344 PRINCIPLES OF CHEMISTRY 



process of carbonisation of vegetable refuse takes place in its trans- 

 formation under water, as shown by the marshy vegetation which 

 forms peat. 6 In this manner doubtless the enormous masses of 

 coal were formed 6 which, following the example set by England, are 



8 When dead vegetable matter undergoes transformation in air, in the presence of 

 moisture and lower organisms, there remains a substance much richer in carbon namely, 

 humus, black earth or mould. 100 parts of humus in a dry state contain about 70 p.c. of 

 carbon. The roots, leaves, and stems of plants which wither and fall to the ground form 

 a soil rich In humus. The non-vital vegetable substances (ligneous tissue) first form 

 brown matter (ulmic compounds), and then black matter (humic substances), which are 

 both insoluble in water ; after this a brown acid is produced, which is soluble in water 

 (apocrenic acid), and lastly a colourless acid also soluble in water (crenic acid). Alkali dis- 

 solves a part of the original brown and black substances, forming solutions of a brown tint 

 (ulmic and humic acids) which sometimes communicate their colour to springs and rivers. 

 The proportion of humus in soil generally has a direct influence on its fertility ; firstly, 

 because putrefying plants develop carbonic anhydride and ammonia, and yield the sub- 

 stances forming the ashes of plants, which are necessary to vegetation ; secondly, because 

 humus is capable of attracting the moisture of the air and of absorbing water (twice its 

 weight) and in this way keeps the soil in a damp condition, which is indispensable for 

 nourishment ; thirdly, humus renders the soil porous, and, fourthly, it renders it more 

 capable of absorbing the heat of the sun's rays. On this account black earth is often 

 most remarkable for its fertility. One object of manuring is to increase the quantity of 

 humus in the soil, and any easily changeable vegetable or any animal matter (composts) 

 may be used. The boundless tracts of black earth soil in Russia are capable of bestowing 

 countless wealth on the country. 



The origin and extent of black earth soil are treated in detail in Professor Dokou- 

 chaeffs works. 



If those substances which produce humus undergo decomposition under water, less 

 ;carbonic anhydride is formed, a quantity of marsh gas, CH 4 , is evolved, and the solid 

 .residue forma an acid humus found in great quantities in marshy places and called 

 <peat. Peat is especially abundant in the lowlands of Holland, North Germany, 

 Ireland, and Bavaria. In Russia it is likewise found in large quantities, especially in 

 the North- West districts. The old hard forms of peat resemble in composition and -pro- 

 perties brown coal ; the newest formations, as yet unhardened by pressure, form very 

 porous masses which retain traces of the vegetable matter from which they have been 

 formed. Dried (and sometimes pressed) peat is used as fuel. The composition of peat 

 varies considerably with the locality in which it is found. When dried in air it does not 

 contain less than 15 p.c. of water and 8 p.c. of ash ; the remainder consists of 45 p.c. of 

 carbon, 4 p.c. of 'hydrogen, 1 p.c. of nitrogen, and 28 p.c. of oxygen. Its heating powe* 

 is about equivalent to that of wood. The brown earthy varieties of coal were probably 

 formed from peat. In other cases they have a marked woody structure, and are then 

 known as lignites. The composition of the brown sorts of coal resembles in a marked 

 degree that of peat namely, in a dried state brown coal contains on an average 60 p.c, 

 of carbon, 5 p.c. of hydrogen, 26 p.c. of oxygen and nitrogen, and 9 p.c. of ash. In 

 Kussia brown coal is met with in many districts near Moscow, in the Governments of 

 Toula and Tver and the neighbourhood ; it is very usually used as fuel, particularly 

 when found in thick seams. The brown coals usually bum with a flame like wood and 

 peat, and are akin to them in heating power, which is half or a third that of the best 

 coal. 



6 Grass and wood, the vegetation of primaeval seas and similar refuse of all geological 

 periods, must have been in many cases subjected to the same changes they now 

 undergo that is, under water they formed peat and lignites. Such substances, pre- 

 served or a long time underground, subjected to the action of water, compressed by the 

 new strata formed above them, transformed by the separation of their more volatile 



