268 On the Manufacture of Optical Glass. [1829. 



the tray-covers replaced, the chamber covers restored to their 

 situation, and the temperature allowed to rise for fifteen or 

 twenty minutes, when the operation may be renewed. 



77. All the precautions against loose particles, dust, and 

 soot, that were before spoken of (66), should be adopted in 

 this operation. In the act of stirring, the instrument should 

 not be struck carelessly against the bottom or sides of the tray ; 

 for the platinum in this highly heated state is very soft, and a 

 hole would readily be forced through it ; nor should it be 

 brought forcibly against the corners, for the metal is in such a 

 favourable condition for welding, that the least blow upon a 

 double part causes adhesion. By merely allowing the stirrer, 

 when ignited, to sink upon the bottom of the tray rather more 

 hastily than usual, it has adhered to the place ; and when, for 

 safety, an underlying plate of platinum was used (50), it was 

 always found welded to the tray at the places which the stirrer 

 had touched a little more forcibly than the adjacent parts, and 

 could not afterwards be separated without leaving holes in the 

 metal. This circumstance was the principal occasion of the 

 advantages afforded by the use of the underlying plate being 

 given up. 



78. The heat which has to be borne during the operation of 

 stirring is very considerable, especially upon the hands ; but 

 at such a moment no retreat from the work, because of mere 

 personal inconvenience, can be allowed. But the circumstance 

 renders the use of a cover for the stirring hand very advan- 

 tageous. I have found a loose linen bag, into which the hand 

 could go freely, more convenient for this purpose than a glove ; 

 for being in contact with the skin at distant parts only, the 

 hand is preserved at a much lower temperature. Two small 

 holes in it, one at the front and the other at the top, allow the 

 handle of the stirrer to pass obliquely through, by which 

 arrangement it is easily held with firmness, and the bag itself 

 prevented from slipping towards the glass. It should not be 

 larger than to cover the wrist, or it will embarrass the move- 

 ments ; and it should be very stiffly starched and ironed, that 

 no fibrous particles may fly from it to the glass during the 

 stirring. 



79. The glass which, adhering to, is brought away with the 

 stirrer, indicates, by its appearance, the general character 



