78 ELEMENTARY PHYSIOLOGY 



bustion of carbon ; the element burnt in each case combining with 

 the oxygen of the air. Nearly all combustible substances contain 

 both carbon and hydrogen ; hence, when these substances burn, 

 they produce both carbonic acid gas and water. We may illustrate 

 this in the following way : 



Hold a dry tumbler over the flame of a candle, and the inside of the glass 

 will be quickly covered with a deposit of water formed by the combustion of 

 the hydrogen in the candle. 



Repeat the above experiment, and then remove the tumbler, closing it 

 quickly with a glass plate or a piece of cardboard, so as to retain some of the 

 gas produced by the burning of the candle. Now open it just a little, pour in 

 a little clear lime-water, close again quickly, and shake it. The lime-water 

 turns milky, which proves the presence of carbonic acid gas. 



The heat of the candle-flame is due to the combustion of 

 carbon and hydrogen ; and the carbonic acid gas and water formed 

 may be termed the products of combustion. 



Organic substances (animal and vegetable substances) are 

 generally prone to decay and putrefaction. This decay is a 

 chemical change by which the organic substance is decomposed 

 into gases which pass off into the air. Thus, a piece of animal 

 flesh contains the elements oxygen, carbon, hydrogen, nitrogen, 

 and small quantities of phosphorus and sulphur. And if this flesh 

 be allowed to putrefy, the following gases are slowly given off into 

 the air : 



Water vapour, composed of hydrogen and oxygen, 



Carbonic acid gas, carbon and oxygen, 



Ammonia, nitrogen and hydrogen, 



Carburetted hydrogen, carbon and hydrogen, 



Sulphuretted hydrogen, sulphur and hydrogen. 



A certain proportion of mineral matter enters into the com- 

 position of all parts of the body, but the proportion varies con- 

 siderably in the various organs and tissues. The amount of mineral 

 substance in any part may be determined by completely burning 

 that part, till nothing but a white ash remains. This ash is the 

 mineral or inorganic substance, all the animal or organic matter 

 having been changed into gases during the combustion. 



Every tissue and fluid of the body is composed of a number 

 of organic and inorganic compounds. 



The organic compounds of the body may be classified into 



(1) Non-nitrogenous, containing carbon, hydrogen, and oxygen 

 only. 



(2) Nitrogenous, containing nitrogen, as well as carbon, 

 hydrogen, and oxygen. 



The non-nitrogenous compounds include/aft and oils, amyloids, 

 and certain organic acids. 



