88 PRINCIPLES OF CHEMISTRY. 



in a glass tube and heated, it will be found to disappear gradually, 

 and we might assume that it has been converted into a gas from 

 which, upon cooling, the red oxide of mercury would be re-obtained. 

 If the apparatus for heating the oxide of mercury be so constructed 

 that the escaping gases may be collected and cooled, we shall not find 

 the red oxide in our receiver, but in its place a colorless gas, while at 

 the same time globules of metallic mercury will be found deposited 

 in the cooler parts of the apparatus (Fig. 35). 



The action of heat consequently has in this case produced an effect 

 entirely different from the effects spoken of heretofore. There is no 

 doubt that the first action of the heat upon the oxide of mercury is 

 an increased velocity of the motion of its molecules and simulta- 

 neously an increase of its volume, but afterward a decomposition of 

 the oxide takes place, and two substances are liberated, each different 

 from the oxide. 



One of these substances is a silvery-w T hite, heavy, liquid metal, the 

 mercury ; the other substance is a colorless, odorless gas, which sup- 

 ports combustion much more freely than does atmospheric air, and is 

 known as oxygen. 



Elements. We have thus succeeded in proving that red oxide of 

 mercury may be converted or decomposed by the mere action of heat 

 into mercury and oxygen. It is but natural to inquire whether it 

 would be possible further to subdivide the mercury or the oxygen 

 into two or more new substances of different properties. To this 

 question, which has been experimentally propounded to Nature over 

 and over again, we have but one answer, viz. : Oxygen and mercury 

 are substances incapable of decomposition by any method or means 

 as yet known to us. They resist the powerful influences of electricity 

 and heat, even when raised to the highest attainable degrees of in- 

 tensity, and they issue unchanged from every variety of reaction 

 hitherto devised with the view of resolving them into simpler forms 

 of matter. 



Therefore we are justified in regarding oxygen and mercury as non- 

 decomposable or simple substances, in contradistinction to compound 

 or decomposable substances, such as the red oxide of mercury. 



All substances which cannot by any known means be resolved into 

 simpler forms of matter, are called elements; all substances which 

 may, by one process or another, be subdivided or decomposed in such 

 a manner that new substances with new properties are formed, are 

 called compound substances or compounds. 



