HYDROCHLORIC ACID 



HYDROGEN CHLORIDE 



great classes known as aliphatic and aromatic. 

 Methane, acetylene and the petroleum hydro- 

 carbons belong to the former group, taking 

 its name from the Greek word aliphe, meaning 

 fat, fats being derivatives of hydrocarbons of 

 this class. The coal-tar products belong to 

 the aromatic group (aroma, a pleasant odor). 

 Examples are benzene (a different substance 

 from benzine), naphthalene (familiar as moth- 

 balls), and anthracene. It is principally from 

 these aromatic hydrocarbons that coal-tar dyes 

 are made. J.F.S. 



Related Subjects. The reader is referred to 

 the following articles in these volumes : 

 Acetylene Coal 



Aniline Coal Tar 



Chemistry 



HYDROCHLORIC ACID , hi dro klo ' rik as' id. 

 See HYDROGEN CHLORIDE. 



HYDROCYANIC , hi dro si an ' ik, ACID . See 

 PRUSSIC ACID. 



HYDROFLUORIC , hi dro floo or ' ik, ACID , 

 the only known compound o^ hydrogen and 

 fluorine, is a very strong, poisonous acid found 

 in liquid form or as a colorless gas. It is 

 chiefly used for etching on glass, and in chem- 

 istry in the analysis of substances containing 

 silica, which it changes into vapor. It attacks 

 most metals violently, but will not affect lead; 

 it is therefore stored in lead, wax or gutta- 

 percha containers. Even when highly diluted 

 with air, it is injurious to the lungs, while to 

 breath the pure vapor means death. In etch- 

 ing, the plate to be etched is coated with wax, 

 the design is scratched through to the glass, 

 and the plate is exposed to the acid. When 

 the wax is melted off, the design is bitten into 

 the glass. See FLUORINE. J.F.S. 



HYDROGEN, hi' dro jen, from Greek words 

 meaning water-producing, is a tasteless, odor- 

 less, colorless elementary gas. It is found in 

 the gases from volcanoes, certain oil wells, the 

 Stassfurt, Germany, salt beds, and in the gases 

 formed in the decay of animal and vegetable 

 tissues. It is also known to be part of the 

 atmosphere of the sun, and has been discovered 

 in meteorites. When water is decomposed it 

 yields one-ninth of its weight of hydrogen. 

 Such decomposition of water is most readily 

 brought about by passing an electric current 

 through the water. Pure water is a very poor 

 conductor of electricity. To make the current 

 pass well we add to the water about one-tenth 

 its weight of sulphuric acid or of caustic soda. 

 Hydrogen then comes off at one of the wires 

 (electrodes or poles) by which the current is 



led into the water and oxygen from the other. 

 The volume of hydrogen obtained is twice as 

 great as that of the oxygen, but its weight is 

 only one-eighth as great, because oxygen is 

 sixteen times as heavy as hydrogen. Hydrogen 

 is the one essential element in all acids; hence 

 it may be obtained by the decomposition of 

 an acid by a metal. It has great affinity for 

 oxygen, with which it forms water. A mixture 

 of hydrogen and oxygen ignited by a flame 

 or an electric spark explodes most violently. 

 Hydrogen is called a reducing agent because 

 of 'its power of withdrawing oxygen from com- 

 pounds containing large proportions of that 

 element. Thus if hydrogen is passed over 

 heated iron rust, the rust is reconverted into 

 iron. 



It burns with a very hot, colorless flame, but 

 other substances will not burn in it. The oxy- 

 hydrogen flame is an application of its affinity 

 for oxygen to industries where very great heat 

 must be obtained. A stream of hydrogen burns 

 in a stream of oxygen, the two uniting without 

 explosion to form water. It will not support 

 life, although breathing it is not directly in- 

 jurious. It has the effect of raising the pitch 

 of the voice to a high, thin squeak. It is the 

 lightest of all gases, and is therefore sometimes 

 employed to raise balloons. See OXYGEN. 



Hydrogen is used to convert liquid fats 

 into more valuable solid fats. This process is 

 called hydrogenation. J.F.S. 



HYDROGEN CHLORIDE, klo 'ride, is the 

 only known compound of hydrogen and chlo- 

 rine. It is a pungent, fuming, colorless gas, one 

 and one-fifth times heavier than air. It dis- 

 solves easily in water, and the solution is the 

 commercial hydrochloric, or muriatic, acid. 

 Hydrochloric acid occurs in the gases of vol- 

 canoes and in the water of rivers flowing 

 through volcanic regions. It is found also in 

 very dilute form in the digestive juices of men 

 and animals, and hence is sometimes given as 

 a tonic. It attacks most metals, evolving hy- 

 drogen and forming the chloride of the metal. 

 A mixture of hydrochloric and nitric acids in 

 the proportion of two to one is called aqua 

 rcgia, meaning royal water, because it will 

 dissolve gold, "the king of metals." 



Hydrochloric is one of the strongest (or 

 most active) acids, and with the exception of 

 sulphuric it is the cheapest of all acids. It is 

 obtained as a by-product in the manufacture 

 of soda from salt, and is used largely for the 

 production of the elementary gas chlorine 

 (which see) and of the chlorides of the metals. 



