RESPIRATION. 133 



fat, and the bile, the solid fibre of the wood, the pulp of 

 the cherry, and the flour of the grains, are all composed, in 

 a great proportion, of this substance. Hydrogen, also, is 

 a very essential element in the composition' of flesh. Com- 

 bined with oxygen, it forms water ; and in this state it is 

 found in the fluids, and in the more solid textures of the ani- 

 mal body. But it is also found, in different combinations, 

 with carbon, oxygen, and nitrogen, in the various kinds of 

 flesh. 



291. It will now be necessary to examine the composition 

 of the air. This is apparently a simple element ; nothing 

 seems purer or less compounded than air, as we breathe it. 

 But chemical analysis shows it to be composed of two ele- 

 ments, oxygen and nitrogen, in about the proportions of 

 twenty-one parts of oxygen to seventy-nine parts of nitrogen. 

 Beside these, there are generally some other gases, a little 

 carbonic acid gas, and a little vapor, amounting to one or 

 two per cent., in the atmospheric air. 



292. Oxygen is one of the most prevalent substances in 

 nature. It enters into the composition of all animal and 

 vegetable matter, and is the perpetually necessary element 

 of life, in all its forms, and in all its stages. It is the essen- 

 tial ingredient of most acids. With sulphur it forms sulphu- 

 ric acid ; with nitrogen, in large proportion, it forms nitric 

 acid, or aqua fortis ; and, in smaller proportion, atmospheric 

 air; with hydrogen, it forms water; and with carbon, car- 

 bonic acid. In air it is a gas ; in water it is a liquid. 

 When separated and alone it is a gas ; in the rust of iron it 

 is solid. 



293. Nitrogen forms about four fifths of the volume of the 

 air we breathe. It is a little lighter than air, and, of course, 

 lighter than oxygen. It unites with oxygen in several propor- 

 tions, forming very different substances, according to the pro- 

 portions of their mixture. Nothing is more mild and bland 

 than air, and few things are more caustic and harsh than 

 aqua fortis, which is a combination of the same elements. 



294 Although nitrogen and oxygen are apparently so 

 12 



