rough a
long series of charges and discharges under strict test conditions. From
the tabulated results of hundreds of tests there were selected
three that showed the best results. These, however, showed only the
superiority of certain PROPORTIONS of the materials. The next step
would be to find out the best QUALITY. Now, as there are several hundred
variations in the quality of nickel flake, and perhaps a thousand ways
to make the hydrate, it will be realized that Edison's methods led to
stupendous detail, for these tests embraced a trial of all the qualities
of both materials in the three proportions found to be most suitable.
Among these many thousands of experiments any that showed extraordinary
results were again elaborated by still further series of tests, until
Edison was satisfied that he had obtained the best result in that
particular line.

The laboratory note-books do not always tell the whole story or meaning
of an experiment that may be briefly outlined on one of their pages. For
example, the early filament made of a mixture of lampblack and tar is
merely a suggestion in the notes, but its making afforded an example
of Edison's pertinacity. These materials, when mixed, became a friable
mass, which he had found could be brought into such a cohesive,
putty-like state by manipulation, as to be capable of being rolled out
into filaments as fine as seven-thousandths of an inch in cross-section.
One of the laboratory assistants was told to make some of this mixture,
knead it, and roll some filaments. After a time he brought the mass to
Edison, and said:

"There's something wrong about this, for it crumbles even after
manipulating it with my fingers."

"How long did you knead it?" said Edison.

"Oh! more than an hour," replied the assistant.

"Well, just keep on for a few hours more and it will come out all
right," was the rejoinder. And this proved to be correct, for, after
a prolonged kneading and rolling, the mass changed into a cohesive,
stringy, homogeneous putty. It was from a mixture of this kind that
spiral filaments were made and used in some of the earliest forms
of successful incandescent lamps; indeed, they are described and
illustrated in Edison's fundamental lamp patent (No. 223,898).

The present narrative would assume the proportions of a history of
the incandescent lamp, should the authors attempt to follow Edison's
investigations through the thousands of pages of note-books away back in
th