382 RECORD OF SCIENCE FOR 1886. 



under varying lengths and pressures. He finds (1) that the resistance 

 l)r()per of the air is sensibly independent of the section of the tube, 

 varies j)roporti()nal to the pressure nearly, and ceases to be measurable 

 for very low pressures j (2) that the resistance at the electrodes in- 

 creases more and more rapidly as the pressure diminishes, becomes 

 strong enough at the lowest pressures to prevent the electricity from 

 enteriug the gas. According to Goldstein the seat of this resistance is 

 the surface of the negative electrode ; according to Hittorf it is in the 

 luminous aureole which surrounds it, while Wiedemann jilaces it in 

 the dark space which appears at low pressures. ( Weid. Ann., xxvi, 85 ; 

 J. Phys., November, 1886, II, v, 543.) 



In his memoirs upon comets, Faye has stated his belief that incan- 

 descent bodies are endowed with an actual repulsive force, and lie gives 

 an experiment which he thinks proves this assumption. Wesendonck 

 has repeated and varied Faye's experiment, heating the platinum strip 

 to incandescence, either by a current or by a flame, and modifying the 

 form of the apparatus, the nature of the gas within it, and the press- 

 ure ; and he concludes that all the phenomeuaobtained can be explained 

 either by the expansion of the gas in the vicinity of the incandescent 

 platinum or by the lateral passage of a part of the discharge through 

 the platinum strip ; and this without resort to repulsion. (Weid. Ann., 

 XXVI, 81 ; J. Phys., November, 1886, II, v, 544.) 



Righi has photographed the electric spark in air and in water, under 

 various conditions. The sparks in water were 7 or 8'^'" long, rose-red, 

 with weak discharges, white and very brilliant with strong ones. They 

 have no aureole. They are often ramified as in air and present over their 

 whole length small sparks, starting to the right and left like a sort of 

 down. Beautiful i)hotographs have also been obtained of the aureoles 

 around the positive and negative electrodes, when the spark has not 

 tension enough to jump. Sometimes two images of the aureole are seen, 

 difleringiu brilliance, projected the one upon theothei, the photographic 

 impression at the point of crossing being paler than the rest of the 

 most distinct image. This the author explains by supposing absorption 

 at a lower temperature by the layers nearest the plate of the light pro- 

 duced by those back of them. (II Nuovo Cimento, xviii, 49; J. Phys., 

 December, 1886, II, V, 575.) 



Kayser has photographed successfully lightning discharges. He has 

 called attention to one particular discharge formed of four flashes of 

 diflerent sizes, parallel through all their sinuosities, decreasing grad- 

 ually in length and brilliance from the first to the fourth. He regards 

 it as an oscillating discharge between a cloud and the earth. He esti- 

 mated the total discharge to have occupied less than a half second. 

 (Weid. Anns., xxv, 131; J. Phys., November, 1886, II, v, 525.) 



Emmott and Ackroyd have exhibited to the London Physical Society 

 an electric-light fire-damp indicator. It consists of two incandescent 

 lamps, one with colorless, the other with red, glass, the circuit being so 



