DEFINITIONS AND DESCRIPTIONS. 47 



gen, carbon and nitrogen. These are called the organic constitu- 

 ents of plants, because when any jjortion of vegetable matter is 

 burned, it either disappears entirely, or leaves behind a small 

 quantity of ash. 



15. The ash is composed of several simple bodies, and hence 

 these latter are caHed the inorganic constituents of plants. 



Sonje knowledge of the organic constituents of plants, ap- 

 pears to be necessary for understanding the subject of this sec- 

 tion, and, for the information of those who have not attended to 

 elementary chemistry, a short description of them is here in- 

 serted. 



1. Oxygen is found in the state of a gas in the atmosphere, 

 mixed with nitrogen, and constitutes one fifth part of its vol- 

 ume ; eight-ninths of water by weight is also oxygen gas. Be- 

 side this, the whole crust of the globe is composed of oxydized 

 substances, that is, of substances combined with oxygen. 



In its pure state, oxygen is a transparent gas, without color, 

 odor or taste, and is a little heavier than the air. It unites 

 chenjically with a great number of substances. If a lighted ta- 

 per is plunged into it, the brilliancy of thd flame is much in- 

 creased, and if heated iron be immersed in a jar of pure gas, 

 the combustion is so intense as to melt and burn the iron. This 

 substance is always one of the agents in all our fires and lights ; 

 hence its importance. Oxygen also is the supporter of the res- 

 piration of animals. No animal can live for any length of time 

 without it.* It is no less essential to the existence of the veg- 

 etable kingdom. 



2. Hydrogen is chiefly found in water, forming one-ninth part, 

 from which it may be obtained by putting into it iron or zinc 

 turnings and sulphuric acid. It is found in most liquids, and 

 in all animai and vegetable bodies. 



Hydrogen in its pure state exists in the form of a gas, no way 

 distinguished in its physical properties from oxygen, with the 

 exception of its being sixteen times lighter, and a much more 

 pow^erful refractor of light. When a lighted taper is inmiersed 

 in it, the hydrogen is set on fire, but the taper is extinguished. 

 If air or oxygen gas is mixed with it, and the flame of a candle 

 brought in contact, the mixture will explode, and the product 

 will be water. Animals are suffocated by it, and balloons ai-e 

 made to ascend. 



Water. One part of hydrogen and eight of oxygen, by weight, 



^ See Gray's Chemistry, p. 131. 



