1829.] On the Manufacture of Optical Glass. 267 



perfect ; for if there be the least difference in different parts, it 

 is liable to form striae : 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 6^ inches in length and f ths 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 en- 

 danger 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 atmo- 

 sphere, 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, 



