perfect; for if there be the least difference in different parts, it is liable to form striaæ: nor are the different portions allowed by the process to arrange themselves according to their specific gravities, in which case one part might perhaps be removed from another, after the glass was finished and cold; but the ascending and descending currents which inevitably take place in the fluid matter, are certain to arrange the irregularities in such a manner as to produce the strongest possible bad effect.
75. The instrument used for stirring has hitherto consisted of a piece of plate platinum, which for the seven-inch glass (taken as illustrating the process) is 6¼ inches in length and ¾ths of an inch in breadth. It is perforated with various irregular holes, that, when drawn through the glass like a rake, it may effectually mix the parts. A piece of thick platinum wire, about 13 inches long, is riveted to it, and the extremity of this screwed into the end of a clean iron rod which answers the purpose of a handle. No small or cellular apertures should be allowed in this stirrer; for they will frequently retain air or moisture, which may cause bubbles in the heated matter and do much harm. A little gold, therefore, should be applied to the part where the stem is attached, and fused, so that all hollows may be filled up. Stirrers of different dimensions are to be provided for different-sized plates of glass. Before being used, they should be steeped in dilute nitric acid, and also heated to redness in the spirit-lamp, just previous to their immersion in the glass for the first time in each experiment.
76. When a stirring is to be performed, the tiles and iron covers are removed from the first part of the chamber (44. 49. 65), the tray-covers also taken off and put into the back part of the chamber (61. 65), the glass quickly examined, to give assurance that all is in good condition, and then the stirring commenced. The stirrer should be put in gently, that no air may be carried down with it, and then drawn through the glass quickly but steadily, so as to mingle effectually, but not to endanger forcing the substance over the edges of the tray or to run the risk of involving air-bubbles. The chamber and its contents are cooled by the necessary exposure to the atmosphere, and therefore, when the agitation has been continued until the glass is so much lowered in temperature as to become thick, it should be discontinued, the stirrer carefully removed,
