860 PRINCIPLES OF CHEMISTRY 



There is one substance known among the saturated hydrocarbons 

 composed of 1 atom of carbon and 4 atoms of hydrogen ; this is a com- 

 pound containing the highest percentage of hydrogen (CH 4 contains 

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

 carbon whose molecule contains but a single atom of carbon. This 

 saturated hydrocarbon, CH 4 , is called marsh gas or methane. If 

 vegetable 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 the 

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

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

 this gas. 29bis 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 



that the group CH 2 , or CH 3 substituted for H, on burning gives out from 152 to 

 159 thousand calories. This is less than that given out by C + H 2 , which is 97 + 69 or 166 

 thousand ; the reason for this difference (it would be still greater if carbon were gaseous) 

 is the amount of heat separated during the formation of CH 2 . According to Stoh- 

 mann, for dextroglucose, C 6 Hi 2 O e , it is 673'7 ; for common sugar, C 12 H 22 O n , 1825'7 ; for 

 cellulose, C 6 H 10 O 5 , 678'0 ; starch, 677'5 ; dextrin, 666'2 ; glycol, C 2 H 6 O 2 , 281'7 ; glycerine, 

 S97'2, &c. The heat of combustion of the following solids (determined by Stohmann) ia 

 expressed per unit of weight : naphthalene, C 10 H 8 , 9,621 ; urea, CN 2 H 4 O, 2,465 ; white of 

 egg, 5,579; dry rye bread, 4,421; wheaten bread, 4,802; tallow, 9,865; butter, 9,192; 

 linseed oil, 9,823. The most complete collection of arithmetical data for the heats of 

 combustion will be found in V. F. Longinin's work, ' Description of the Various Methods 

 of Determining the Heats of Combustion of Organic Compounds ' (Moscow, 1894). 



The number of units of heat given out by unit weight during the complete combustion 

 and cooling of the following ordinary kinds of fuel in their usual state of dryness and 

 purity are : (1) for wood charcoal, anthracite, semi-anthracite, bituminous coal and coke, 

 from 7,200 to 8,200 ; (2) dry, long flaming coals, and the best brown coals, from 6,200 to 

 6,800 ; (3) perfectly dry wood, 3,500 ; hardly dry, 2,500 ; (4) perfectly dry peat, best kind, 

 4,500 ; compressed and dried, 8,000 ; (5) petroleum refuse and similar liquid hydrocarbons, 

 about 11,000 ; (6) illuminating gas of the ordinary composition (about 45 vols. H, 40 vols. 

 CH 4 , 5 vols. CO, and 6 vols. N), about 12,000 ; (7) producer gas (see next Chapter), con- 

 taining 2 vols. carbonic anhydride, 30 vols. carbonic oxide, and 68 vols. nitrogen for one 

 part by weight of the whole carbon burnt, 5,800, and for one part by weight of the gas, 

 910, units of heat ; and (8) water gas (see next chapter) containing 4 vols. carbonic anhy- 

 dride, 8 vols. N 2 , 24 vols. carbonic oxide, and 46 vols. H 2 , for one part by weight of the 

 carbon consumed in the generator 10,900, and for one part by weight of the gas, 8,600 

 units of heat. In these figures, as in all calorimetric observations, the water produced 

 by the combustion of the fuel is supposed to be liquid. As regards the temperature 

 reached by the fuel, it is important to remark that for solid fuel it is indispensable to 

 admit (to ensure complete combustion) twice the amount of air required, but liquid, or 

 pulverised fuel, and especially gaseous fuel, does not require an excess of air ; therefore, 

 a kilogram of charcoal, giving 8,000 units of heat, requires about 24 kilograms of air (3 

 kilograms of air per thousand calories) and a kilogram of producer gas requires only 

 0'77 kilogram of air (0'85 kilo, of air per 1,000 calories), 1 kilogram of water gas about 

 4'5 of air (1'25 kilo, of air per 1,000 calories). 



89 bb Manure which decomposes under the action of bacteria gives of! CO 2 and 



