Oxygen can combine with hydrogen in but two proportions--peroxid of hydrogen (H₂O₂) and water (H₂O)--while carbon and hydrogen can combine in more than a hundred different compounds. The simpler of these are acetylene (C₂H₂) and marsh gas or methane (CH₄), which is the fire-damp in mines. The compounds containing carbon, hydrogen, and oxygen number into the thousands. A great many substances formed in plants contain these three elements, such as fruit-acids, alcohol, sugar, and fats. CLASSIFICATION OF ORGANIC CARBON COMPOUNDS Only a few of the most important groups of the organic or life-formed carbon compounds will be considered in this work, namely: a Hydrocarbons b Alcohols c Glycerin d Aldehydes and ethers e Organic acids f Carbohydrates g Fats a HYDROCARBONS [Sidenote: Uses of hydrocarbons in industrial chemistry] Hydrocarbons are compounds of the two elements carbon and hydrogen. These compounds are very important in industrial chemistry. They are found in petroleum, coal-tar, etc., which were originally formed from decaying and petrifying masses of plants. Gasoline, benzin, naphtha, acetylene, methane, etc., are some of the industrial forms by which hydrocarbons are known in commerce. [Sidenote: Coal-tar products] The industries based upon the chemistry of these hydrocarbons are very complex and interesting. Coal-tar yields, by repeated distillation and chemical reaction, thousands of compounds, many of which find important industrial usages. Coal-tar dyes are very numerous and of wonderful coloring power. They have been extensively used in the artificial coloring of manufactured foods. The Federal Pure Food Law attempted to prohibit this. In fact, it was the pernicious effect and extensive use of these poisons that stimulated the passage of the "Food and Drugs Act." Another interesting product of the coal-tar industry is saccharin. Saccharin has no food value whatever, but it is 280 times sweeter than cane-sugar, and is therefore used as a substitute in sweetening some prepared foods. b ALCOHOLS [Sidenote: Varieties of alcohol] To the ordinary mind the term alcohol refers only to the intoxicating element in liquors. To the chemist, alcohol has a much broader significance. There are many varieties of alcohols, of which ethyl alcohol (C₂H₅.HO), which is found in liquors, is only one example. Another form of alcohol which is fairly well known is wood or methyl alcohol (CH₃.OH). [Sidenote: Formation of higher alcohols]