32 
MR. FARADAY ON THE 
purity hereafter, or of cementing the tray and the earthenware together in a 
very inconvenient and injurious manner. 
G8. If, also, it should be observed, at this time, that there is a superabun- 
dance of glass in the tray, and not sufficient distance between its surface and 
the edges of the platina, the excess should be ladled out (28), an operation 
easily performed, but which must be done with care. 
69. When the glass is ascertained to be in a proper condition, and that there 
is no appearance of any portion of it outside the tray, the covers are to be re- 
placed, the chamber closed, and the heat continued. If the covers be glazed, 
some precaution is required in their arrangement ; for on putting the second 
cover over the first, if they are left in contact by a portion of glazed sur- 
face, they will be found, upon their next removal, to adhere at that place. 
They should never be put in contact therefore with each other, or, if that can- 
not be avoided, a piece of old platina foil should be laid upon the place where 
the contact is necessary (58). 
70 . Whilst the glass is covered and subjected to a high temperature, there is, 
as before stated, an inward current of fresh air passing continually to and about 
it through the air tube, during the whole time of the experiment (55). 
It was necessary to apply a valve to the external orifice of this tube to regu- 
late the supply ; for the draught was so considerable, that the glass was cooled 
by it, and much dust carried in. Finding reason to believe that even when very 
much diminished, the quantity of soots and dust in a London atmosphere, and 
especially in that portion of it taken from an experimental room in which a 
powerful furnace was at work, were competent to do much harm in eighteen 
or twenty-four hours, by giving colour and forming striae, experiments were 
made on the means of cleansing the entering air. It was found easy to effect 
this, by the assistance of two or three Woulfe’s bottles, or two or three jars, 
inverted one within another, using at the same time portions of diluted sul- 
phuric acid, or such solutions of salts in the vessels as would not supply any 
moisture to the air, but rather hike water with the dust from it. In these 
oases the air did not bubble through the liquid, but only passed close to its 
surface, and had time to deposit its dust during its passage through the in- 
closed spaces above the fluid: but, finally, a still simpler arrangement was used, 
consisting merely of a plug of clean dry sponge fitted into the end of the tube, 
