521 , Mr. Shelf ord Bidwell [March 11, 



WEEKLY EVENING MEETING, 



Friday, March 11, 1881. 



William Spottiswoode, Esq. M.A. D.C.L. Pres. R.S. &c. 



in the Chair. 



Shelford Bidwell, M.A. LL.B. M.B.I. 



Selenium and its applications to the Photophone and Telephotography. 



Before entering upon my subject, I must claim your indulgence upon 

 two grounds. A week ago I had not the remotest idea that I was to 

 have the honour of addressing you hero this evening; the time 

 which I have had for preparation has, therefore, been exceedingly 

 limited. In the second place, it is my desire (in accordance with 

 the traditions of this Institution) not merely to give a description 

 of the experiments in which I have for the last few months been 

 engaged, but, as far as possible, to reproduce them before you. Now 

 these experiments are mostly of a very delicate nature. In the quiet 

 of a laboratory — where time is practically unlimited, and where an 

 operation, if it should fail at first, may be repeated an indefinite 

 number of times — success is tolerably certain to be finally obtained ; 

 but in exhibiting delicate experiments before an audience, one is 

 working under the most unfavourable conditions, and, in case of 

 failure in the first instance, the attempt cannot generally be repeated. 

 Moreover, the substance with which we are chiefly concerned, sele- 

 nium, is apparently extremely capricious in its behaviour. This 

 appearance is, of course, really due to our present ignorance of its 

 properties ; but the fact remains that, on account of the great uncer- 

 tainty of its action, it is a very difiicult substance to deal with. 



Selenium is a rare chemical element which was discovered in the 

 beginning of the present century. In many of its properties it closely 

 resembles sulphur, and, like sulphur and some other substances, it is 

 capable of existing in more than one form. 



The ordinary form is that called vitreous. Selenium in this con- 

 dition is as absolutely structureless as glass, and in appearance 

 resembles nothing so much as bright black sealing-wax, with, 

 perhaps, somewhat of a metallic lustre; its real colour, however, 

 when seen in thin films, is ruby red. Its melting-point is a little 

 higher than 100° C. In its second modification selenium is crystal- 

 line. When in this form its surface is dull, its fracture is metallic 

 (not unlike that of cast iron), its colour is grey or leaden, and it is 



