248 On the Manufacture of Optical Glass. [1829. 



the glass is either transferred by a platinum ladle into trays 

 roughly turned up out of old platina foil, or into a clean deep 

 white earthenware vessel containing much distilled water. In 

 the latter case it is obtained in a divided state, and when drained, 

 is dried on the sand-bath, and put up in clean bottles. 



29. When a crucible has been emptied of its first portion of 

 glass, it will serve, if carefully used, for a second, third, fourth, 

 or for many operations; but it should be watched for cracks 

 and casualties, that the running of the glass into the furnace 

 may be prevented, and, if necessary, another vessel taken. 



30. The rough glass thus prepared is in the next operation 

 to be converted into an annealed and finished plate. The size 

 must therefore be determined upon, and we will assume it as 7 

 inches square, and 8 tenths of an inch thick, that being the 

 dimension of the largest plate as yet made. For the purpose 

 of making a competent platinum vessel, a plate of that metal will 

 be required at least 10 inches square; but if larger, it should 

 not be cut, but either made into a tray with higher sides than 

 is absolutely needful, or else used first in the manufacture of 

 a larger plate of glass than the one to be described. It should 

 be of such thickness as to weigh at least 17'5 grains to the 

 square inch; and it is important that in its preparation a 

 good ingot, or the good part of an ingot, of platinum has been 

 selected, and that it has been rolled very gradually and care- 

 fully without the formation of any holes by the adhesion of 

 dirt or hard particles, or by the dragging of the metal in the 

 mills. The desired perfection is, I understand, best obtained 

 by rolling the platinum between two clean plates of good copper. 



31. The plate, being laid upon clean paper or a cloth on a 

 smooth table, is to be cleansed with a cloth and a little water 

 or alcohol, and then to be ignited at every part by a large 

 spirit-lamp. It must next be carefully examined as to its state, 

 and the occurrence of places upon its surface where holes are 

 likely to exist. If the metal seems dragged in any place, an 

 effect indicated by a roughness upon the surface, or by short 

 lines parallel to each other but perpendicular to the course of 

 rolling, such place should be noted or marked, for which 

 purpose a dot of ink will be convenient. If a scale appears, 

 or a small portion is apparently folded over, it should also be 

 marked ; and if a black spot is visible (and they are sometimes 



