90 Van-Marum's Gaxometer 
means sof plafter of Paris, which, that it may afford no > pats 
fage to the atmofpheric air, is coated over with common 
cement above the edge of the band aa, as may be feen at 
£5; and this cement is covered with a ftripe of linen ¢c, a 
quarter of an inch in breadth, dipped in the white of an egg 
mixed with lime. The rim of copper aa, has a copper ring 
dd, foldered to it at right angles; and on which the plate 
of copper ee is ground, in fuch a manner that it is fuf- 
ficient to greafe {lightly the upper furface of the ring dd, 
when a vacuum is to be made, taking care to apply fo little 
greafe that it may not penetrate into the balloon. To ex- 
hauft the balloon there is a cock which communicates by a 
bent tube with an air pump placed behind the balloon ; and 
to prevent the working of the air pump from faa the. 
balloon, this bent tube is made in part of elaftic gum. 
*' The plate of copper ¢e¢ is kept down upon the ring dd 
by fix ferews, two°of which may be feen at ff, placed at 
equal diftances around the neck of the balloon, and by 
means of which the plate ee may be made to prefs very 
ftrongly on the ring ¢d, on turning thefe ferews by the help 
of akey. Both a the gazometers are made to fhut in the 
fame manner. 
The inferior furface of the plate ee is covered, as far as It 
covers the opening of the balloon, by a thin plate of pure 
filver, in order that the vapours formed during the experi- 
ment may not touch the copper. For the fame reafon the 
bent tube /m, in the balloon, is alfo made of pure filver. 
At the extremity of this tube is a piece of platina, having a 
very fmall aperture, fearcely fufficient to afford a paffage to 
a very fine needle. The part 2, which ferves as a conductor 
to kindle the hydrogen gaz by an eleétric fpark, is alfo made 
of platina, as far as it is not enclofed in the olafs tube 60, by 
which it is infulated. I have preferred platina for thefe 
two parts, in order to prevent the oxydation of the metal, 
which the heat produced by the combuftion of the hydrogen. 
gaz in oxygen might occafion, Fhe Sg Pp pr, which 
ferve 
