22 
MR. FARADAY ON THE 
chemical injury it may have suffered. Any part which is altered in appear- 
ance, or has been attacked by the acid, or which tarnishes when heated to 
redness by the spirit lamp, has been thus affected; and it will depend upon the 
extent of the action whether the plate is unfit for further use. No chemical 
injury is occasioned by the proper and successful performance of an expe- 
riment. 
42. An examination for holes by the candle or lamp must next be made, 
especially in the folds at the corners and where adhesion of the platina from 
welding may have occurred, and any that are discovered are to be marked 
as before (31). The plate should then be flattened by being put between 
two sheets of writing paper upon a smooth table, and the edge of a folding 
knife or some other smooth substance drawn over it ; but if this be done 
whilst old glass adheres to the plate, it is almost certain to produce injury. 
The holes are then to be soldered and mended, the patches being applied upon 
the same side as before. The guage for the new tray is to be applied to the 
plate, shifted, if there be occasion, from its old position, as before intimated 
(35), and the folding of the tray, its completion and examination, to take 
place as before. 
43. It is desirable never to cut the platina smaller than can be helped, but 
always to make the largest plate upon it for which it is competent. Then, when 
operated with a second or third time, smaller guages may be used, and the 
folds will not be repeated in the same place ; and if injury occurs to the metal, 
being generally at the sides of the tray, the middle part will still be left for the 
preparation of smaller plates of glass. 
If such large plates of platina are required for trays as can hardly be rolled 
at once, there is no difficulty in making a folded joint and rendering it tight 
by soldering with gold. 
44. A kind of furnace, unlike the former, is now required for the completion 
of the glass, and its delivery in the state of an annealed plate. This furnace 
shall be described accurately in the Appendix. It may here be sufficient to 
state that it consists of a fire-place in which coals are burnt ; of a part beyond, 
acting both as furnace and flue, in which coke is used; and of a chamber above, 
to be heated by the fire, though out of the course of both flame and smoke. It 
is in this chamber that the glass is made ; so that, by the arrangement adopted, 
