258 On the Manufacture of Optical Glass. [1829. 



crucibles are manufactured. These we obtained through the 

 intervention of our President; they were purposely manu- 

 factured for us by Mr. Michell of Caleneck in Cornwall, a 

 gentleman who has been ever willing and anxious to assist 

 us in our inquiries, by supplying us with vessels of any size 

 or form, or any other article which it was in his power to 

 produce. 



53. The Cornish plates have not much cohesion, and feel 

 tender in the hand. They may be rubbed down to a flat sur- 

 face, and resist any heat which can be applied to them in these 

 or in much more powerful furnaces. They are therefore 

 readily brought to any thickness, and when of about f ths of 

 an inch, and supported in the furnace as before described (47), 

 have strength to bear any weight required to be placed upon 

 them. They do not crack, nor do they force themselves to 

 pieces by expansion ; but they are porous, as indeed are in a 

 greater or smaller degree all the materials of which the chamber 

 and its sides are now composed. 



54. The porosity of these materials was of great importance ; 

 for it allowed of the passage of gaseous matter, and that even 

 of a reducing nature, from the fire into the chamber. I have 

 frequently had evidence that the sides and bottom might be 

 considered as a very sieve-like partition between the fire, the 

 flue, and the ' space called the chamber ; for when the upper 

 aperture has been closed, there has been a current through 

 the chamber in the direction of the flame, the gaseous matter 

 entering at the extremity nearest the fire, and passing out at 

 the end towards the flue. In one or two cases, oxide of lead 

 was actually reduced, and the glass thus rendered cloudy. 



55. Hence it became necessary to use some certain means of 

 maintaining an oxygenating atmosphere about the glass ; to ob- 

 tain which, and also to prevent any other injurious vapours from 

 the fire entering the space beneath and within the earthenware 

 covers (51), the expedient was adopted of allowing a current 

 of fresh air to pass continually into that space and circulate 

 about the glass. To effect this, a clean earthenware tube, glazed 

 within, was inserted horizontally into the side of the furnace, 

 in such a manner that one extremity was flush with the inside 

 of the chamber, and of such height, that its lower edge corre- 

 sponded with the level of the bottom upon which the glass in 



