306 A. G. Bell—Production of Sound by Light. 
light, vibrations on selenium (and probably on the other sub- 
stances), we control the quality of the sound, and obtain all 
varieties of articulate speech. We can thus, without a con- 
ucting wire as in electric telephony, speak from station to sta- 
tion wherever we can project a beam of light. We have not 
eam of light can be flashed from one observatory to another. 
The necessary privacy of our experiments, hitherto, has alone 
prevented any attempts at determining the extreme distance at 
which this new method of vocal communication will be avail- 
le. 
I shall now speak of selenium. 
Selentum.—In the year 1817, Berzelius and Gottlieb Gahn 
made an examination of the method of preparing sulphuric 
acid in use at Gripsholm. During the course of this examina- 
tion they observed in the acid a sediment of a partly reddish, 
partly clear brown color, which under the action of the blow- 
pipe gave out a peculiar odor, like that attributed by Klaproth 
to tellurium. 
As tellurium was a substance of extreme rarity, Berzelius 
attempted its production from this deposit, but he was unable 
ter many experiments to obtain farther indications of its 
presence. He found plentiful signs of sulphur mixed with 
mercury, copper, tin, zinc, iron, arsenic and lead, but no trace 
of tellurium. 
It was not in the nature of Berzelius to be disheartened by 
this result. In science every failure advances the boundary of 
knowledge as well as every success; and Berzelius felt that if 
the characteristic odor that had been observed did not proceed 
from tellurium, it might possibly indicate the presence of some 
substance then unknown to the chemist. Urged on by this 
hope he returned with renewed ardor to his work. 
e collected a great quantity of the material and submitted 
the whole mass to various chemical processes. He succeeded 
in separating successively the sulphur, the mercury, the copper, 
the tin and the other known substances, whose presence ha 
been indicated by his tests; and after all these had been 
eliminated, there still remained a residue, which proved upon 
examination to be what he had been in search of—a new 
elementary substance. 
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