"You will observe that this checking of the current is attained in various ways in different burners," continued the chemist as he unscrewed and dissected the samples before him. "In some it is done by a perforated metal disk in the orifice; in others, by a bit of wool, which checks slightly a slow current, and by the pressure of a strong one becomes compacted and forms a more effective obstacle. In most cases, however, it soon becomes solid with condensed matters from the gas. Another form of check is a small cap having perpendicular slits at the sides. The cylinder of the cap, being smaller than the orifice of the burner, screws down into it; the openings being shortened or lengthened according as the cylinder is screwed up or down. One objection to this is the trouble required in regulating. Here is another burner, in which the orifice ends in a cap whose sides, near the bottom, are pierced with four pin-holes directed downward. This reverses the direction of the current of gas, which then escapes through the pin-holes downward into a chamber, then turns upward along its sides to the tip, on entering which it again turns. Each burner is able to consume economically a flow of gas peculiar to itself, which can be ascertained by a minute's experiment, and then regulated by the tap in the pipe. But this requires much care, and is apt to be neglected. A very small tap in the burner (as in the Wood and Ellis burners), which can be adjusted so as to require no further attention, seems the best method of effecting this graduation."