282 Dr. Hare’s Improved Barometer Gage Eudiometer. 
gage tube, as represented at B in the figure, its orifice communica- 
ting with the mercury of the same receptacle, and placing zero of 
the descending column of graduations on a level with the surface of 
the mercury in the Torricellian tube, the quantity of air in the re- 
ceiver will always be as the number of degrees, between the surface 
of the mercury in the gage and the surface of the same metal in the 
Torricellian tube. 
The scale comprises ten divisions, each containing ten subdivisions. 
The whole scale may therefore be otithated to divide the capacity 
of a receiver into ten volumes, or into one hundred, whenever the 
zeros of the right and left hand columns of degrees coincide simulta- 
neously, the one with the surface of the mercury in the receptacle, 
and the other with that of the Torricellian column. But on this it 
were vain to rely, since the altitude of the Torricellian column is 
liable to vary while the scale remains unchanged. ‘This difficulty is, 
however, easily surmounted by restricting the length of the graduated 
part of the scale to the minimum height of the mercurial column, or 
twenty-seven inches; and employing an excess of hydrogen when 
the quantity of oxygen is to be ascertained, and an excess of oxy- 
gen when the quantity of hydrogen, or hydrogen and carbon, are in 
estion; the excess in either case, being made equal to the differ- 
ence between twenty-seven inches, and the height of the Torricellian 
column. With this precaution, the quantities introduced or with- 
drawn, will always be to each other as the changes which they pro- 
duce in the column of mercury in the gage tube. The rise of the 
mercury in the tube, will cause the sartios of it in the receptacle D 
to be lower; but the breadth of this vessel is so great, and the de- 
scent of the mercurial surface in it is so inconsiderable, that no error 
worthy of attention is thus created. 
I ought to mention, that the cavity of the gage tube ought to be 
so small in proportion to that of the receiver, as to create no error 
worthy of attention. 
Description of the Galvano Ignition Apparatus. 
An iron cylinder, of about an inch in bore, includes another con- 
centric cylinder, or tube of glass. A platina wire, which, by being 
made — subject of a galvanic discharge, is employed to ignite the 
gasey the cavity ofthe glass. Opposite to it, two 
op snings: are made i in the 3 iron, which serve for windows, enabling the 
operator to see the progress of the i ignition, and consequently, to know 
when to break the am circuit, in order to avoid fusing the wire- 
Ses 
