1895.] on Atmospheric Electricity. 463 



can be watched for a long time. It will be noticed that the flame, 

 being altogether surrounded by a tube of the same potential, cannot 

 be active in this case, but the conductivity must be due to the gas as 

 it escapes from the chimney. 



It follows from these experiments that every fire burnt on the 

 surface of the earth, and every chimney through which products of 

 combustion pass, act like very effective lightning conductors, and 

 would consequently discharge, slowly but surely, any electrification 

 of the surface of the earth. The peculiar immunity of factory 

 chimneys against damage by lightning appears from statistics col- 

 lected by Hellmann in Schleswig-Holstein,* for while 6 '3 churches 

 per thousand were struck, and 8*5 windmills, the number per 

 thousand of factory chimneys struck was only ■ 3. 



Franklin was acquainted with the action of flames ; he also dis- 

 covered that no charge could be given to a red-hot iron ball, a fact 

 which seems to have been forgotten until re-discovered in our own 

 times by Guthrie. Franklin also tried the action of sunlight, but 

 obtained no result. Had he performed the experiment with carefully- 

 cleaned zinc, he would have anticipated one of the most striking of 

 Hertz's discoveries. We now know that a negatively-charged surface 

 will discharge into air when illuminated by strong violet light, and 

 sunlight will be sufficient with specially sensitive materials. This 

 action has been investigated in detail by Elster and Geitel, who have 

 not, however, succeeded in obtaining results with sunlight acting on 

 such bodies as we know the earth's crust to be made of. So far, then 

 we have no experimental evidence to include light as an active agent 

 in the phenomenon of atmospheric electricity. 



We possess in the electric discharge itself a very powerful, and 

 probably very generally active means of breaking down the insulating 

 power of air. Some of the experiments t which I described some 

 years ago to prove this, were objected to on the ground that it might 

 not be the discharge itself, but the ultra-violet light sent out by the 

 luminosity of the discharge, which was active. The following form 

 of the experiment conclusively shows that the discharge acts inde- 

 pendently of light. 



In Fig. 2, K represents a Ehumkorff coil entirely surrounded by 

 a metallic box B, which is connected to earth. The terminals of 

 the coil lead to two electrodes inside a metallic tube T, which is 

 also kept at zero potential. This tube is arranged so that a current 

 of air can be blown through it. The air, on escaping through the 

 tube, is made either to impinge on or to pass near a metallic plate 

 connected to a charged electroscope. Under these circumstances, the 

 electroscope is not discharged either by a current of air alone, or by 

 the coil alone. But as soon as the air is blown through the apparatus 

 while the sparks are passing and then made to impinge on the 



* ' Veroffentl. des Kgl. Preuss. Stat. Bureaus,' 1886, p. 177, quoted by Bebber, 

 ' Meteorologie,' p. 245. t Proc. Roy. Soc, vol. xlii. 



