VOL. LXXXI.] PHILOSOPHICAL TRANSACTIONS. 95 



very delicate one. I once directed a person to calcine together antimony and 

 bone shavings, in the usual manner, to that state in which the white powder 

 may be produced by a due degree of fire ; but instead of a snow-white mass, I 

 could not by any degree of fire obtain any colour but a dirty whitish or light 

 stone colour ; though repeated calcinations were employed. The reason of the 

 failure was, that the earthen dish had been broken during the calcination, and a 

 few very small pieces of it had scaled off, and being mixed with the powder oc- 

 casioned this disappointment with respect to colour. The same disappointment 

 has been also occasioned by using a rusty iron rod in calcining the mixture. 



The bone-ashes procured from the sal ammoniac and spirit of hartshorn ma- 

 nufactories, frequently failed in producing a white powder; and so did sometimes 

 the bone-ashes, called prepared hartshorn, sold by the druggists. Even after a 

 fine white-coloured mass had been made, if it was pulverized in an iron mortar 

 that had extremely little calx on its surface, or dirt, the powder was not white. 



The yellow coat and glaze on the sides of the crucible and surface of the cal- 

 cined mixture of bone and antimony, in these experiments, is to be ascribed 

 rather to the fusion of the clay of the crucible with the antimonial calx, 

 than to the greater degree of fire in the part of the crucible in which it takes 

 place; or than to the calx of iron and siliceous earth of the vessel : because the 

 same yellow coat and glazing are produced on the Wedgwood pyrometer-pieces, 

 which are placed in the middle of the charge, and where the degree of heat 

 cannot be so great as nearer the side of the crucible, and yet a snow-white mass 

 is produced between these clay pieces and the sides of the crucible. This effect 

 of clay, in forming a yellow coat and glaze, is shown by the observation of 

 what happens when the calcined mixture is put into a Wedgwood's crucible, 

 which is made of much purer clay than other vessels of this kind, and when it 

 is set in a larger Hessian crucible with the space between the 2 vessels filled with 

 the same calcined mixture. After exposure to a sufficient degree of fire, viz. 

 about 120*^ of "Wedgwood's scale, the inside and outside of the inner crucible 

 will be covered with a yellow vitreous coat and glaze, as well as the inside of the 

 outer crucible in contact with the charge, while the rest of the matter within 

 these vessels is of a snowy whiteness. This yellow coat is one reason for the 

 powder being of a shade of yellow in some specimens. 



Supposing the fusibility of the antimonial calces to be diminished the more 

 they are calcined ; the following experiment shows, that the antimonial calx in 

 James's powder is more calcined than that in Exper. 2. 



Exper. 7. 70igrs.of calcined antimony, as prepared in Exper. 2, triturated 

 with 534- SI'S- of calcined bone, formed a powder of a bluish cast, which being 

 exposed in a close crucible, for half an hour, in a melting furnace, the degree of 



