16 NATURE OF MATTERS RECEIVED. 



ganic systems, whether vegetable or animal, whether humble or elaborate- 

 An animal ere- \y developed, possess no power of creating material. Their 

 buToSy trfns- function is of necessity limited to the mere transformation of 



forms the sub- substances furnished to them. From this it follows, even in 

 ceives. the case of man, that the substances dismissed from the sys- 



tem are metamorphosed forms of those which have been received, and 

 that, whatever their appearance may be, they must have arisen from the 

 reaction of the food, water, and air upon one another. 



This reaction we may proceed to view as a purely chemical result ; 

 for, casting aside all the vain hypotheses of the older physiology, and per- 

 mitting ourselves to be guided by the harmonies of nature, we should ex- 

 pect to recognize in the changes taking place in organic systems, and in 

 the phenomena which attend those changes, the same results which arise 

 in the artificial or experimental reaction of food, water, and air on each 

 other. A very superficial examination of the facts shows at once the 

 The chemical correctness of this expectation. On such an examination we 

 mauersre-^ now enter > premising it with some general remarks needful for 

 ceived. our purpose on the nature and properties of food, water, and air. 



1st. OF FOOD. No article is suitable for food except it be of a com- 

 bustible nature. Its chemical constitution must be such that if its tem- 

 perature be raised to a proper degree with a due access of atmospheric air 

 it will take fire and burn, and the products of its combustion must be car- 

 bonic acid gas and water, or those substances with nitrogen or its com- 

 pounds. 



2d. OF WATER. This may be taken as the type and representative 

 of all the various liquids used as drinks. It evaporates at any tempera- 

 ture, even at those which are lower than its freezing point, and in this 

 evaporation produces cold. Water vaporizing from the skin absorbs 1114 

 degrees of heat, and hence exerts a most powerful refrigerating action. 

 Over saline substances there are few bodies which exercise so general a 

 solvent effect. In virtue of this property, it is enabled to introduce in 

 the dissolved state such compounds as are wanted for the nutrition of the 

 system, and in the same manner to carry away the wasted products of 

 decay. 



3d. OF ATMOSPHERIC AIR. The active principle of the air is oxygen 

 gas, the effects of which are moderated by the presence of a large quanti- 

 ty of nitrogen- four fifths of the air consisting of this latter substance. 

 Physiologically, we often use the terms atmospheric air and oxygen syn- 

 oifymously. 



The chief materials which a living being receives from the external 

 world are, therefore, COMBUSTIBLE MATTER, WATER, OXYGEN GAS ; and out 

 of the action of these upon one another all the physical phenomena of its 

 life arise. 



