1 829.] On the Manufacture of Optical Glass. 253 



be examined by a clean dry point of bibulous paper, wbich 

 soon shows* by its change of appearance, the transmission of 

 any fluid : but attention is required that no false indication be 

 produced by carelessly bringing the paper near the upper 

 edges of the platinum, especially in the folded places. These 

 minute holes do not occasion much harm in the furnace, but 

 no fault should be allowed to pass which care can correct. 



40. When the tray is faulty, the alcohol must be removed 

 by a small siphon, the holes soldered in the manner before 

 described (32), and the tray again tried. When it proves 

 good, it is, after the removal of the alcohol, to be heated red- 

 hot in every part by the flame of a large spirit-lamp, and then 

 reserved with care in a clean place until required. 



4-1. If the platinum has been used before, it should first be 

 ascertained that none of the glass from the former experiment 

 remain on it. If there be any portion, the plate must be 

 returned to the weak acid or pickle out of which it has been 

 taken. If free from glass, it should then be examined as to 

 any chemical injury it may have suffered. Any part which is 

 altered in appearance, or has been attacked by the acid, or 

 which tarnishes when heated to redness by the spirit-lamp, has 

 been thus affected ; and it will depend upon the extent of the 

 action whether the plate is unfit for further use. No chemical 

 injury is occasioned by the proper and successful performance 

 of an experiment. 



4.2. An examination for holes by the candle or lamp must 

 next be made, especially in the folds at the corners and where 

 adhesion of the platinum from welding may have occurred, and 

 any that are discovered are to be marked as before (31). The 

 plate should then be flattened by being put between two sheets 

 of writing-paper upon a smooth table, and the edge of a folding 

 knife or some other smooth substance drawn over it ; but if 

 this be done whilst old glass adheres to the plate, it is almost 

 certain to produce injury. The holes are then to be soldered 

 and mended, the patches being applied upon the same side as 

 before. The gauge for the new tray is to be applied to the 

 plate, shifted, if there be occasion, from its old position, as 

 before intimated (35), and the folding of the tray, its comple- 

 tion and examination, to take place as before. 



43. It is desirable never to cut the platinum smaller than can 



