December 18, 1896.] 



SCIENCE. 



893 



face of the dish, and only a few millimeters 

 distant, was placed a grounded piece of 

 wire gauze, which served to protect the 

 dish from the influence of the atmos- 

 pheric electricity. With this apparatus a 

 marked efiect could be observed both with 

 direct and diffused sunlight. Wires of 

 Zn, Mg, or Al carefully cleaned and at- 

 tached to an Exner electroscope were found 

 to give a simple and sensitive portable 

 apparatus. 



75. Elster and Geitel appear to have been 

 at first interested in the phenomenon chiefly 

 on account of its important bearing upon 

 meteorology, atmospheric electricity, etc. 

 Much has in fact been written on this 

 phase of the subject, which can only be 

 mentioned here. They were anxious to 

 obtain some instrument by which the actinic 

 intensity of sunlight could be measured 

 from hour to hour and day by day. Since 

 the efiect is influenced so greatly by slight 

 changes in the character of the sensitive 

 surface, it was difiicult to find any active 

 substance which would remain permanent. 

 Amalgamated zine was however found to 

 fulfill the requirements, being much more 

 permanent than pure zinc, and also more 

 sensitive. 



76. Elster and Geitel called attention to 

 the fact that only electro-positive metals 

 appear to possess ' actino-electric ' sensitive- 

 ness.* It seemed therefore natural to ex- 

 pect that metals that are even more posi- 

 tive than Zn, Al and Mg, should show the 

 effect in more marked degree. K and Na 

 were tested, but, on account of the rapid 

 oxydation of the surface, no action could be 

 observed. The same trouble was met with 

 in the case of solid amalgams of K and ISTa. 

 By using a dilute solution of K or Na in 

 mercury, and allowing the liquid to flow 

 continuously from an aperture, a clean 

 surface could however be maintained, and 

 under these circumstances the efiect of 



*See § 34. 



light upon a negative charge was very 

 marked. With positive electricity there 

 was no action. Pure mercury was entirely 

 inactive. 



77. The most sensitive metals as regards 

 the action of sunlight were, in the order of 

 sensitiveness : 



K, Na, Al, Zn, Sn. 

 This series is exactly the same as the Volta 

 contact series. 



78. Sunlight produced no effect on un- 

 amalgamated but clean surfaces of 



Sn, Cd, Pb, Cu, Brass, Fe, C, Pt. 



79. From a meteorological standpoint 

 experiments with surfaces of water ap- 

 pear of considerable interest. The zinc 

 dish described above was therefore filled 

 (1) with pure rain water; (2) with: 

 hot water (rapidly evaporating) ; (3) 

 with salt water. No action could be de- 

 tected from sunlight in any case. If any 

 action exists it must be hundreds of times 

 smaller than that observed with zinc. 



80. In the course of these experiments 

 Elster and Geitel obtained a noticeable 

 efiect with a powder used in making lumi- 

 nous paint (Balmain's Leuchtfarbe) . 



81. The action in the case of sunlight 

 seems to depend upon somewhat longer 

 waves than those of the extreme ultra-violet. 

 Glass and mica, for example, do not stop 

 the effect completely. 



82. Elster and Geitel describe, at the close 

 of their article, convenient forms of appa- 

 ratus for showing the fundamental phe- 

 nomena.* Lecture experiments are also 

 suggested by Hallwachs.f 



83. A method of conveniently using the 

 highly sensitive sodium amalgam without 

 deterioration of its surface was devised soon 

 after by Elster and Geitel. | The amal- 

 gam was prepared and kept in a vacuum 



* An improved form of apparatus for this purpose 

 is described in Wied. Ann. 42, p. 564. 

 t Wied. Ann. 41, p. 161. 

 J Wied. Ann. 41, p. 161. 



