138 Introduction to the Study of Science 



the constituents of the atmosphere is apparently not disturbed 

 by the return of the vast quantities of carbon dioxid from coal 

 and the hydrocarbons. The carbon dioxid may be absorbed, 

 and the balance thus maintained, by the rapidly increasing extent 

 and amount of vegetation which is being grown under man's 

 control. 



SUMMARY 



The candle is a miniature gas plant. 



Its flame consists of burning gas. 



Two products of combustion are water vapor and carbon dioxid. 



That water is composed of oxygen and hydrogen may be proven by 

 burning magnesium in steam. 



Water is composed of two volumes of hydrogen and one volume of 

 oxygen as shown by electrolysis. 



Hydrogen, discovered by Cavendish, burns in air, producing hy- 

 drogen oxid, or water. 



Hydrogen may be liberated from water by certain metals. 



Hydrogen is a colorless, odorless gas, much lighter than air. It 

 burns with a hot, colorless flame, but does not support combustion. 



Hydrogen may be liberated from sulfuric or other acids by zinc 

 or certain other metals. 



Hydrogen is an essential part of many compounds, such as water, 

 hydrogen peroxid, ammonia, acids, foods, and fuels. 



The presence of hydrogen in the composition of the candle and 

 other substances may be proved by the production of water when they 

 burn in the oxygen of the air. 



Carbon dioxid, another product of combustion, is a colorless, odor- 

 less gas. 



Its presence is indicated by limewater. 



By the products of its combustion the candle is proven to contain 

 carbon and hydrogen. 



Mineral matter that does not burn is left as ash from many kinds 

 of fuel. 



Smoke, consisting of invisible gases and soot, is due to imperfect 

 combustion and represents a direct loss of fuel. 



Soot is unburned carbon. It may be commercially produced and 

 sold as lampblack. 



The invisible gases of smoke contain hydrogen and carbon monoxid, 

 which are valuable fuels. 



Charcoal and coke are impure forms of carbon produced by de- 

 structive distillation of wood and bituminous coal. 



