[The principal kinds of soap manufactured in this country are,--white soap, composed chiefly of tallow and soda, but for some purposes of olive oil and soda; yellow soap, made of tallow, rosin and soda, a little palm oil being occasionally added; mottled soap, formed of tallow, kitchen stuff and soda, its peculiar appearance being communicated by dispersing the lees through it towards the end of the operation; brown soap, made from palm oil, rosin and soda. Soft soap is made with potash and drying oils, either alone or mixed with tallow, and other coarse fatty matters. The fatty matter is mixed with the alkaline ley, and the whole boiled gently for some time, until the fat is completely saponified, which may be known by its becoming clear and transparent, and its susceptibility of being drawn into long threads. A quantity of common salt is then added to the boiling mixture, until the soap loses its thready character, and drops from the spatula in short thick lumps. The soap is then removed, either after cooling, or at once ladled out. Common fatty matters, as tallow, fat-oils, &c., are compounds of a fatty acid with a base, thus resembling salts; the base is a peculiar sweet principle, glycerine; by ebullition with the caustic lye, the neutral fatty compound is decomposed, the fatty acid combining with the base soda, and forming the soap, whilst the glycerine with the excess of alkali remains in the liquid. The so-called _silicated_ soap, of which large quantities are now manufactured, is made by combining silicate of soda with hard soap in the hot and pasty state; in this way from 10 to 30 per cent. of the silicate may be introduced. Such soap possesses, according to Dr. Ure, very powerful detergent qualities, but it is apt to feel hard and somewhat gritty in use. The silicated soda is obtained by boiling ground flints in a strong caustic lye. Many substances are used to adulterate soap, such as potatoe-starch, clay, &c., for which _improvements_, as they are termed, numerous patents have been granted in this country. In Great Britain the hard kind of soap is chiefly made at Liverpool and London, but in considerable quantities also at Runcorn, Bristol, Brentford, Hull, Bromsgrove, Plymouth and Sethwick, and at Glasgow and Leith in Scotland; the soft soap is made principally at Liverpool, Glasgow and Bradford; and silicated soap is likewise extensively manufactured at Liverpool. From the excise returns, it appears that 140,712,535 pounds of hard, 9,788,851 pounds of soft, and 3,921,862 pounds of silicated soap were made in England in 1841; and 10,708,464 pounds of hard, and 4,535,030 pounds of soft soap in Scotland; making in all 169,666,742 pounds, which is an increase of about 30 per cent. since 1832[273].