Dr. Hare’s improved Eudiometers. 25 
should this method of producing ignition be deemed desirable 
for the sake of varying the experiments, or for the purpose of 
illustration. This glass screws on to the socket (S), the other 
being removed. ‘The wires (WW) remain, but should be of - 
such a height as not to interfere with the passage of the elec- 
tric spark. ‘The instrument is operated with as usual, ex- 
cepting the employment of an electrical machine, or electro- 
phorus, to ignite the gaseous mixture, in lieu of a calorimotor. 
For the travelling chemist the last-mentioned mode of igni- 
tion may be preferable, because an electrophorus is more por- 
table than a galvanic apparatus. 
In damp weather, or in a laboratory where there is a pneu- 
matic cistern, or amid the moisture arising from the respira- 
tion of a large class, it is often impossible to accomplish ex- 
plosions by electricity. 
Of the Mercurial Sliding-Rod Eudiometer with a Water Gauge. 
The eudiometer which I have described, though satisfactory 
in its results, and in its conveniency, when used with water, 
has not been found so when used over mercury. The great 
weight of this fluid caused the indications to vary in conse- 
quence of variations of position, during manipulation, too slight 
to be avoided. The instrument represented in the following 
cut (fig. 4) is furnished with a water gauge, which being ap- 
pealed to, enables us to render the density of the gases within 
in equilibrio with the air without. Hence we can effect their 
measurement with great accuracy. 
Let us suppose that this eudiometer has been thoroughly 
filled with mercury, the sliding-rod being drawn out to its 
greatest extent, and that it is firmly fixed over a mercurial 
cistern in the position in which it is represented in the drawing, 
the little funnel-shaped part at the bottom descending into 
the fluid to the depth of half an inch. Above this part is seen 
a cock (C), the key of which, in addition to the perforation 
usual in cocks, has another, at right angles to, and terminating 
in, the ordinary perforation. When the lever (L) attached to 
the key of this cock is situated as it is seen in the drawing, the 
tube containing the sliding rod communicates with the re- 
cipient, but not with the mercury of the reservoir. Supposing 
the lever moved through a quarter of a circle, to the other 
side of the glass, the tube in which the rod slides will com- 
municate at the same time with the recipient and the reservoir. 
By means of the gauge-cock (C) the passage between the gauge 
and the recipient is opened or shut at pleasure. 
As subsidiary to this eudiometer, another is provided with 
Vol. 67. No. 333. Jan. 1826. D a rod 
