348 PRINCIPLES OF CHEMISTRY 



pound containing the highest percentage of hydrogen (CH 4 contains, 

 25 per cent, of hydrogen), and at the same time it is the only 

 hydrocarbon whose molecule contains a single atom of carbon This 

 saturated hydrocarbon, CH,, is called mnrxh-f/nx or methane. If vege- 

 table or animal refuse suffers decomposition in a space where the air 

 has not free access, or no access at all, then the decomposition is 

 accompanied with the formation of marsh gas, and this either at an 

 ordinary temperature, or at a comparatively much higher one. On this 

 account plants, when decomposing under water in marshes, give out 

 this gas. It is well known that if the mud in bogs be stirred up, the act 

 is accompanied with the evolution of a large quantity of gas bubbles ; 

 these may, although slowly, also separate of their own accord. The 

 gas which is evolved consists principally of marsh-gas. 30 If wood, coal, 

 or many other vegetable or animal substances are decomposed by the 

 action of heat without access of air that is, are subjected to dry dis- 

 tillation they, in addition to many other gaseous products of decom- 

 position (carbonic anhydride, hydrogen, and various other substances), 

 evolve a great deal of methane. Generally, the gas which is used for 

 lighting purposes is obtained by this means, and, therefore, always 

 contains marsh gas, mixed with hydrogen and other vapours and gases, 

 although it is subsequently purified from many of them. 31 As the 



cated to the boiler, and to the surrounding air. Subtracting 27'5 units of heat for 

 radiation, we have 160 units of heat per second communicated to the water. If the 

 pressure in the boiler be 5 atmospheres, = 152 (page 54), and the temperature of the 

 feed water 12, then (the heat of warming and evaporation of 640 units of heat, see 

 page 53) 0'25 kilo, of steam at 150 per second (900 kilos, per hour) will be produced. 

 If this steam does its work without waste of any kind, and be condensed in the con- 

 denser at 50, then, according to the second law of the mechanical theory of heat, not 

 more in any case than 160 (150 50) *- (150 + 273) or 37 - H units of heat, will be converted 

 into work, which (according to the first law of the mechanical theory of heat) will equal 

 424 x 87'8 or 15927 kilogrammetres of work per second, or 212 horse-power (one horse 

 power =75 kilogrammetres per second). 



But as at least 35 p.c. (in the best machines) is lost in inevitable loss of heat and 

 through friction, only about 140 horse-power can be obtained with the above-mentioned 

 quantity of fuel in a steam-engine of excellent construction. Generally, 1 kilo, of coal 

 per horse-power per hour is consumed in the best steam-engines (with expansion and 

 condensation). Only in gas engines at present does the work of the fuel approach the 

 theoretical limit. 



30 It is easy to collect the gas which is evolved in marshy places if a glass bottle be 

 inverted in the water and a funnel put into it (in water) ; if the mud of the bottom be 

 now agitated, the bubbles which rise may be easily caught in the funnel. 



31 Illuminating gas is generally prepared by heating gas coal (see Note 6) in oval 

 cylindrical horizontal cast-iron or clay retorts. Several such retorts BB (fig. ~>>~ 

 disposed in the furnace A, and heated together. When the retorts are heated to a 

 red heat, lumps of coal are thrown into them, and they are then closed with a closely- 

 fitting cover. The illustration shows the furnace, with five retorts. Coke (see Note 

 1, dry distillation) remains in the retorts, and the volatile products in the form of vapours 

 and gases travel along the pipes d, rising from each retort. These pipes branch above 

 the stove, and are hermetically sealed into the receiver/ (hydraulic main) placed above 



