 tion of potassium hydroxide, which is capable of absorbing the 

 resulting gas. The oxygen enters from the left. When the sul- 

 phur is heated, it burns in the oxygen, and the gain in weight of 



