274 On the Manufacture of Optical Glass. [1829. 



pulled outwards by their extremities, will usually open, so that 

 the platinum becomes single again. Then proceeding from 

 corner to corner, the platinum 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 platinum 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 platinum, 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 platinum 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, perhaps even to 

 the full extent of our desires ; but it will require time and 

 patience to effect it. As I have before said, we are in the 

 course of our experiments only ; and up to the last have seen 

 reason to vary the arrangements, and still intend to make altera- 

 tions. Everything agrees to convince me that the size of the 

 plate is not a circumstance involving any additional difficulty ; 

 but that, on the contrary, it will probably be safer to make 

 a large than a small experiment. We can at pleasure obtain a 

 glass perfectly free from striae^ unexceptionable in hardness, 

 and with less colour than crown glass ; but it is the simul- 

 taneous absence of all striae and bubbles, with at the same 

 time that degree of hardness and colour which will render the 

 glass fit for optical purposes, that I am aiming at, and that I 

 trust shortly to obtain. 



95. As soon as the plates of glass are removed from the 



