Sampling.—Samples should always be drawn from at least 10 sacks or separate parts of the bulk, and, in the case of valonia, special care should be taken to have a fair average quantity of " beard." If soveral sacks are apread in layers on a level floor, and then portions going quite to the ground are taken from eeveral parts of the floor, this will be accomplished. Where samplus must be dealt with which have not been specially drawn, it might be safest to weigh out from each the same proportion of beard and whole cupe, bearing in mind that the beard is always the richest part of the valonia. In sampling myrabolans, it ehould be remembered that the poor am] light nute will rise to the top, and hence the hand should be plunged well into the sack. Grinding when practicable is probably best done in a small disinteg,rator, fitted with gratinga. The material, of which some pounds must be used, is screened over a sieve of say 15 wires per in., and all coarser parts are returned to the mill till they will pass. The mill must grind inte a close box, that no dust may be lost. The advantage of this plan is that samples can be ground without previous dryiug, and thus in many cases time may be saved and eeparate determination of moiature avoided. When this is not practicable, the sample of some lb. at least is ground in an ordinary bark-mill, well mixed, spread out flat on a floor or table, and several portions are taken as already described, say 50-100 grm. in all, and dried in a water- or air-oven at 100° (212' F.). The moisture is best determined, to ease time, in a small eeparate portion of 10 grm., which must be dried till it ceases to lose weight, and the leas taken as moisture. It must be weighed in a covered capsule, as it is very hygroscopic. When the larger portion of the sample has been dried some houra, it is passed twice through a good coffee-mill, and then returned to the oven till thoronghly dried, for which, 12-24 hours is generally sufficient. Another method sometimes convenient is to take eaCh acorn, or each piece of bark of the eample to be tested, and enip a piece from it with a pair of tinners' shears, taking care that in the case of valonia the section runs right to the centre of the cup ; end in bark, that fair sharee of the outer and inner layers are taken. 'The reason for drying before grinding is, that unless hard dried, tanning materials cannot be passed through a small mill. Bark and valonia usually contain 12-16 per cent. of moieture.
LOwenthaTs Process : Exhaustion.-10 grm. of valonia, 20-30 grm. of bark, or corresponding quantities of other materials, are boiled for 10 minutes with 250 cc. of distilled water, great care being taken at first to avoid frothing and boiling over. The clear liquid is then poured into a gauged flask, the residue is boiled up for 10 minutes, a second and a third time, with 250 ce. of water, and is finally rineed into the flask, allowed to cool to 15° (59° F.), and made up to 1 litre. In the ease of sumach, a little more boiling even than this is clearable. Another method is to boil for hour with 250 cc. of water, then pour the whole on a filter, waah with boiling water so long as a drop of the filtrate bluekene paper moistened with a dilute eolution of ferric acetate, and finally make up to 1 litre. Many materiala, however, clog the filter to such an extent that washing is almost impossible. Kathreincr uees 15 litPes of water, and correaponding quantities of material, in a large steam-jueketed copper pan, for each exhauation, making the weight up finally to 15 kilos., aud obtains very uniform and excellent results. With all materials which deposit ellagic acid or other insoluble derivatives, on cooling and standing, considerably higher mulls will be obtained if the titration be made as soon as the liquor ia cold, than if it be allowed to etand 24 hours; in this respect, a uniform practice 'Mould be adhered to. Addition of
cc. of glacial acetic acid renders the infusions less liable to change.
Reagents.—Solutions are required of (1) Pure permaganate of potaeh, 1 grm. per litre. (2) Pure sulphindigotate of potash, 6 grm., and concentrated eulphurie acid, 50 grm. per litre. (3) Pure oxalic acid, 10 grm. per litre. The sulphindigotate of potaah (indigo carmine), must be filtered, and when oxidized by permanganate, el:mild give a pure clear yellow, free from any trace of brown or orange. Any contamination with indigo purple, which gives brown oxidation-products, is quite fatal to the aceuraey of the analysis. The permanganate eolution is standardized by measuring 10 cc. of the (deeinortnal) oxalic acid solution, adding a little pure sulphuric acid and distilled water, warming to 58° (136° F.), and running in the permanganate till a faint permanent pink is produced, for which about 50-51 cc. should be required. The indigo-carmine solution should be of such strength that 14-16 cc. of permanganate are required to bleach the quantity employed, which may be 20-25 ee., as convenient. (4) Gelatine eolution : 25 grm. of gelatine or finest glue are dissolved in 1 litre of water, and saturated with table-salt. (5) Salt and acid solution : to a saturated solution of common salt, are added 50 cc. of concentrated hydrochloric acid per litre.
Actual Analyais.-25 cc. of indigo solution are diluted to about litre with distilled or good drinking-water in a white porcelain baain, and the permanganate solution is added drop by drop from a burette with constant stirring till the last trace of green disappears. Call the quantity of permanganate used A. 25 cc. of indigo solution and 10 cc. of the tannin infusion are treated aa above. Let the quantity of permanganate be B. (If it exceeds 25 cc., lees tannin infusion must be taken, and the results calculated accordingly.) 100 cc. of the tannin infusion are well mixed with 50 cc. of gelatine solution, and then with 100 cc. of salt solution, shaken for some minutes, allowed to stand several houre, and filtered. The filtrate ehould be perfectly bright and clear. 50 cc. of this filtrate ( =20 of original infusion) are titrated with permanganate and indigo as beforo described, the permanganate being run in very slowly, 5-6 minutes being taken for the process. Let this be called ; it is the amount required to oxidize the " not tannin." Then B — A — (C — A) ie the quantity of permanganate required to oxidize the tannin. 4.156 grm. of pure gallotannic acid (Neubauer) and 6.236 grm. of oak-bark tannin (Oser) are said to be equivalent to 6.3 grm. of oxalic acid, and these numbers may be used to calculate percen tages ; but as the equivalents of most tannins are nnknown, it would probably be better simply to calculate out the equivalent percentage of oxalic acid, which does not differ sensibly from that of oak-bark tannin. Then calling T the permanganate coneutned by the tannin of 10 cc. infusion, 0 that required by 10 cc. of the oxalic eolution, and x the percentage, and supposing 10 grm. of substance to be employed, 0 : T : : 100 : x; or the quantity of permanganate required to oxidize the tannin, divided by that required to oxidize the oxalic acid, and multiplied by 100, gives the percentage. Gallic acid needs more permanganate than the same weight of tannic acid, or even than the quantity of tannic acid from which it is derived.