ammonia are well known; not only is it largely employed in medicine, but, when mixed with some aromatic substance, it is used in scent-bottles, thus affording a striking instance of the transmutations effected by scientific agency, the foetid liquid of the gas-works being transformed into a scent used by ladies as a cherished luxury. Besides sal-ammoniac, the manufacture of which from gas-water exceeds 4,000 tons annually, about 5,000 tons of sulphate of ammonia are also produced by adding sulphuric acid to the gas liquor. This is of great value as a manure, and is also one of the principal component parts of alum, so largely used in dyeing and calico printing.

This compound alum consists of the earth alumina in combination with ammonia and sulphuric acid. Chemically speaking it is a double salt of sulphate of alumina and sulphate of ammonia, and is prepared among other ways by acting upon a mineral containing alumina with sulphuric acid, and then mixing the resulting liquid with sulphate of ammonia. For the purposes to which alum is applied the sulphate of ammonia is superfluous, as all that the dyer wants is a solution containing the earth alumina; but it must have no impurities in it. Now the only practicable way of purifying a salt is by crystallization, and it so happens that alumina salts are about the most uncrystallizable in the whole range of chemistry. Fortunately, however, the double salts which alumina forms with alkalies possess, like most double salts, very strong crystalline tendencies, and therefore manufacturers add sulphate of ammonia to the sulphate of alumina in order to obtain a compound which is capable of ready purification owing to its eminently crystallizable properties.

But the alum manufacturer is not only indebted to gas makers' by-products for his ammonia, but likewise for the sulphuric acid. We mentioned that one of the chief impurities in gas was sulphur, and alluded to a method of removing this body by a mixture of sawdust and iron. The action of the sulphur compound upon this is to produce water and sulphide of iron; when it has in this manner absorbed as much sulphur as it can, air is passed through the mixture; the effect of this is to convert the sulphide of iron back again into oxide of iron, the sulphur being separated in the form of powder. The mixture being in this way revivified is ready to absorb a fresh quantity of sulphur-impurity from the gas, and thus the processes go on alternately until the pores of the mixture are completely choked up with the deposited sulphur.

This spent material is now used for the manufacture of sulphuric acid by burning it in properly constructed furnaces, and allowing the products of combustion to mix with nitrous vapours and aqueous vapour in enormous leaden chambers. The sulphur

is caused to combine with the greatest possible amount of oxygen, becoming converted into sulphuric acid, or oil of vitriol as it is generally termed when in a concentrated form. Sulphuric acid, which is also made from other sources of sulphur, is one of the most important chemical manufactures of the country, and upon it depends the equally important manufacture of soda from sea salt. An eminent philosopher* of our times has expressed the opinion that the commercial prosperity of a country may be estimated by the amount of sulphuric acid which it produces, and that the consumption of soda affords a fair criterion of a nation's civilization, inasmuch as soap and glass, the manufacture of which depends upon these former, are inseparably connected with our ideas of cleanliness and comfort. Respecting the soap-test, there may be some reasons to question its accuracy, but the production of sulphuric acid lies at the root of nearly every industrial art or manufacture in which chemical agencies are employed, and may thus be looked upon as a fair test of the intellectual activity of a nation.

We now pass on to the oily matter which is obtained in the distillation of coal. This is a very complex body, containing a large number of substances of different degrees of volatility, and varying greatly in their characteristics. Some are alkaline, as, for example, the now well known substance aniline, others are acid, but by far the greater portion are neutral. By appropriate chemical and mechanical means these components of the crude coal tar are each obtained in a state of purity. The lighter portions, known by the name of coal naphtha, consist in great measure of benzol, a liquid which has been for several years past applied to a considerable number of uses in the arts. For the present we will confine ourselves to the part which it plays in the production of the brilliant colouring matters, the manufacture of which is so well illustrated in the Eastern Annexe of the International Exhibition. When benzol is acted upon with strong nitric acid it becomes converted into a heavy oily liquid, of a yellowish colour, which has an intensely sweet taste, and an odour powerfully recalling that of oil of bitter almonds. This is called nitro-benzol. It is employed to some extent in perfumery and in scenting soap, as well as in confectionary, where, owing to its non-poisonous properties, it forms a good substitute for the highly poisonous oil of bitter almonds. But the chief interest which attaches to this substance is on account of the transformation which it undergoes under the influence of certain chemical reducing agents. To follow these different changes step by step would require us to devote some little space to chemical explanations and formula, but a general idea.

* Baron Liebig: Familiar Letters on Chemistry.

of the transformations may be given without taking up much space. Benzol consists of the elements carbon and hydrogen in the proportion of twelve of the former to six of the latter. The action of the nitric acid upon this is to remove one of the hydrogen atoms and put in its place an atom of peroxide of nitrogen. When this nitro-benzol is acted upon by certain chemical agents of the class called reducing (as for instance a mixture of iron-filings and acetic acid which is now generally used), the whole of the oxygen which the peroxide of nitrogen has brought into the nitro-benzol is removed, and two parts of hydrogen are added, so that the original benzol becomes transformed into a body containing twelve parts of carbon, seven parts of hydrogen, and one part of nitrogen. This is aniline, a substance which illustrates in a striking manner the effect that demand exerts upon supply. Some years ago all the laboratories in Europe did not contain a pound weight of it, whereas it is now manufactured by thousands of gallons at a time. There is still another stage to be passed before we get to the colouring matter, but here the change is by no means well understood, and the best processes are kept scrupulously secret. The action is, however, the reverse of the one just now described, being the addition instead of subtraction of oxygen, and it is by the employment of different oxidising agents that we get mauve, magenta, roseine, azuline, bleu de Paris, and other gorgeous dyes which have received arbitrary names. Mauve or aniline purple was first discovered by Mr. Perkin, and in his case at the Exhibition may be seen a very complete and beautiful collection showing the different stages of the manufacture, from the crude coal oil up to a gigantic block of the pure dye itself upwards of a cubic foot in bulk, and for the production of which the distilled products from 2,000 tons of coal were consumed. The tinctorial properties of this dye are very strong. Mr. Perkin illustrates this by exhibiting a gallon jar filled with a beautiful violet solution, the colour of which is communicated to the water by one grain only of the dye. To render this illustration more striking there is placed near it a similar sized jar filled with crude coal-tar, the whole of which would have to be employed to produce this single grain of colouring matter.

The gigantic scale upon which the manufacture of these colouring matters is carried on and the perfection to which it is brought are strikingly illustrated by Messrs. Simpson, Maule, and Nicholson. This firm have succeeded in producing the beautiful colour known as magenta in a crystalline form, and one of the most striking objects in the whole department is the magnificent crown of acetate of rosaniline (the chemical name for magenta), which occupies so prominent a position in their

One of the most curious points about this is the colour which it exhibits. The rich, deep, rose tint which the body communicates to silk, &c., instead of being concentrated and intensified until deepened almost to a black, here shines and glistens from the facets of the beautiful crystals with a rich metallic green lustre, sparkling in the sunshine like the plumage of tropical birds, or the wing cases of certain beetles. Crystals such as these are unattainable except when working on a manufacturing scale. Laboratory experimentalists had already ascertained the fact that magenta was capable of assuming a regular form; but crystals such as compose this crown-the planes in some being nearly an inch across-can only be developed by manufacturers whose crystallizing vats hold upwards of £2,000 worth of colouring matter.

Before leaving this subject, we may draw attention to the important branch of national industry which this manufacture is assuming, and pay our tribute of admiration to the skill and intelligence of the chemist who has succeeded in converting the most nauseous and repulsive by-products of gas manufacture into such lovely colouring agents. Through his exertions, England will cease to import colouring matters, and will become a dye-exporting country.

Another product of the distillation of coal now claims our attention. In 1841, Liebig said that it would certainly be one of the greatest discoveries of the age if any one should succeed in condensing coal-gas into a white, dry, odourless substance, portable, and capable of being placed upon a candlestick or burned in a lamp. Ten years afterwards, Mr. Young showed, in the Great Exhibition of 1851, a single candle made from paraffin, a waxy-looking solid, which was known to be obtained in small quantities from the distillation of peat, wood, or coal. Liebig's prediction was here fulfilled. Paraffin is absolutely identical in composition with the most luminiferous portion of coal-gas, and only differs from it in being in a more condensed state; its per-centage composition of carbon and hydrogen is, in fact, identical with that of olefiant gas. Another ten years, and the commercial manufacture of paraffin has assumed gigantic proportions, and Mr. Young's establishment for its production, at Bathgate, ranks among the largest chemical works in the world. The composition of paraffin, indeed, renders it pre-eminently adapted for the production of light. It is a beautiful wax, melting at about 130°, and when heated to a considerably higher temperature (as when burning in the wick of a candle) it decomposes into true olefiant gas, producing a beautiful white light. A paraffin candle amounts, therefore, to a perfectly-constructed and portable gas works; its flame is not like that of an ordinary candle, but is identical with that of the

most perfect coal-gas, which is, indeed, self-produced as it is wanted, without any costly or complicated apparatus, and in a state of purity unattainable by ordinary means.

Intermediate between olefiant gas and the solid paraffin in condensation, but of the same per-centage composition, are a variety of other bodies which assume the form of oils more or less volatile in proportion to the number of atoms which are compressed into the same bulk. They are known by the name of paraffin oils, and their inflammability, volatility, and consequent danger, when used as household illuminants, increase as they approach in rarity the gaseous extremity of the scale, and diminish as they draw near the density of the solid. True paraffin oils must be carefully distinguished from spurious bodies of the same name ;* these contain various volatile bodies, the vapours of which take fire at the ordinary temperature, and which are as dangerous as common turpentine or benzol on the approach of a light. True paraffin oil will only burn in the presence of a wick, and is, therefore, perfectly safe. When burning, it also splits up into olefiant gas, and, therefore, produces a brilliantly-white light.

We stated at the commencement of this article that some of the products of the distillation of coal were acid bodies. The most important of these is carbolic acid, which is one of the principal constituents of creosote. This has recently been employed for sanitary purposes. Its antiseptic properties are remarkable, and when united with lime, it forms one of the most powerful disinfectants known. Carbolic acid is now prepared in enormous quantities from coal-tar, and has been applied with eminent success in several towns to the disinfection of sewage. It acts, not by destroying putrefactive principles as chlorine does, but it completely arrests decomposition. Animal matter washed over with a solution of this acid may be exposed for any length of time to the atmosphere without putrefying; and Dr. Lyon Playfair mentions instance in which a human body was preserved for two months in this way until it reached a distant grave.

an

When carbolic acid is acted upon by nitric acid a somewhat similar effect is produced as with benzol under these circumstances; part of the hydrogen is replaced by peroxide of nitrogen, and a body known as carbazotic acid is produced. Within the last few years this body has been extensively employed as a dye. The colour which it communicates to silk or wool is of

Nor must they be confounded with petroleum oil, which is now being imported largely from Canada and Pennsylvania. This substance will cause a great revolution in our modes of illumination, especially amongst the poor. It will be treated in a special article.-ED.