50 
MR. FARADAY ON THE 
or gum lac. If one of these plates of glass, without any previous warming 
and drying, be lightly brushed or wiped with flannel or silk, it instantly be- 
comes strongly electrical, and retains its electricity for a long time ; but it 
would be almost impossible to develop electricity by such slight means with 
flint or plate or even crown glass in a similar state. Hence the glass makes 
as good an elcctrophorus as lac or resin, and may probably be found hereafter 
to answer many useful electrical purposes. But the great point at present in 
view, is the proof which such electrical properties give of the absence of that 
film of moisture which is so constant upon other glasses. 
1 13. All these circumstances are favourable to the opinion that the heavy 
glass will not be found objectionable in the construction of telescopes, because 
of any undue tendency to tarnish, and especially when precautions are taken 
to protect it from sulphuretted vapours in the manner before described (107). 
No difficulty can be anticipated in preserving the air within a limited and in- 
closed space free from such a contamination : to preserve it dry, if that had 
been necessary, under the different circumstances of varying temperature and 
the inevitable change of the air more or less frequently, would have been a far 
more difficult task. 
1 14. The other kind of superficial change, i. e. the corrosion or crystalliza- 
tion which takes place principally on plate glass, is doubtless also due to the 
alkali present. Sometimes, indeed, specimens of glass may be found where 
the alkali being too abundant, a similar but more extensive action has taken 
place over the whole of the surface, and the glass falls off in scales. Whether 
the alteration be due to the action of the alkali on the water only, or on the 
carbonic acid and other substances it finds in the air, or to its united action on 
all together, is of little consequence at present, as the substance on which it 
depends is altogether absent from the glass under consideration. 
1 15. Among the great number of glasses made, there arc several of different 
composition, which have been selected, because of their general characters and 
properties, for more extensive trial and investigation when time will permit. 
Of these it would be useless to speak at present, as what might be stated of 
them now would probably require correction from future experiments. Up to 
this period the attention has been devoted, as it still must be for a while, to 
the establishment of a process which, competent to produce with certainty a 
