432 
ME. AV. E. CAEE OX THE LAAA'S GOA^EEXIXG 
iniiiiiniiiu spark ])oteiitial, found tliat a very small pei-centage of oxvgeii increased 
their values for nitrogen by an amount out of all proportion to the ([uantity of the 
denser gas jiresent. 
I he calculated values for the minimum .spark potential in vater vapour and in 
ammonia are 439 volts and 543 volts respectively, and the values found bv Capstick 
for the cathode fall in these gases are respectively 469 and 582 volts. When we 
consider that the values in the one case are calculated from the measurements made 
on one effect, wliile the values in tlie second case are the direct experimental results 
on an entirely different effect, this com])aratively close agreement not only forms a 
corroboration of Strutt’s conclusions, but also lends support to the view that the 
mlnimuin spark j>otential has to do with the atoms rather than the molecules of a 
gas, and is determined, in any special case, by the a])plication of a simple additive law. 
In this connection it may be mentioned that the value found by Strutt"^ foi' tlie 
cathode fall in the monati.anic gas argon, 167 ^■olts, corresponds very closely with 
the constants which we have ascribed to the atoms of the various gases mentioned 
above. His value, 226 volts, for the monatomic gas helium, how’ever, is considerably 
larger than any of the atomic constants we have deduced. 
In performing these ex])eriments, all ordinary precautions Avere taken to ensure the 
purity of the gases. The nitrous oxide was prepared by heating ammonium nitrate 
111 a flask, and the gas Avas collected over Avater, but Avas Avell dried Avith phosphorous 
pentoxide before being passed into the disclnarge tube. The sulphur dioxide was 
prepared from copper and suliihuric acid. In order to purify it the better, the gas 
AA as diied and then licpiefied. It aatis further dried by being passed through a 
phosphoric pentoxide tube before reaching the discharge apparatus. 
Acetylene aatis obtained in the usual AAvay by the action of AA-ater on calcium 
caibide, and Avas carefully dried AAuth sulpliuric acid and phosphoric pentoxide. 
Hydrogen sulphide Avas prepared in a Kipp apparatus from ferric sulphide and 
sulphuric acid. It Avas sloAvly bubbled through Avash-bottles of Avater and then 
carefully dried in the usilal Avay. 
In every case, as in the early part of the experiments, the gas remained in the 
discharge chamber In the pre.sence of phosphoric pentoxide for several hours before 
any readings Avere taken. 
X. Summary of Results. 
1. The laAA’ governing electric discharges between parallel plates, in a uniform field, 
in any gas, foi piessures at and beloAA^ the critical pressures, is that AA'hich Paschex 
found to hold Avith spherical electrodes for higii pressures, viz., tliat, Avith a gHen 
spark jrotential, the pressure at AA^liicli discharge occurs is in\’er.sel\' proportional to 
the distance betAA'een the electrodes. 
* Strutt, ‘Phil. Mag.,’ March, 1900. 
