He stood still till Tom had passed through the gate and

It is said that Edison replied rather incoherently and changed the topic of conversation.

This innate modesty, however, does not prevent Edison from recognizing and classifying his own methods of investigation. In a conversation with two old associates recently (April, 1909), he remarked: "It has been said of me that my methods are empirical. That is true only so far as chemistry is concerned. Did you ever realize that practically all industrial chemistry is colloidal in its nature? Hard rubber, celluloid, glass, soap, paper, and lots of others, all have to deal with amorphous substances, as to which comparatively little has been really settled. My methods are similar to those followed by Luther Burbank. He plants an acre, and when this is in bloom he inspects it. He has a sharp eye, and can pick out of thousands a single plant that has promise of what he wants. From this he gets the seed, and uses his skill and knowledge in producing from it a number of new plants which, on development, furnish the means of propagating an improved variety in large quantity. So, when I am after a chemical result that I have in mind, I may make hundreds or thousands of experiments out of which there may be one that promises results in the right direction. This I follow up to its legitimate conclusion, discarding the others, and usually get what I am after. There is no doubt about this being empirical; but when it comes to problems of a mechanical nature, I want to tell you that all I've ever tackled and solved have been done by hard, logical thinking." The intense earnestness and emphasis with which this was said were very impressive to the auditors. This empirical method may perhaps be better illustrated by a specific example. During the latter part of the storage battery investigations, after the form of positive element had been determined upon, it became necessary to ascertain what definite proportions and what quality of nickel hydrate and nickel flake would give the best results. A series of positive tubes were filled with the two materials in different proportions--say, nine parts hydrate to one of flake; eight parts hydrate to two of flake; seven parts hydrate to three of flake, and so on through varying proportions. Three sets of each of these positives were made, and all put into separate test tubes with a uniform type of negative element. These were carried through 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 cer- tain 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 the eighties and early nineties. Improvement of the lamp was constantly in his mind all those years, and besides the vast amount of detail experimental work he laid out for his assistants, he carried on a great deal of research personally. Sometimes whole books are filled in his own handwriting with records of experiments showing every conceivable variation of some particular line of inquiry; each trial bearing some terse comment expressive of results. In one book appear the details of one of these experiments on September 3, 1891, at 4.30 A.M., with the comment: "Brought up lamp higher than a 16-c.p. 240 was ever brought before--Hurrah!" Notwithstanding the late hour, he turns over to the next page and goes on to write his deductions from this result as compared with those previously obtained. Proceeding day by day, as appears by this same book, he follows up another line of investigation on lamps, apparently full of difficulty, for after one hundred and thirty-two other recorded experiments we find this note: "Saturday 3.30 went home disgusted with incandescent lamps." This feeling was evidently evanescent, for on the succeeding Monday the work was continued and carried on by him as keenly as before, as shown by the next batch of notes.

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