334 Cutbush on the Voltaic Lamp. 
tion has been completed, and sulphate of iron or of zine. 
formed, as the case may be, the quantity of water will al- 
Ways prevent its crystallization. Hence it is a matter of 
some moment to have the acid sufficiently diluted. 
In consequence of some defect or imperfection in the 
electrophorus cake, or of its splitting, which sometimes 
happens, I have found it necessary to remelt it, or to make 
a new one, by melting the best yellow rosin, and adding a 
small quantity of Spanish brown. Having melted the rosin, 
it was poured into a shallow dish made of tin plate, and left 
to cool undisturbed, not permitting however any bubbles to 
appear on its surface. 
The electrophorus belonging to my apparatus, when ex- 
cited will retain its effect for many months. I excited it in 
the usual manner, by gently warming it, and rubbing it with 
a foxtail, catskin, or silk handkerchief ; ‘the former of which 
T found preferable. When thus excited, I have been suc- 
cessful in producing a spark even in the dampest weather. 
Tt requires, however, that the box, which contains the elee- 
trophorus, should be kept as tight as possible. 
There is one defect which in fact is inseparable from the 
construction of the cock; namely, that however perfectly 
tight it may be, by frequent use it becomes loose, and suf- 
fers the gas to escape gradually. To prevent this, and te 
make the cock as tight as possible, I have used various ex- 
pedients, but the following I find preferable : mix a portion 
of tallow with finely pulverized plumbago, so as to render 
the whole as stiff as possible; then apply it to the-cock. 
From observation I have found, that hydrogen gas pre- 
pared by using zinc, makes its escape more readily than 
that prepared ‘with iron filings; for, under the same cir- 
cumstances, the former I have discovered to disappear 
sometimes in twenty-four hours, while the latter has re- 
mained more than a week. In consequence of this cir- 
eumstance, I employ iron filings in preference to those of 
zinc, although we know that the gas from the latter is 
much purer and consequently lighter, whereas that procur- 
ed by using iron filings contains more or less carbon, and is 
consequently impure. 
The lamp answers every purpose, and I find it more cer- 
for lighting a candle than any other contrivance, and 
herefore preferable to any which I have tried ; having used 
