OCTOBEE 4, 1895.] 



SCIENCE. 



435 



die and four minutes of electrification gave 

 nearly maximum effect. The greatest de- 

 flection observed was 936 scale divisions. 

 To find, from this reading, the electric den- 

 sitj of the air in the can, we took a metal- 

 lic disk, of 2 cms. radius, attached to a long- 

 varnished glass rod, and placed it at a dis- 

 tance of 1.45 cm. from another and larger 

 metallic disk. This small air condenser 

 was charged from the electric light conduc- 

 tors in the laboratory to a difference of po- 

 tential amounting to 100 volts. The insu- 

 lated disk thus charged was removed and 

 laid upon the roof of the large insulated 

 can. This addition to the metal in connec- 

 tion with it does not sensibly influence its 

 electrostatic capacity. The deflection ob- 

 served was 122 scale divisions. The capac- 

 ity of the condenser is approximately 



i^ XI '45' 



1 

 1-45' 



The quantity of electricity with which it 

 was charged was 



1 

 1-45 



1 



4-35 



electrostatic unit. Hence, the cxuantity to 

 give 936 scale divisions was 



J^ X^-1 7637 

 4-35 ^ 122 - ^■^'''^^■ 



The bellows was worked vigorously for 

 two and a-half minutes, and in that time 

 all the electrified air would be exhausted. 

 The capacity of the can was 16,632 cubic 

 centimetres, which gives, for the quantity 

 of electricity per cubic centimetre, 



The electrification of the air in this case 

 was positive ; it was about as great as the 

 greatest we got, whether positive or nega- 

 tive, in common air when we electrified it 

 by discharge from needle points. This is 

 about four times the electric density which 



we roughly estimated as about the greatest 

 given to the air in the inside of a large 

 metal vat, electrified by a needle point and 

 then left to itself, and tested by the poten- 

 tial of a water dropper with its nozzle in 

 the centre of the vat, in experiments made 

 two years ago and described in a communi- 

 cation to the Eoyal Society of date May, 

 1894.* 



§ 4. In subsequent experiments, electri- 

 fying common air in a large, gasholder over 

 water by an insulated gas flame burning 

 within it with a wire in the intei'ior of the 

 flame kept electrified by an electric machine 

 to about 6,000 volts, whether positively or 

 negatively, we found as much as 1'5 x 10~* 

 for the electric density of the air. Electri- 

 fying carbonic acid in the same gasholder, 

 tvhether positively or negatively, by needle 

 points, we obtained an electric density of 

 2-2 xlO-^ 



§ 5. We found about the same electric 

 density (2*2 x 10"'') of negative electricity 

 in carbonic acid gas drawn from an iron 

 cylinder lying horizontally, and allowed to 

 pass by a U-tube into the gasholder with- 

 out bubbling through the water. This elec- 

 trification was due probably not to carbonic 

 acid gas rushing through the stopcock of 

 the cylinder, but to bubbling from the liquid 

 carbonic in its interior, or to the formation 

 of carbonic acid snow in the passages and 

 its subsequent evaporation. When carbonic 

 acid gas was drawn slowly from the liquid 

 carbonic acid in the iron cylinder placed 

 upright, and allowed to pass, without bub- 

 bling, through the U-tube into the gas- 

 holder over water, no electrification was 

 found in the gas unless electricity was com- 

 municated to it from needle points. 



§ 6. The electrifications of air and car- 

 bonic acid described in Sections 4 and 5 

 were tested, and their electric densities 

 measured by drawing by an air pump a 



* 'On the Electrification of Air, ' by Lord Kelvin 

 and Magnus Maclean. 



