106 CHEMISTRY. [Lesson IV. 



part of the oxygen of the air within the 

 glass. Then the carbon of the candle 

 united with a part of the oxygen of the air, 

 and formed carbonic acid gas, which is not 

 a supporter of combustion ; and as the 

 oxygen, the chief supporter of combustion, 

 was withdrawn, the candle went out. 



i'.AL QUESTIONS ON LESSON III. 

 1. Can we burn any substance ? 

 '1. What is the difference betweeft com- 

 bustible and incombustible substances ? 

 '.. What is incandescence ? 



a substances be destroyed .' 



LESSON IV. 



WE have seen that the oxygen of the air is a supporter of combustion, and hence it 

 follows, that in order to maintain combustion, a fresh supply of air must be established, 

 and the smoke allowed to escape. It is therefore evident, that combustion, after all, 

 is nothing more than a process of oxidation, or combination of oxygen with the body 

 consumed. Now combination is not merely mixing, it is more ; it is the intimate and 

 close union of substances. For example, [Experiment 11] we have some oil in this 

 bottle, and when I add some water to it, you will see that the two substances mix, but 

 they do not combine, because when they have stood a short time, the oil will separate 

 itself from the water. [The bottle is allowed to stand, and the oil is then seen floating 

 on the top of the water]. I will cause these two bodies to combine, by adding another 

 substance potash, and then we shall get a soapy compound, differing from the oil, the 

 water, or the potash. 



The elevation of temperature generally promotes the chemical action of bodies, but 

 nevertheless, the effect will depend, in certain cases, on the degree of heat adapted for 

 particular purposes. For example, if we expose mercury in a proper vessel, and in 

 contact with the air to a heat of 680 Fahrenheit, it becomes converted into the red 

 oxide of mercury. If we expose the red oxide of mercury to 980 Fahrenheit, the 

 mercury will be reproduced in its metallic state, and a gas given off, which is oxygen. 

 This naturally leads us to reflect, what caused the red oxide of mercury to resolve 

 itself into the metal, mercury and the gas, oxygen? Heat. It could not have 

 been effected by any other means than heat. We therefore learn that heat is an 

 important agent in chemical combination. 



When elements unite with elements, chemists regard the union as combinations of 

 ihejirst order as acids and bases. When these combine, to form sails, we have com- 

 binations of the second order. When we find double salts resulting from the union of 

 salts with salts, we regard them as combinations of the third order. Bodies do not 

 generally combine chemically, otherwise than in fixed proportions ; hence every body, 

 the result of chemical combination, generally contains fixed proportions. For example, 

 water always consists of one atom of hydrogen, and one atom of oxygen. 



[The pupil should read Lessons III., IV., and V., of Natural Philosophy, before entering upon 

 the question of this Lesson.] 



QUESTIONS. 



37. T. What is the atomic theory ? 

 P. It is the theory which explains the 

 manner in which the atoms of bodies 



combine chemically in certain proportions 

 to form new substances. For example, we 

 know that an atom of hydrogen and an 



