2. The second process consists in exposing the goods to the action of alkaline legs.
The alkali universally employed by the bleachers in Great Britain is Russian or American potash, which contains about two-thirds of its weight of caustic potash, according to the experiments of Vauquelin (Annales de Chemie, XL. 273). The other ingredients are sulphate of potash, muriate of potash, carbonic acid, and siliceous earth. It has been alleged that the potash of commerce is often adulterated ar tificially with common salt. This the bleacher should always ascertain before employing it. Indeed, every bleacher, who wishes to be exact, ought to be in pos session of a mode of determining the exact quantity of potash which the alkali that he intends to use contains. There are two methods which may be employed for this purpose. The first is to dissolve a certain quantity of the potash in water, and to try how much acid of a known strength is requisite to saturate the alkali contained in this solution. Ample directions for reducing this method to practice, are contained in the dissertation of Vauquelin above re ferred to. The second mode of testing the alkali is more rapid ; but would be less convenient for the bleacher, unless he were in possession of a mercurial pneumatic trough; but if he is supplied with this part of chemical apparatus, the method is very easy ; and, perhaps, in the hands of persons not very conversant with chemical experiments, more to be depended on than the first described method. It is this. A glass tube of the capacity of 10 or 12 cubic inches, shut at one end, and flat at the other, so as to stand on the mercurial trough when filled with mercury, is to be graduated into cubic inches and tenths. The tube, when the strength of the alkali is to be tried, is to be filled with mercury, and placed inverted on the trough. Then let up 20 grains of the alkali to be examined, which will rise to the top of the tube. Add now about 0 grains of sulphuric acid. As soon as the acid comes in contact with the potash, an ef fervescence takes place, and the carbonic acid is ex tricated. Observe the number of cubic inches and tenths of this gas extricated, taking care to sink the tube so far in the trough that the mercury in the tube and trough are upon the same level. Multiply the bulk of the carbonic acid by the number 0.46818 ; the product is the weight of carbonic acid present in grains. Multiply this weight by 2.18 ; the pro duct is the weight m grains of real potash contained in 20 grains of the pearl ashes under examination.
This last method is founded upon several chemi 'cal facts which have been sufficiently established.
The potash, as it exists in American or Russian potash, is combined with carbonic acid in such a proportion, that one atom of acid is combined with one atom of alkali. An atom of carbonic acid weighs 2.750, and an atom of potash 6. The weight of a cubic inch of carbonic acid gas is 0.46313 grains. Hence, if we multiply the bulk of carbonic acid in cubic inches and tenths, by 0.46313, we obtain its weight in grains.. The numbers 2.750 and 6, are very nearly to each other as 1 to 2.18. Hence, if we multiply the weight of carbonic acid, found by 2.18, we obtain the weight of potash with which it was combined. It is proper to know, that this method will give the proportion of potash rather below the truth, because, a little of the carbonic acid will be held in solution by the acid employed.. If we add such a quantity of sulphuric acid, that, after the expulsion of the gas, the whole shall remain in a liquid state, the result will be almost perfectly ex. act, if the bulk of the liquid be added to that of the gaseous product, and the whole be considered as car bonic acid gas.
Formerly, the Irish bleachers were in the habit of using barilla instead of potash. But there are two objections to the use of this alkali. In the first place, the weight of real alkali contained in the same proportion of barilla, is much smaller than in pearl ash ; and, in the second place, the weight of an atom of soda being greater than that of an atom of potash, it is probable, that the second will. go farther in bleaching than the former. When to this fact we add the difference of the price, which is always in favour of potash, there can be no hesitation in af firming, that no bleacher who studies the principles of economy, would make use of barilla if he can be supplied with pearl-ash. In trying the strength of barilla, the second of the two methods above de scribed cannot be employed, because barilla contains both carbonate of lime and carbonate of magnesia. Hence the quantity of carbonic acid will always be much greater than it would be, if barilla contained only carbonate of soda. According to the experi ments of Kirwan, a great proportion of the soda in barilla is in a caustic state. But if we attend to the way in which this substance is procured by burning the salcola vermiculata, we shall scarcely be induced to adopt this opinion. For, during the combustion of vegetable substances, carbonic acid is always evolved in considerable quantities, this acid would, of course, combine with the alkali, and the heat of the combustion is insufficient to decompose the car bonate of soda when once formed.