Fire and Heat 129 



constitute such substances as natural gas, coal gas, coal tar, 

 and petroleum with its derivative products, such as kerosene, 

 gasoline, naphtha, benzine, and paraffin. The compounds of 

 this group are generally designated as hydrocarbons. These 

 are all very combustible, and because of the large proportion of 

 hydrogen they contain, yield an especially large quantity of 

 heat. Some of them, such as gasoline, benzine, and naphtha, 

 are so highly combustible as to be explosive; they contain 

 unusually large proportions of volatile hydrocarbons. 



Many useful compounds are formed of carbon, hydrogen, 

 and oxygen, as for example, wood, lignite, peat, coal, and 

 alcohol. The table on page 144 shows the percentage of hy- 

 drogen and carbon in these substances and also in those grouped 

 under the hydrocarbons. 



Nature often works in seemingly contradictory ways. By 

 a change in the proportions of the elements, a very different 

 series of substances results. Carbon combines with hydrogen 

 and oxygen to produce such familiar compounds as the various 

 kinds of sugar and starch. These are fuels, but of a class quite 

 different from that given above. They make up a large part 

 of the fuel or food supply of mankind. This group of carbon 

 compounds is called the carbohydrates, the oxygen and hy- 

 drogen being united in the same ratio as in water. The same 

 three elements are combined, moreover, in such common 

 substances as vinegar, acetic acid, citric acid abundant in 

 lemons and oranges, and tartaric acid especially abundant in 

 grapes. 



51. Carbon and hydrogen in paraffin. The products of 

 the burning candle have been captured and the secret of their 

 immediate source discovered. Water and carbon dioxid have 

 revealed the elements of which they are composed. The little 

 ash from the burned wick may be neglected. But a new problem 

 is presented. How can the candle be composed of carbon and 

 hydrogen? Carbon as shown by soot or charcoal is black. 

 Hydrogen is invisible as a gas and does not become a liquid or 



