owed by experiments with the same kind of carbon, but in vacuo
by means of a hand-worked air-pump. This time the carbon strip burned
at incandescence for about eight minutes. Various expedients to prevent
oxidization were tried, such, for instance, as coating the carbon with
powdered glass, which in melting would protect the carbon from the
atmosphere, but without successful results.

Edison was inclined to concur in the prevailing opinion as to the easy
destructibility of carbon, but, without actually settling the point in
his mind, he laid aside temporarily this line of experiment and entered
a new field. He had made previously some trials of platinum wire as
an incandescent burner for a lamp, but left it for a time in favor of
carbon. He now turned to the use of almost infusible metals--such as
boron, ruthenium, chromium, etc.--as separators or tiny bridges between
two carbon points, the current acting so as to bring these separators
to a high degree of incandescence, at which point they would emit a
brilliant light. He also placed some of these refractory metals directly
in the circuit, bringing them to incandescence, and used silicon in
powdered form in glass tubes placed in the electric circuit. His notes
include the use of powdered silicon mixed with lime or other very
infusible non-conductors or semi-conductors. Edison's conclusions on
these substances were that, while in some respects they were within the
bounds of possibility for the subdivision of the electric current, they
did not reach the ideal that he had in mind for commercial results.

Edison's systematized attacks on the problem were two in number, the
first of which we have just related, which began in September, 1877, and
continued until about January, 1878. Contemporaneously, he and his
force of men were very busily engaged day and night on other important
enterprises and inventions. Among the latter, the phonograph may be
specially mentioned, as it was invented in the late fall of 1877. From
that time until July, 1878, his time and attention day and night were
almost completely absorbed by the excitement caused by the invention and
exhibition of the machine. In July, feeling entitled to a brief vacation
after several years of continuous labor, Edison went with the expedition
to Wyoming to observe an eclipse of the sun, and incidentally to test
his tasimeter, a delicate instrument devised by him for measuring
heat transmitted through immense distanc