It was decided at Liège, 1901, and Leeds, 1902,[196] that the “Shake Method” with chromed hide-powder, of the American Association of Official Agricultural Chemists, 1901 (A.O.A.C.), should be employed in the detannisation of used tanning liquors, as with these the filter method is apt to give too high results owing to the amount of non-volatile acids which they contain. The method of the A.O.A.C. is given in [Appendix C].
[196] Procter and Blockey quoted experiments at the Leeds Conference, proving that gallic acid and some other non-tanning substances were largely absorbed by the hide-powder filter, though probably not permanently retained by leather; while the error, though still considerable, was much less when the chromed hide-power shake method was employed. Where only gallotannic and gallic acids are present, as in the case of sumach and commercial gallotannic acids, the most accurate quantitative estimation is probably that by the Löwenthal method carried out as described L.I.L.B., p. 123, but considerable skill is required in its execution.
Analysis of Spent Tans.
It was decided at Leeds, 1902, that spent tans must be analysed like fresh tanning materials; but where the prescribed strength of solution cannot otherwise be obtained it is permissible to concentrate the entire solution by evaporation. It is advisable, where suitable apparatus is available, to concentrate in vacuo; but failing this, an ordinary flask may be used, in the neck of which a funnel is placed.
APPENDIX B.—THE DECIMAL SYSTEM.
The metrical system of weights and measures, and the Centigrade thermometer scale have been generally used throughout the book, as more international and scientific than the complicated systems still unfortunately in use in this country. They have been fully explained in the Author’s ‘Laboratory Book,’ p. 2; but as this is not always at hand, a short sketch may be permitted here.
The basis of the metrical system is the “meter,” which is approximately 1⁄10,000,000 of the distance from the earth’s pole to the equator, and is equal to 39·3708 English inches, and for many practical purposes may be roughly reckoned as 40 inches. The meter is divided into 10 parts or “decimeters,” 100 parts or “centimeters,” and 1000 parts or “millimeters.” The standard of capacity is a cube of 1 decimeter, or about 4 inches, and consequently contains 1000 cubic centimeters, and is denominated a “liter.” The standard of weight is 1 cubic centimeter of water (at 4° C.), which is called a “gram.” Hence 1 liter of water weighs 1 “kilogram,” or 1000 grams. 1 cubic meter of water contains 1000 liters, and weighs 1000 kilograms, or 1 metrical ton (2200 lb. English). For purposes of reduction, the following figures may be given:—
| 1 | gram | = | 15·431 | grains. |
| 1 | lb. av. | = | 453·6 | grams. |
| 1 | liter | = | 0·22 | gallon. |
| 1 | gallon | = | 4·543 | litres. |
Actual reduction is, however, generally unnecessary if the question be treated as one of proportion. Thus a solution of 1 gram per liter is of the same strength as one of 1 lb. per 100 gallons (1000 lb.), and very approximately, as one of 1 oz. avoirdupois per cubic foot. In the case of pits, it is often simplest to measure them directly with a meter rule; length, breadth and depth, measured in decimeters and multiplied together, giving the contents in liters, and, in the case of water, the weight in kilograms.