340 EXPERIMENTS ON THE RELATION 
dexterity, we transfer the gas into the sealed leg of the syphon. 
When we conceive enough to have been passed up, we remove 
the finger, and next bring the mercury to a level in both legs, 
either by the addition of a few drops, or by the displacement 
of a portion, by thrusting down into it a small cylinder of 
wood. We now ascertain, by careful inspection, the volume 
of included gas. Applying the fore-finger again to the orifice, 
so as also to touch the end of the platina wire, we then ap- 
proach the pendent ball or button to the electrical. machine, 
and transmit the spark. Even when the included gas is consi- 
derable in quantity, and of a strongly explosive power, we feel 
at the instant nothing but a slight push or pressure on the tip 
of the finger. After explosion, when condensation of volume 
ensues, the finger will feel pressed down to the orifice by the 
superincumbent atmosphere. On gradually sliding the finger 
to one side, and admitting the air, the mercurial column in the 
sealed leg will rise more or less above that in the other. We 
then pour in this liquid metal, till the equilibrium be again re- 
stored, when we read off as before, wi:hout any reduction, the 
true resulting volume of gas. 
As we ought always to leave two inches or more of air be- 
tween the finger and the mercury, this atmospheric column 
serves as a perfect recoil spring, enabling us to explode very 
large quantities without any inconvenience or danger. The 
manipulation is also, after a little practice, as easy as that of 
the single tube. But a peculiar advantage of this detachable 
instrument, is to enable us to keep our pneumatic troughs and 
electrical machine at any distance which convenience may re- 
quire ; even in different chambers, which, in the case of wet 
weather, or a damp apartment, may be found necessary to en- 
sure electrical excitation. In the immediate vicinity of the 
water pneumatic cistern, we know how often the electric spark 
refuses 
