CHAPTER VII 



THE BURNING OF INFLAMMABLE AIR, AND THE COMPOSITION 

 OF WATER 



Priestley (1781) notices the formation of water as a 

 product of combustion. In ancient times the theory was 

 held that all matter was derived from four " elements, "- 

 earth, air, fire, and water. The discovery of the composite 

 nature of earths (such as the metallic oxides) and of air has 

 been described in Chapter III ; in the same chapter an 

 account has been given of the experiments which proved 

 that fire is not a material substance, and cannot of itself add 

 to, or subtract from, the weight of things. The composite 

 nature of water was not suspected 1 until a later period, 

 although it was supposed to give birth to many material 

 substances, and even to vegetable and animal tissues. 2 



1 "There are, I believe, very few maxims in philosophy that have 

 laid firmer hold upon the mind, than that air, meaning atmospherical 

 air (free from various foreign matters, which were always supposed to 

 be dissolved, and intermixed with it), is a simple elementary substance, 

 indestructible and unalterable, at least, as much so as water is supposed 

 to be" (Priestley, Experiments on Air, II. 30 31). 



2 Van Helmont (1577 1644) planted a willow tree weighing five 

 pounds in 200 pounds of dry earth, and watering it with rain or 

 distilled water, found that after five years it weighed 169 Ibs., although 

 the earth had only lost two ounces. He knew nothing of the air as a 

 source of the combustible part of a plant, and concluded that 164 

 pounds of wood, bark, roots, etc., had been produced from the water 

 alone. 



