Of the numerous known dyes, few can be applied to either animal or vegetable fibre without some preparation beyond that of cleansing the stuff, and immersing it in the dyeing liquor. When colours can be fixed on cloth without any previous preparation, they are called substantive colours, such is indigo; when they cannot be so fixed, but require to be saturated with some preparation, such as acetate of alumina, or a metallic oxide, &c. they are called adjective colours; of this kind are madder, cochineal, &c. The substances with which cloths are impregnated, previously to being dyed, are called mordants, because they are supposed to bite or lay hold of the colour which is applied.
The chief difference between vegetable and animal substances is, that animal (as for instance wool) contains a small portion of carbon, and a large quantity of hydrogen and nitrogen; while vegetables contain a very large proportion of carbon, less hydrogen, and, in general, no nitrogen.
It is the interest of every dyer to acquire as much information as possible concerning the nature of alum, iron, carbon, nitrogen, hydrogen, the alkalies, acids, &c. in order to prevent or obviate the consequences of an incorrect application of these agents in the various departments of his art, and also to apply them with the greatest success. We shall, therefore, enter a little into the nature and combinations of some of these bodies, and state some of the leading facts with which the modern discoveries in chemistry have made us acquainted.
Carbon, or charcoal, is considered an elementary body, because, as yet, no means have been found adequate to decompose it; it forms the skeleton of vegetables or their woody fibre.
We must now direct the attention of the reader to oxygen gas, the discovery of which was made by Dr. Priestley in the year 1774, and by him called dephlogisticated air; the most important discovery that was, perhaps, ever made in chemistry. When a metal is exposed to atmospheric air, at almost every temperature, it loses its metallic lustre, and acquires the form and appearance of an earthy substance. If this change be produced in a given quantity of air, the oxidation can only be carried on to a certain degree; and on examining the air which remains, we shall find that it has lost the whole of its oxygen, and that nothing remains but nitrogen gas. What was formerly called the calcination of metals is nothing but the process of their union with oxygen, which is now therefore properly called their oxidation.
If charcoal be mixed with the metallic oxide, and a suitable heat be applied to the mixture, it will unite with the oxygen and form carbonic acid, which will fly off in the form of gas, while the metal will assume its metallic form. From this we learn that oxygen is a part of atmospheric air, and that nitrogen constitutes another portion of the same air. Ammonia is a combination of nitrogen and hydrogen. Combustion, or the burning of any combustible body, cannot take place, at least under ordinary circumstances, without the presence of oxygen. Nitrogen gas, (called by its discoverers azotic gas), constitutes about three fourths of atmospheric air; the other fourth consists of oxygen, besides a small fraction of carbonic acid gas. Oxygen decomposes and destroys all fugitive colours. Oxygen is, besides, the basis of almost all the acids, and hence is one of the most universal agents in nature.
Hydrogen, formerly called inflammable air, was discovered by Mr. Cavendish in 1767; it is called hydrogen, because it is one of the component parts of water; or, more properly, it is the base of water. It is obtained in the most pure state from the decomposition of water by means of metals, thus: pass one hundred parts of water through a red hot iron tube, a gun barrel for instance, fifteen parts of hydrogen gas will be produced, while the inside of the tube will be found converted into an oxide, and to have gained eighty five parts in weight.
Again, when eighty five parts of oxygen gas are burned with fifteen of hydrogen gas, both gases vanish, and one hundred parts of water are the result. Hydrogen gas, when in a pure state, is about fifteen times lighter than atmospheric air; hence its use for inflating balloons. Hydrogen, if inhaled, destroys animal life; combined with nitrogen, it forms ammonia, or the volatile alkali, as we have before stated.
We have mentioned the fixed alkalies in a preceding section. We may add here, that by the discoveries of Sir Humphry Davy, in the year 1807, the base of caustic, or pure potash, is now known to consist of a light, white metallic substance, to which the name of potassium has been given; it is of the consistence of soft wax; at a freezing temperature it is hard, brittle, and solid; when thrown upon water it instantly takes fire, hydrogen gas escapes, and an oxide of potassium, or caustic pot-ash, is produced. The potash and pearl-ash of the shops we must not forget, are combinations of carbonic acid and pot-ash, hence they effervesce with all the acids; but caustic pot-ash, containing no carbonic acid, combines with any of the acids without effervescence.
The SODA, as obtained from barilla, is a carbonate of soda; pure soda, or caustic soda, was, till the discoveries of Sir Humphry Davy, supposed to be, as well as potash, a simple substance. It is now, however, known to consist of a metallic substance of the colour of lead, but, nevertheless, lighter than water; upon which, when thrown, it produces, like potassium, violent action, yet does not, in general, like potassium, inflame. It is called sodium; pure soda consists therefore of sodium and oxygen, hence it is an oxide of sodium. These discoveries of the composition of the fixed alkalies are of infinite importance in the arts. The alkalies contain some very striking properties: