38 
MR. FARADAY ON THE 
tion of the bubbles. When reduced to powder, it should be again heated upon 
a piece of platina foil in the flame of a spirit lamp. 
84. The quantity of powdered platina which I have usually employed has 
been about 7 or 8 grains for every pound weight of glass. But in order to 
effect its more general and perfect diffusion, I have usually mixed it with ten 
or twelve times its bulk of pulverized glass. For this purpose, some of the 
rough glass, the same in composition with that to be perfected, has been crushed 
small in a clean agate mortar, and the finer parts separated from the coarser on 
an inclined and shaken sheet of paper. The former have been then mixed little 
by little with the platina, and rubbed slightly with the finger, to effect perfect 
separation of the metal, and then the coarser parts have been added, to increase 
the bulk. In this state it was ready for use. 
85. The time of introducing this prepared platina is, like the times of stir- 
ring, as yet under investigation. It has usually been sprinkled from the pla- 
tina ladle (28) over the surface of the well-fused and highly-heated glass, 
at the period of the first stirring. This method has the advantage of bringing 
the assisting 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 addition 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 interstices of the 
powder. 
86. On other occasions the prepared mixture of platina and glass has been 
introduced into the tray at the period when it was charged with the due quan- 
tity 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 ultimately prove the better method of 
proceeding, both for the greater length of time during which the platina can 
act, and for the facility and convenience of its introduction. 
87 . In either mode of appliance the platina has been found highly service- 
able; and in every case since its use, where stirring has not been necessary, the 
resulting glass has proved to be perfectly free from bubbles. 
ss. As already mentioned, the best periods for stirring and repose have not 
