COMBUSTION 211 



remains that the weight of all the substances resulting from 

 combustion is exactly equal to the weight of the original sub- 

 stance plus the oxygen which has united with it. 



Here, then, we have a statement of the modern explana- 

 tion of combustion. It is an explanation which rests upon 

 good evidence, for the products of combustion have repeat- 

 edly been carefully collected and weighed with results as 

 indicated. You have already learned in your study of air 

 that gas has weight which can be measured. So the 

 gaseous, liquid, and solid substances which result from 

 combustion, when all weighed, and their weights added, 

 have always been found to exceed the weight of the original 

 substance, the difference being the weight of the oxygen 

 which has been taken on. 



Inasmuch as the proof of this explanation of combustion 

 depends upon very careful weighings, it does not surprise 

 you to learn that this proof was one of the results which 

 followed the perfection of balance scales, scales so perfected 

 that they record the most delicate differences in weight. 

 If the older scientists had had better scales, the right 

 explanation of combustion would doubtless be much older 

 than it is. So it has been with many of the greatest dis- 

 coveries of science; they have resulted from the perfection 

 of some mechanical device which enables man to get more 

 evidence than he could get before. Thus the develop- 

 ment of modern biology had to await the perfection of the 

 microscope. To the physicist and chemist, delicate bal- 

 ances are quite as important as the microscope is to the 

 biologist or the telescope is to the astronomer. One of 

 the first discoveries made by the use of a delicate balance 

 was that metal oxides, such as iron oxide, weigh more than 

 the metals from which they are formed. In the light of 



