Key's eyes concentrated suddenly, the listless look left his face.

His fingers trembled lightly as he again let the salt water fall into the solution, with exactly the same result! Again and again he repeated it, until the bottom of the glass was quite gray with the fallen precipitate.

And his own face grew as gray.
His hand trembled no longer as he carefully poured off the solution so as not to disturb the precipitate at the bottom.

Then he drew out his knife, scooped a little of the gray sediment upon its point, and emptying his tin cup, turned it upside down upon his knee, placed the sediment upon it, and began to spread it over the dull surface of its bottom with his knife.

He had intended to rub it briskly with his knife blade.