MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
41 
the experiment has been well conducted, every thing- will be found loose, and 
unaltered in disposition from what they were when first arranged. The earthen- 
ware supports are to be removed, and the tray taken out. After examining 
the glass itself, the exterior of the tray should be carefully observed, whether 
there be any appearance of leakages either through imperceptible holes or at 
the corners, and such places as can be rectified by a patch should be noted in 
reference to the future use of the platina. 
92. An operation which, to be successful, requires much care, is then to be 
performed ; namely, the stripping off the platina from the glass. The tray 
should be placed on clean smooth paper upon a cloth. The corners are one 
by one to be opened by a blunt smooth knife, or some softer instrument, from 
the side towards which they were folded ; and being then carefully pulled out- 
wards by their extremities, will usually open, so that the platina becomes single 
again. Then proceeding from corner to corner, the platina will peel or strip 
easily from the sides of the glass, and will remain adhering by the bottom 
only. From time to time as fragments of glass are formed, they should be 
blown away or otherwise removed, that they may not cut the metal. If now 
the glass be placed a little over the edge of the table and firmly held, the pla- 
tina may gradually be separated from the bottom in the same manner as from 
the sides, and the glass and the metal finally divided from each other without 
any injury to the former, and very little to the latter. 
93. Immediately upon the separation of the platina, and before it can receive 
any mechanical injury beyond what it was impossible to avoid, it is to be put 
into a pickle consisting of nitric acid and water, and left there for several days. 
The dilute acid acts upon the adhering glass, dissolving and loosening it, and 
the plate is thus rendered fit for future operations (41). The stirrers also, 
when no longer required in an experiment, should be taken from their iron 
handles and put into the same pickling liquor. In this way the platina is 
perfectly cleaned, and being afterwards washed carefully in pure water and 
ignited, is again ready for use. 
94. Such is the nature of the process as practised at present, by which plates 
of heavy optical glass seven inches square and eight pounds in weight have been 
prepared. I am encouraged to believe that it will admit of improvement, per- 
haps even to the full extent of our desires ; but it will require time and patience 
MDCCCXXX. 
G 
