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MR AV. R. CARR ON THE LAM'S GOVERNING 
A like conclusion must be drawn from the curve shown in fig. 3, which graphically 
represents the numbers in Table IT. In plotting this curve the products of spark 
lengths and discharge pressures were taken as abscissse and the sparking potentials 
as ordinates. The regularity of the curve which represents the products for the five 
different electrode distances shows clearly that there can be no doubt regardino’ the 
applicability of Paschen’s law to electric discharges, in air, at pressures at and below 
the critical point as well as at pressures above it. 
IV. Ex'periments in Hydrogen. 
In order to demonstrate, if possible, the generality of the law which has just been 
proven to hold for discharges in air, a series of measurements were made on the spark 
potentials in the gases hydrogen and carbon dioxide. 
In these experiments exactly the same apparatus was used as in the previous 
experiments in air. 
Preparatory to making the measurements in hydrogen the apparatus was first 
exhausted of air to a pressure of 1 millim. of mercury, or less, and then filled with 
hydrogen to atmospheric pressure. It was then exhausted and refilled with hydrogen 
several times to make certain that all air was removed. 
The hydrogen was prepared from zinc and sulphuric acid in a Kipp apparatus, and, 
in order to ensure purity and freedom from moisture, was passed through wash- 
bottles containing potassium permanganate and caustic potash, and through a tube 
tightly packed with phosphoric pentoxide, before being led into the discharge 
chamber. 
Also, just as in the experiments in air, the gas was always allowed to stand for 
several hours, at a pressure of about 20 millims. of mercury, in the pi'esence of 
phosplioric pentoxide liefore any readings were recorded. 
