102 PRACTICAL LESSONS IN SCIENCE. 



when bodies burned the phlogiston made its escape, and the resi- 

 due was regarded as the substance with which the phlogiston 

 had been combined. The British chemist, Black, about 1750, 

 began to use the balance as an aid in chemical investigation. 



About 1774 Dr. Joseph Priestly discovered and isolated 

 oxygen, but called it dephlogistigated air. Scheele, a Swedish 

 chemist, discovered oxygen the next year, and the French chem- 

 ist, Lavoisier, claimed to have discovered the same gas independ- 

 ently of the others. He at least understood the nature of the 

 gas better than the others, and in 1781 gave it its present name, 

 oxygen, " acid-former," as he had recognized that it was an 

 essential ingredient in several important acids. Lavoisier pointed 

 out the fallacies of the phlogiston theory, and gave the received 

 explanation of combustion. He also introduced system into 

 chemistry and chemical research, and by many is called the 

 father of modern chemistry. 



The discovery of oxygen has been called the capital discovery 

 of the last century, rivaling in importance the great discovery 

 of Newton in the previous one. It formed one of the great eras 

 in human progress. It resulted in gradually putting away old 

 and fanciful theories, the outgrowth of limited knowledge, and 

 laid the foundation of modern chemical science. It opened a 

 way to a knowledge of the composition of the air and of water, 

 of plants and animals. It may be said to have changed the 

 manners, customs and business habits of the world. More than 

 anything else it has explained the phenomena and opened up 

 the resources of the material universe. 



The introduction of instruments for weighing has been of im- 

 mense value in chemical science, even rivaling the discovery of 

 oxygen. One of the greatest discoveries of modern times is the 

 truth that nature works with the same exactness on the small 

 scale as on the large. It is the glory of Newton to have proved 

 that the movements of all celestial and terrestrial bodies are 

 regulated by mathematical law. "It has been established by 

 chemists that the minutest particles of matter, in their actions 

 and reactions, obey law, and that every chemical compound has 



