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 divisionsj each containing ten subdivisions. 

 The whole scale may therefore be estimated 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 

 question ; 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- 

 dravi^n, will always be to each other as the changes wiiich they pro- 

 duce in the column of mercury in the gage tube. The rise of the 

 mercury in the tube, will cause the surface 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 Gahano 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 the subject of a galvanic discharge, is employed to ignite the 

 gaseous mixture, occupies the cavity of the glass. Opposite to it, two 

 openings are made in the iron, which serve for windows, enabhngthe 

 operator to see the progress of the ignition, and consequently, to know 

 when to break the galvanic circuit, in order to avoid fusing the wire. 



