Products of simpler composition are not invariably formed. Frequently at the moment of decomposition several nuclei unite together to form more complex products, as, for instance, in the dry distillation of salts of organic acids, where, especially in the case of alkaline and alkaline earthy salts, a carbonate is left, whilst the remaining nuclei of two original molecules unite to form a new compound, e.g. sodic acetate :

63. Not seldom these processes occur without formation of essential quantities of bye-products, but mostly, especially in the dry distillation of complex bodies, the number of products is very large, and includes bodies of both greater and less complexity. The distillation products of the same body differ essentially in nature, quantity, and state of aggregation according to the temperature at which decomposition is effected.

In the dry distillation of wood, which has the formula C6H10O5, or more probably some multiple, there are first given off gases and an aqueous liquid. The first contains much CO2, later follow carbon monoxide, then gaseous hydrocarbons, such as CH4, C2H4, C2H2, &c., and on very strongly heating, hydrogen also. Water at first passes

over in largest quantity, but is soon followed by acetic acid, C2H1О2, wood spirit, CH,O, acetone, C3H6O, &c., which remain dissolved in the former. When the oxygen of the wood has been in great part removed in the form of these and similar highly oxygenated compounds, there follow (generally at the same time as the gaseous hydrocarbons) more difficultly vapourisable compounds, poorer in oxygen, and of more complex constitution, such as phenol CHO, cresol C,H,O, &c., as also hydrocarbons of higher molecular weight, e.g. benzol C6H6, naphthalene CoHs, anthracene C4H10, &c. These all condense in cooled receivers, and form two layers, an aqueous and an oily. The latter is nearly invariably of a dark colour, and consists of solutions of solid bodies in oily products, and often mixed with bodies still solid. This layer is termed tar. From tar the various constituents which boil at different temperatures can be generally separated by fractional distillation. As a rule, those bodies which are liquid at ordinary temperatures distil sooner than solids. If the first are distilled off, a brownish black resinous mass, termed pitch, is left.

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In the original distillation vessel a residue of charcoal is left, still containing all the mineral constituents of the original substance, which are left as ash on complete combustion. In the

dry distillation of nitrogenous bodies, much ammonia is evolved, and is found as carbonate in the aqueous distillate, whilst the tar then invariably contains nitrogenous liquid bases, such as aniline, pyridine, &c., and the residual charcoal still contains some nitrogen, which cannot be separated from the carbon even at a white heat.

Putrefaction, Decay, and Fermentation of Organic Bodies.

64. If organic bodies are left in a damp condition at ordinary temperatures, many of them suffer apparently completely spontaneous changes, whose products show some similarity to those of dry decomposition. There are formed, namely, in addition to gases and volatile bodies, frequently of unpleasant odour (odour of putrefaction), also aqueous and even oily products and tarlike masses, or at least a darkcoloured residue, rich in carbon (humus). In general, the resemblance of these products to those of dry distillation, are the closer, the more completely oxygen has been excluded during their formation.

These changes are, however, in all probability never entirely spontaneous. To their initiation an exposure, however short, to air is essential; they are prevented by high or very low temperatures, by absence of water, and by the presence of certain poisonous bodies-antiseptic media. Amongst these latter are arsenious acid, mercury and zinc salts, tannin, creosote, and also common salt in concentrated solution.

65. During the exposure of putrescible bodies to air, they come in contact with the germs of microscopic organised beings, which by their evolution and multiplication are the primary cause of decomposition. As their vegetation occurs most vigorously at temperatures of 20°-30°, they most readily cause decomposition at these temperatures. Below 0° these beings lose the power of growth for at least the time of duration of that temperature. At temperatures near the boiling point of water they are killed like all other organisms, as also by antiseptic poisons.

66. Putrefaction and decay cannot be sharply separated from one another; by the first term are meant those decomposition processes which occur under the aid of organisms, and without action of oxygen, whilst in decay intense oxidising action occurs at the expense of the atmospheric oxygen.

To putrescible bodies especially belong the nitrogenous and sulphuretted constituents of animals and plants, the albuminoïd bodies or proteid substances: e.g. albumen, caseïn, the substance of muscle and membrane, &c.

67. A very minute amount of the above-mentioned putrefactionexcitants can cause the decomposition of very considerable quantities of decomposable bodies, the latter probably serving as the nutritive material for many quickly following generations of the first. The decomposition products must then be considered as the excreta of these organisms. Such reactions as these are termed fermentations, and the bodies causing the change, whilst they themselves apparently take no visible part, are termed ferments.

68. One and the same body may undergo very different decompositions, according to the nature of the ferment acting on it, or according

to the variations in the growth of the ferment produced by temperature, amount of moisture, &c.

Cane sugar, e.g., in contact with many organised ferments, is converted-with combination of the elements of water-into glucose:

whilst the solution of the latter is converted by the ferment of putrid proteid bodies into lactic acid:

which is further converted into butyric acid, with evolution of carbonic anhydride and hydrogen:

2C,H,O,C,H2O2 + 2 CO2 + 2H2.

(Butyric acid.)

If any unaltered glucose be present during this latter fermentation it is converted into mannite:

CH12O6+ 2HC,H,,O, (mannite).

=

These and similar fermentations occurring without direct participation of atmospheric oxygen, may be considered as putrefactive processes, as also the decomposition of glucose into carbonic anhydride and alcohol under the influence of yeast:

CH1206 = 2CO2 + 2C2H6O (alcohol).

In other fermentations the atmospheric oxygen is actively engaged, as in the acetic fermentation of alcohol:

C2H,0 +02 = H2O + C2H ̧O, (acetic acid).

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These, however, more closely resemble the decay processes of complex albuminoïd bodies.

69. As already mentioned, the ferments are frequently organised, often single cells of plant-like nature, sometimes also having the power of motion, as in the butyric ferment. There are also some ferments which are not organised, such as synaptase and diastase ; these are soluble in water, and under favourable circumstances can cause large quantities of certain other compounds to suffer specific changes.

The amygdalin of bitter almonds in presence of water is decomposed, by the synaptase contained in the same seeds, into bitter almond oil, glucose, and hydrocyanic acid:

C20H27NO11 + 2H2O = CNH + 2C6H12O6 + C2H2O.
(Amygdaline.)

No other ferment is capable of producing this decomposition.

Not seldom inorganic compounds can act on organic bodies in a ferment-like manner, especially strong acids and sometimes strong basic hydrates.

70. By processes of putrefaction, decay, and fermentation, animals

and plants gradually vanish after death, only the non-volatile mineral constituents remaining behind. By long-continued addition of oxygen they are converted into volatile products, especially into carbonic anhydride, water, and ammonia, which then serve anew as nourishment to the vegetable world.

It is often of great economical importance to preserve unchanged the very putrescible nitrogenous foods. This may be effected either by placing them under conditions under which fermentation is impossible, even in the presence of ferments, as, for instance, by freezing, or by destroying all ferments present.

By bringing bodies capable of putrefying into contact with salt, sugar, alcohol, or similar media, the water necessary for fermentation is removed, and they therefore remain unchanged. The same result is arrived at by drying at high temperatures. By smoking another result is obtained in addition to drying, the substances getting saturated with creosote, volatile oils, &c., which kill the organisms causing putrefaction.

The conservation of foods by Appert's method, which consists in heating them in tins to the temperature of boiling water, and then hermetically closing the latter whilst still at that temperature, depends on the destruction of all putrefactives at the boiling heat. So long as no air can reach the food to convey fresh germs to it, it remains unaltered at ordinary temperatures; from the moment of contact with air putrefactive action starts, which can be again destroyed by heating to boiling.

In order to preserve anatomical preparations, they are treated with solutions of mercuric chloride, zincic chloride, arsenious acid, &c.

The chief mass of wood, the cell substance, is not capable of putrefaction, but from the presence of albuminous substances in the wood, which can go into putrefaction under favourable conditions, the woody fibre is often destroyed. This is prevented either by washing the putrescible substances out of the wood (by steaming under pressure), or by forcing substances into it which prevent putrefaction (antiseptic media). The wood is saturated with cupric sulphate or corrosive sublimate solutions, or with 'pyrolignate of iron,' in which latter case the presence of creosote is especially active.

CYANOGEN COMPOUNDS.

71. By the name cyanogen the group CN (= Cy) is understood, which acts generally as a monovalent radical. It can exist in several modifications, accordingly as the combining metal is united to the carbon or to the nitrogen atom.

In the first case, the two elements are united to each other by three bonds:

CEN

R

In that case the radical is termed true cyanogen (generally cyanogen only) or carbonitrile, and, similarly to ammonia, possesses the power,

phuric acid and forty parts of water in a flask (fig. 16, A), connected, by means of the bent tube b, with the condensing apparatus D E. The flask is heated until the liquid begins to boil, and the distillate is collected in the receiver B.

There is thus obtained an aqueous prussic acid, which readily decomposes on keeping; the addition of a drop of sulphuric acid renders it much more stable.

By the action of sulphuric acid upon potassic ferrocyanide, only one-half of its cyanogen is evolved as hydrocyanic acid, there being formed at the same time a white insoluble cyanide of iron and

potassium of the formula K2Fe2C6N6, which is not attacked by dilute sulphuric acid. The reaction taking place is represented by the equation:

2K4Fe(CN) + 3H2SO, = 3K2SO, + 6HCN + K2Fe2(C¿N6).

From the aqueous hydrocyanic acid, the anhydrous acid can be obtained by fractional distillation, and treatment with calcic chloride; the pretty concentrated aqueous solution is slightly warmed, and the vapour of the easily volatile acid condensed in a receiver cooled by means of ice, and containing fused calcic chloride in coarse powder. The receiver is tightly stoppered, and allowed to stand till the salt has united to all the water; then, by application of gentle heat, the anhydrous acid is distilled, and collected in a receiver cooled by a freezing mixture of ice and salt.