Analyßs, 5 01 rally ignited them, though probably not long enough . Weftrumb has alfo conftantly detcrminen tne weights of his eartbs by ignition, and in many inttances has fhewn the difFerences of their weights when heated in the ufual manner, and after ignition-, he has proved that even ponderous fpar requires ignition to dry it perfedlly ; but he does not feem to have regulated the time they fhould' be expofed to a red heat. Dr. Withering dried his argill in a^ low red heat for io minutes; theother earths he feems to have barely dried in the heat of the atmöfpherc. Klaproth conftantly afcertains the weight of all his earths, after low ignition for half an hour. As in moft ftones the component earths are in an exceeding dry ftatc, and, lf not, the weight of water and air they contain fhould be afcertained previous to their analyfis; it fhould feem that the deficcation occafioned by ignition is that which approaches moft to their natural ftate, and confequently fhould always beobtained. 4. The difficulties accompanying the ?.p- plication of the Pruffianteft have occafioned many errors. Weigleb conftantly employed the Llood lye-f: this is lcarce ever perfedlly faturated, and, at leaft after a fhort time, precipitates earths as well as metals; alfo, by reafon of the vitriolic acid it contains, calx cannot efcape it; hence perhaps he has detetfed none where others have found it; befides, he makes no deduftion for the iron contained in this alkali. * ift Chyin Ann. 1786, 117, iftdo. 1787, 19. And 1788, 49. ‘ t ift Crell. Beytrage, 5?. K k 3 Mr.