400 THE CANADIAN NATURALIST. [Vol. 



IX 



incident with the maximum of the heat rays ; tut, according to^ 

 Adams, the maximum effect is produced in the greenish-yellow or 

 most luminous part of the spectrum. Lord Rosse exposed sele- 

 nium to the action of non-luminous radiations from hot bodies, 

 but could produce no effect : whereas a thermopile under similar 

 circumstances gave abundant indications of current. He also cut 

 off the heat rays from luminous bodies by the interposition of 

 liquid solutions, such as alum, between the selenium and the 

 source of light, without affecting the power of the light to reduce 

 the resistance of the selenium ; whereas the interposition of these 

 same substances almost completely neutralize the effect upon the 

 thermopile, Adams found that selenium was sensitive to the 

 cold light of the moon, and Werner Siemens discovered that in 

 certain extremely sensitive varieties of selenium, heat and light 

 produced opposite effects. In Siemens's experiments, special 

 arrangements were made for the purpose of reducing the resis- 

 tance of the selenium employed. Two fine platinum wires were 

 coiled together in the shape of a double flat spiral in the zig-zag- 

 shape, and were laid upon a plate of mica so that the discs did 

 not touch one another. A drop of melted selenium was then placed 

 upon the platinum-wire arrangement, and a second sheet of mica 

 was pressed upon the selenium, so as to cause it to spread out 

 and fill the spaces between the wires. Each cell was about the 

 size of a silver dime. The selenium cells were then placed in a 

 paraffiue bath, and exposed for some hours to a temperature of 

 210° C, after which they were allowed to cool with extreme 

 slowness. The results obtained with the cells were very extra, 

 ordinary ; in some cases the resistance of the cells, when exposed 

 to li"ht, was only one-fifteenth of their resistance in the dark. 



Without dwelling farther upon the researches of others, I 

 may say that the chief information concerning the effects of light 

 upon the conductivity of selenium will be found under the names 

 of Willoughby Smith, Lieutenant Sale, Draper and Moss, Professor 

 W. Gr Adams, Lord Rosse, Day, Sabini, Dr. Werner Siemens 

 and Dr. C. W. Siemens. All observations by these various 

 authors had been made by means of galvanometers; but it occur- 

 red to me that the telephone, from the extreme sensitiveness to 

 electrical influences, might be substituted with advantage. Upon 

 consideration of the subject, however, I saw that the experiments 

 could not be conducted in the ordinary way for the following 

 eason : The law of a udibility of the telephone is precisely an- 



