CHAMBERS'S INFORMATION FOR THE PEOPLE. 



of vegetable matter under water ; the gas which 

 bubbles up when the bottom of a pond is dis- 

 turbed consists of marsh gas mixed with carbonic 

 acid. It also occurs in cavities in coal-seams, and 

 forms the ' fire-damp ' of our coal-mines, and is 

 produced by the distillation of many organic sub- 

 stances. It is one of the constituents of coal-gas. 

 It is a combustible gas, and when mixed with the 

 proper proportion of oxygen, forms an explosive 

 mixture. The products of the combination are 

 carbonic acidandwater,CH 4 -|-2O 2 = CO 2 + 2H 2 O. 

 From this equation it will be seen that one 

 volume of marsh gas requires two volumes of 

 oxygen for its combustion. As air contains about 

 20 per cent, of oxygen, one volume of marsh gas 

 requires 10 volumes of air for its complete com- 

 bustion. The explosions which occur in coal- 

 mines are caused by the ignition of such a mixture, 

 and explosion takes place even when the two gases 

 are not in the exact proportions given above. It 

 has been found by experiment that no explosion 

 takes place when there is less than three volumes, 

 or more than 18 volumes of air, to one of marsh 

 gas. It will be remembered that a 

 red-hot body will cause explosion of 

 a mixture of hydrogen and oxygen 

 the temperature at which marsh gas 

 takes fire is much higher contact 

 with a flame or white-hot body being 

 necessary. It is upon this fact that 

 the efficacy of Davy's safety-lamp 

 depends. In this lamp the flame is 

 inclosed in a cylinder of wire-gauze. 

 If such a lamp be placed in an ex- 

 plosive mixture of marsh gas and air, 

 the mixture inside the cylinder, im- 

 mediately in contact with the flame 

 of the lamp, will burn ; but this com- 

 bustion will not extend to the mixture 

 outside, as the gases, by passing 

 through the meshes of the wire-gauze, 

 are cooled down below the tempera- 

 ture necessary for kindling marsh 

 gas. The enlargement of the flame, 

 due to the marsh gas burning inside 

 the cylinder, is a warning to the miner of the pres- 

 ence of ' fire-damp.' 



Olefiant gas contains carbon and hydrogen in 

 the proportion 6 : 1 ; its formula is C 2 H 4 . It is 

 a colourless transparent gas of specific gravity 

 0-978. It is inflammable, burning with a strongly 

 luminous flame, and producing carbonic acid and 

 water, C 2 H 4 + 3O 2 =2CO 2 + 2H 2 O. It occurs in 

 coal-gas, and the luminosity of coal-gas is to a 

 great extent due to its presence. It can also be 

 prepared by heating a mixture of four parts of 

 strong sulphuric acid and one of alcohol ; the 

 action which takes place is represented by the 

 following equation : 



Fig. 2. 



C S H 6 = H 2 



Alcohol. Water. 



C a H 4 . 



Olefiant Gas. 



The sulphuric acid in this case acts as a dehy- 

 drating or water-removing agent. 



CHLORINE has been already spoken of (p. 313) 

 as a simple or elementary salt-radical. It occurs 

 in nature in combination with metals, forming 

 metallic chlorides. Of these the most important 

 is chloride of spdium or common salt (NaCl). 

 Common salt is by far the most abundant of the 

 salts dissolved in sea-water, and is obtained from 



Uft 



sea-water by spontaneous evaporation in shallow 

 ponds or marshes in the neighbourhood of the sea 

 in warm countries. It also occurs in extensive 

 beds as rock-salt, and in solution in many mineral 

 waters. When acted on by sulphuric acid, it yields 

 sulphate of soda and hydrochloric acid gas 

 (2NaCl + H 2 SO 4 = 2HC1 + Na 2 SO 4 ). Hydro- 

 chloric acid gas is a colourless transparent gas, of 

 specific gravity i -27. It is very soluble in water ; 

 at ordinary temperatures, water dissolves about 450 

 times its volume of the gas. The solution is denser 

 than water, and the gas cannot be wholly expelled 

 by boiling. The solution is generally called hydro- 

 chloric acid, or muriatic acid, or spirit of salt. 

 Hydrochloric acid acts upon such metallic oxides 

 as are bases, producing a salt and water ; thus, 

 CaO + 2HC1 = CaCl 2 + H 2 O. But if we take an 

 oxide such as peroxide of manganese, which is not 

 a base, and has no corresponding chloride, the 

 action is different, thus : MnO 2 + 4.HC1 = MnCl 2 

 + C1 2 + 2H 2 O. Instead of obtaining MnCl 4 , we 

 have MnCl 2 and C1 2 ; that is, free chlorine. It is in 

 this way that chlorine gas is prepared. Chlorine is 

 a greenish yellow gas, of specific gravity 2-45, 

 having a pungent smell. As it is much heavier 

 than air, it may be collected by displacement, the 

 tube frorn which it issues being led to the bottom 

 of the receiving vessel. The chlorine collects at 

 the bottom of the vessel, and gradually fills it, 

 driving out the air (see fig. 3). It cannot be 



Fig- 3- 



breathed unless very much diluted with air, and 

 has a very irritating effect on the lining mem- 

 brane of the air-passages. Water dissolves about 

 twice its volume of chlorine gas. Chlorine unites 

 very readily with hydrogen ; a jet of hydrogen 

 kindled in chlorine burns readily, forming hydro- 

 chloric acid. A mixture of equal volumes of 

 hydrogen and chlorine can be kept for any length 

 of time in the dark, but explodes violently when 

 exposed to sunlight. In diffused light, the two 

 gases combine more slowly. This tendency of 

 chlorine to combine with hydrogen renders wet 

 chlorine, or chlorine along with water, a very 

 powerful oxidising agent ; the chlorine attracting 

 the hydrogen of the water, the oxygen of which 

 thus becomes more easily separated, and can 

 therefore readily attack oxidisable substances. 



It is in this way that chlorine acts as a bleaching 

 arfd as a disinfecting agent. Many colouring-mat- 

 ters, and many offensive gases and vapours, are 

 oxidisable, and can, by oxidation, be converted into 

 colourless substances, or into inodorous bodies. 

 In these cases, they are destroyed by wet chlorine 

 the chlorine taking the hydrogen of the water, 

 while the oxygen oxidises the colouring or offen- 

 sive substances. Chlorine unites with slaked 



