Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 3, 1802).djvu/95

77&#93; LEA rewarded with their gold medal. ~Jt consists of a vessel 1 2 feet in length, six feet wide, and three feet ten inches deep, in which a pair of brass rollers is fixed, one above the other : the centre of these is about ten inches beneath the top of the vessel 5 and, one inch lower, a covering of oak-boards or riddles, about an inch thick, is inserted in a groove on the inside ot the vessel, so that it may be occasionally re- moved. Theseboards are perforated in the centre w'ith several holes, each of which is about iivc-eighths of an inch in diameter. Previouslv to the working of this machine, the vessel is tilled with water, about tliree inches above the oak-boards ; when the whole of the lower brass-roller, and half the tipper roller, are completely im- mersed : the lead, on being cor- roded, is passed through these ■ cyhnders ; and, by stirring the metal with a copper rake^ the ce- ■ russe is forced throng! i the riddles, and the blue or nncajcined metal remains above. Thus, white-lead is prepared ; and, by such useful contrivance, the minutest particles ■ are prevented from ascending, and consequently their pernicious in- fluence on die health of the work- men is effeftually obviated. Among the different patents re- ' gistered for the manula6ture of white-lead, we shall mention only those granted to Mr. James Tuk- NER, in 1780; to Mr. Richard FisHwicK, in 1/8/; and to the Earl of DuNDo>.-ALD; of which the reader will find ample specifi- cations in the different volumes of the "Repertory," ike. — A patent was also granted in l7Qg, to Mr. John "W'lLKiNsoN-, for a new inethod of making white-lead : he direds any quantity of litharge to be ground LEA 77 yery fine in sea-water, or other sa- line mixture ; and, by repeatedly triturating, washing, and bleach- ing it, the patentee asserts, that white-lead of the best quality may be obtained. No vinegar, or other acid mixture, is necessary in this process ; as levigation, repeated ablutions, and drying, are amply sufficient, provided more time be allowed for the operation, by the medium of the common atmos- phere. The last patent we shall notice, was obtained, at the commence- ment of 1801, by Mr, Thomas Grace, for a contrivance of mak- ing an acid to corrode lead, and ialso for a new process of manu- facturing white-lead. — In either re- .spcci, however, the usual method is but little varied : the acid re- quired for the purpose is generally prepared by fermenting melasse,? and v.-ater, together with other materials, which are well known to vinegar-makers, and which it would be needless todetviil. Among other articles, he makes economical use of the S0ZJ7-S, or water in which wheat has been steeped for con- verting it into starch ; as likewise of the v.ater employed for distihing oil of turpentine ; both these U- quids possess a considerable j;or- tion of acidity, which has hitherto been generally wasted. All the ditferent methods of pre- paring white-lead, however, arc extremely pernicious, as well to the manuflidurer as to those who use vessels that are glazed with it- Hen ce we have already (vol, ii. p. 3/7) pointed out proper sub- 'stitutes for this destrudive metal, which were invented by fcjreigii chcmistsj and shall, therefore, con- clude with an ^iccount of the pa- tent grunted in l^QO to Mr. James