[carr] AN EXTENSION OF PASCHEN’S LAW 173 
The hydrogen was prepared from zinc and sulphuric acid in a 
Kipp apparatus, and, in order to insure purity and freedom from 
moisture, was passed through wash-bottles containing potassium per- 
manganate 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 mm. of mercury 
in the presence of phosphoric pentoxide, before any readings were 
recorded. 
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In the experiments with this gas readings were taken for the same 
electrode distances, 1, 2,3, 5, and 10 mm., and the values of the spark 
potentials and their corresponding pressures are given in Table III. 
These numbers are also graphically set forth in Fig. IV. 
We see from this table that the readings corresponding to the 
spark potential 1800 volts are:— 



Pessina 7 Miineres 



Distance between electrodes °  Discharge pressure in 
in mm. mm. of mercury. 
1 2°60 
2 1°33 
3 861 
5 “516 
10 ‘264 

which pressures are in the ratio 9-9: 5-0: 3-2: 1-9: 1. 
