SLOW OXIDATION OF WOODY FIBRE. 149 conchoidal. The coke obtained from it differs little in appearance from the original coal either in bulk or in weight. Anthracite is extremely abundant over vast tracts of North America, but much of it is so compact that it is scarcely applicable as a fuel. A large quantity of excellent anthracite is obtained from the South Wales coal field. The tables on the following page will give an idea of the composition of wood, and will illustrate the progress of its decay. They also furnish a synoptic view of the composition of some of the principal varieties of coal, in the order of their occurrence in successive geological stages. The proportion of oxygen, it will be seen, diminishes rapidly, and that of the hydrogen more slowly, as the coal passes from lignite towards anthracite, in which form it consists of nearly pure carbon. 34 48 Observation has proved that during these changes oxygen in small quantity is absorbed, whilst carbonic anhydride and carburetted hydrogen are evolved in large quantities. It may be worth while to point out the simple mode in which we may from these facts theoretically explain the conversion of wood into coal under the influence of a very limited supply of oxygen, whilst the vegetable matter is submerged in water at a moderate temperature. If we assign to wood the empirical formula H22, founded upon the analysis of oak given in the table which follows, the approximative empirical formulæ for peat will be €20H228; for Bovey lignite, H; for Wigan Cannel, €26H20; and for the Welsh anthracite, H. Now, if a small amount of oxygen, such as might be supplied by solution in water, be supposed to act upon the woody tissue, each of these varieties of fuel might be formed by the separation of marsh gas, carbonic anhydride, and water, in the following proportions :— Wood. 40 Peat. Ꮎ 4 €34H18022 + 602 = 4¤20H22→3 + 24H, + 32 CO2 + 4 H ̧Ð 434H4022+20=4H2 +8 €H, +20 0, + 24 H2O 49 Wood. 28 Cannel. 4 2 434H48022 + 402 = 4¤26H20→2 + 8 ¤H, + 24 ¤¤2 + 40 H2O 434H48022 + 5022 €40H160 + 24 ¤H, + 32 €0, + 32 H2O. The amount of ash left by different kinds of coal is very variable. In the finest species of coals the fixed ash appears to be disseminated pretty evenly through the mass, but in the coarser 150 COMPOSITION OF WOOD AND COAL. varieties, thin seams of iron pyrites, and at other times, as in the Newport coal, crystallized lamina composed of the mixed calcic, magnesic, and ferrous carbonates are found. It is obvious that where these seams occur, their components must have been carried into the mass by infiltration. The ash of pure coal furnishes but minute quantities of the salts of the alkalies; it usually contains but little calcium or magnesium, the bulk of it consisting of aluminic silicate, with variable amounts of oxide of iron. Vaux also found traces of copper and lead in most of the coals which he analysed. Composition of Wood and Coal. Variety Composition of different Varieties of Coal-(continued). . Parrot. Cannel. Caking. Dry. Dry. Steam. Anthracite. § IV. ON FERMENTATION. (1112) Under the term fermentation, frequent mention has been made of a particular change which saccharine bodies undergo, in consequence of which they become converted into carbonic anhydride and alcohol. Many other substances, however, besides sugar, are susceptible of an analogous change, and the term fermentation is now applied to various transformations which organic compounds experience under the influence of a small quantity of organized matter, which is itself in a state of active alteration. This active substance is termed a ferment, and at the close of the operation appears neither to have imparted anything to, nor received anything from the substance which is undergoing fermentation. During this change, the body which is experiencing fermentation is gradually broken up into two or more substances of simpler composition; in certain cases the elements of water are assimilated, in others they are separated, whilst in a third class of decompositions water is neither separated nor assimilated. For example, when fruit sugar is placed in contact with ordinary yeast, the sugar is simply converted into carbonic anhydride and alcohol, the yeast acting as the ferment: No assimilation or separation of water occurs in this case. 14 But when grape sugar, H.,, is subjected to fermentation, carbonic anhydride and alcohol are formed as before, and the elements of water are eliminated : EH14 : Again, when cane sugar is submitted to fermentation, it assimilates the elements of water, first becoming changed into fruit sugar, and this substance is afterwards broken up into carbonic anhydride and alcohol as before:— €12H22O11+H2O=2ЄH12; and ЄH1206=2 Є→2+2 ¤¿Н ̧Ð. The different varieties of fermentation receive their distinctive. name from the principal products which they furnish; for example, when spirit of wine is formed from sugar, the change is called the alcoholic or vinous fermentation; when lactic acid is converted into butyric acid, the process is termed the butyric fermentation, &c. 152 ALCOHOLIC FERMENTATION. In all cases of fermentation the following conditions are essential:-1, the presence of water; 2, a temperature ranging between the extremes of 41° and 113° (5° and 45° C.); 3, a peculiar ferment (the nature of which varies in the different cases); 4, exposure for a longer or shorter period to the air; and 5, the body which is to undergo fermentation. It may be remarked that in most instances the compounds susceptible of fermentation are crystallizable products. It is especially to be observed, although the ferment does not contribute any of its components to the results of fermentation, yet that it is itself destroyed during the operation; the products of its decomposition being generally discernible mixed with the results of the fermentation. This fact has an important bearing upon the theory of the process, but its significance will be best understood by tracing the operation through its different stages in the formation of alcohol. (a) Alcoholic Fermentation. (1113) When the juices of plants or of fruits containing sugar, such for instance as must (the juice of the grape), are kept at a temperature of about 68° (20° C.) for some hours, a change begins to show itself. The liquid becomes turbid, and small bubbles rise to the surface; or in popular language, it begins to work or to ferment. This change is due in the first instance to the alterations produced in the albuminous and azotised matters, which all such juices contain, under the combined influence of atmospheric oxygen, warmth, and moisture. This azotised matter is decomposed, and an intestine change commences, which after it has once begun, continues in vessels to which the further access of air is prevented. As the fermentation proceeds, a continual disengagement of heat is maintained, whilst a constant extrication of gas continues; this gas, if collected, is found to consist entirely of carbonic anhydride. After a time the escape of gas diminishes, and at length ceases; if the liquid be now examined, the sweet taste of the sugar will have disappeared, and the solution will have acquired a flavour more or less spirituous. If this liquid be distilled, the first portions that come over will be found to contain an inflammable product lighter than water, which is easily recognized as dilute spirit of wine or alcohol. (1114) Properties of Yeast.-Upon examining the liquid which remains after fermentation is complete, it is found to contain a multitude of small oval organized bodies which do not exceed of an inch, or o'1 millimetre, in diameter, and which when PROPERTIES OF YEAST. 153 viewed under the microscope, are seen to consist of nucleated cells presenting the appearance shown in fig. 383, page 155. These cells form the essential constituent of yeast, a fungus to which the term Torula cerevisiae has been applied, and which is obtained in great abundance during the fermentation of wort in the manufacture of beer. Beer yeast is the substance which possesses, in the highest degree, the power of producing the alcoholic fermentation. Its efficacy in this respect is easily proved by the following experiment:-Dissolve 4 parts of pure cane sugar in twenty parts of water, and add 1 part of fresh yeast; then expose the mixture to a temperature of about 77° (25° C.). In less than an hour fermentation will commence, and carbonic anhydride will be evolved in abundance. Yeast, when in its active condition, always exhibits a slightly acid reaction; if thoroughly washed, the globules are much less active, but they again acquire activity by exposure for a few hours to the air, during which time the acidity is again developed. The addition of a minute quantity of some of the vegetable acids, such as the acetic or the tartaric acid, to washed yeast, immediately restores its activity, but if a larger quantity of the acid be employed the process of fermentation is arrested. The strong mineral acids, particularly the sulphuric acid, when present, even in small quantity, immediately put a stop to the alcoholic fermentation; the same effect is also produced by solutions containing traces of sulphurous acid. It is also checked by the addition of a small quantity of free alkali. Many other circumstances likewise put an immediate stop to the process of fermentation. For example: a solution which contains more than one-fourth of its weight of sugar cannot be made to ferment; the presence of 20 per cent. of alcohol or upwards, also puts an end to the process; the addition of a strong solution of common salt, or the presence of many metallic salts in the liquid, such for instance as nitrate of silver, corrosive sublimate, and sulphate or acetate of copper, check it immediately. The occurrence of fermentation is also prevented by the presence of small quantities either of black oxide of manganese, or of mercuric oxide, of strychnia, or of quinia. Small quantities of kreasote, of oil of turpentine, and of many other essential oils, have a similar effect. If, however, fermentation has once commenced, the vegetable bases have no specific power of arresting the process. On the other hand, neither solution of arsenious acid, of acetate of lead, nor of tartar emetic exerts any retarding effect |