1829.] On the Manufacture of Optical Glass. 271 



stirring. This method has the advantage of bringing the as- 

 sisting substance into contact with the glass when the latter is 

 highly disposed to throw off its adhering gaseous matter, and 

 also allows of thorough mixture ; but it also causes the addi- 

 tion of fresh glass after the concoction of the materials has 

 been proceeding for many hours ; and it likewise occasions the 

 introduction of many bubbles formed by the air in the inter 

 stices of the powder. 



86. On other occasions the prepared mixture of platinum and 

 glass has been introduced into the tray at the period when it 

 was charged with the due quantity of rough glass, and before 

 the application of fire. Particular attention was then paid to 

 its general diffusion throughout the charge, and on these occa- 

 sions its action commenced the moment the glass in contact 

 with it was fluid. I am inclined to believe the latter will ulti- 

 mately prove the better method of proceeding, both for the 

 greater length of time during which the platinum can act, and 

 for the facility and convenience of its introduction. 



87. In either mode of appliance the platinum has been found 

 highly serviceable ; and in every case since its use, where stirring 

 has not been necessary, the resulting glass has proved to be 

 perfectly free from bubbles. 



88. As already mentioned, the best periods for stirring and 

 repose have not been finally determined. Stirring introduces 

 bubbles, and therefore should, if possible, be avoided towards 

 the conclusion of the experiment. Rest, or at least that con- 

 dition in which there is no other motion than what is due to 

 the currents produced by slight differences of temperature, 

 causes striae even after very careful mixture (71. 72), and is 

 therefore equally to be feared ; and whatever other variations 

 may have been adopted, I have always found it important to 

 apply a careful concluding stirring. The following may be 

 considered as the order of an experiment. If the spongy pla- 

 tinum has not been introduced into the tray with the rough glass, 

 then about the sixth hour after lighting the fire it is added in 

 the manner already directed (85), and the glass well stirred (76). 

 At about the twelfth hour the stirrings are recommenced, for 

 the purpose of making the mixture perfect, and are repeated 

 every 20 or 30 minutes, according to the fusibility of the glass 

 and the state of the heat (60), for eight or nine times. The glass is 



