RESPIRATION. 53 



The air we breathe (called atmospheric air) is a compound of about one 

 part of ox'ygen gas to four parts of ni'trogen gas, and a very much smaller 

 proportion of carbo'nic acid gas, together with some watery vapour. 



Ox'ygen and ni'trogen are simple substances, that is, chemists have not 

 been able to decompose them ; but carbonic acid gas is a compound sub- 

 stance, that is, it consists of more than one material or substance. 



This name, ox'ygen, is formed from the Greek, oxus, acid, and geinomai, 

 I beget, and was so called because it was believed, without it, there could 

 be no acid. Although there are acids which contain no ox'ygen, we know- 

 that without its presence every living thing, animal or plant, would die, and 

 all fire would be extinguished. It is indispensable to respiration and com- 

 bustion, j 



The word ni'trogen was formed from the Greek, nitron, nitre, and 

 geinomai, I beget, because it was discovered to be one of the essential con- 

 stituents of nitre, and also of nitric acid. It was also called azote (from </, 

 privative, and zoe, life), because it would not support animal life. 



Carbo'nic acid consists of carbon and ox'ygen. 



Carbon (from the Latin, carbo, coal) is the name of a simple substance 

 or element. It occurs naturally in the form of the diamond (which is pure 

 carbon), of plumbago or black-lead, anthracite and bituminous coals ; it is 

 an elementary constituent of all wood ; it seems to be the true food of plants, 

 without which they die. Lamp-black and charcoal are forms of impure 

 carbon. The chief action of vegetable organization is to obtain and form 

 carbon. 



Carbo'nic acid exists in the atmosphere as the product of combustion, and 

 of the respiration of animals ; the frothing of beer, and the sparkling of 

 champagne and " mineral water," depend on its presence.] 



36. The leaves and other green parts of plants also absorb 

 the carbonic acid gas contained in the air, and by the process of 

 respiration, this fluid, as well as the carbonic acid formed in the 

 interior of the plant, is decomposed ; its carbon remains in the 

 tissue of the plant, and nourishes it, while the oxygen is thrown 

 off and mingles with the atmosphere. 



37. We now see that the relations of plants with the air are 

 more complicated* than those of animals with the same fluid. 

 The latter absorb oxygen, and in its place exhale carbo'nic acid ; 

 plants absorb ox'ygen and carbo'nic acid, and exhale the ox'ygen 

 arising either from the quantity of this gas previously absorbed, 

 or from the decomposition of the carbo'nic acid derived from the 

 atmosphere. 



38. In general it is the last phenomenon, that is, the absorp* 

 tion of carbonic acid, its decomposition and the exhalation of 

 ox'ygen, that is designated under the name of respiration of 

 plants. Its effect, as we see, is to destroy the carbo'nic acid, 



36. What parts of plants absorb carbo'nic acid gas from the atmospheric 

 air ? What becomes of the constituent elements of the carbo'nic acid of 

 plants ? 



37. How does the respiration of animals differ from that of plants ? 



38. What constitutes the respiration of plants ? What is the effect of 

 the respiration of plants ? How does it purify the atmosphere ? 



5* 



