_Thiosulphates_ are detected by precipitating any sulphite and sulphate with barium chloride, filtering, acidifying, and adding a few drops of potassium permanganate solution, when in the presence of a mere trace of thiosulphate, the solution becomes cloudy. _Sulphides._--Lewkowitsch recommends testing for these by replacing the mercury bichloride with lead acetate paper in the Gutzeit arsenic test. Any sulphide causes a blackening of the lead acetate paper. _Sugars_ may be tested for both before and after inversion, by boiling with Fehlings' solution, when no reduction should take place, if pure. _Fatty acids_ are detected by the turbidity they produce when the diluted glycerine is acidified. CHAPTER XI. STATISTICS OF THE SOAP INDUSTRY. Until the year 1853 the amount of soap produced annually in this country was readily obtainable from the official returns collected for the purpose of levying the duty, and the following figures, taken at intervals of ten years for the half century prior to that date, show the steady development of the industry during that period:-- _______________________________________________________________ | | | | | | | Year. | Manufactured. | Consumed. | Exported. | Duty per Ton. | |_______|_______________|___________|___________|_______________| | | | | | | | | Cwts. | Cwts. | Cwts. | L | | 1801 | 509,980 | 482,140 | 26,790 | 21 | | 1811 | 678,570 | 651,780 | 26,790 | 21 | | 1821 | 875,000 | 839,290 | 35,710 | 28 | | 1831 | 1,098,210 | 955,360 | 142,850 | 28 | | 1841 | 1,776,790 | 1,517,860 | 258,930 | 14 | | 1851 | 1,937,500 | 1,741,070 | 196,430 | 14 | |_______|_______________|___________|___________|_______________| Since the repeal of the soap duty, the revenue from which had reached about L1,000,000 per annum, no accurate means of gauging the production exists, but it is estimated that it has nearly quadrupled during the last fifty-five years, being now some 7,000,000 or 8,000,000 cwt. per annum. The number of soap manufacturers in the United Kingdom is nearly 300, and the amount of capital invested in the industry is roughly estimated to approach L20,000,000 sterling. Official figures are still available for the amount and value of soap annually imported and exported to and from the United Kingdom, the returns for the last eight years being:--