[12] See _Edinburgh Transactions_, vol. ix., p. 435. A monochromatic lamp for producing yellow light may be constructed most effectually, by employing a portable gas lamp, containing compressed oil gas. If we allow the gas to escape in a copious stream, and set it on fire, it will form an explosive mixture with the atmospheric air, and will no longer burn with a white flame, but will emit a bluish and reddish light. The force of the issuing gas, or any accidental current of air, is capable of blowing out this flame, so that it is necessary to have a contrivance for sustaining it. The method which I used for this purpose is shown in Fig. 22. A small gas tube _a b c_, arising from the main burner MN of the gas lamp PQ, terminates above the burner, and has a short tube _d e_, moveable up and down within it, so as to be gas-tight. This tube _d e_, closed at _e_, communicates with the hollow ring _f g_, in the inside of which four apertures are perforated in such a manner as to throw their jets of gas to the apex of a cone, of which _f g_ is the base. When we cause the gas to flow from the burner M, by opening the main cock A, it will rush into the tube _a b c d_, and issue in small flames at the four holes in the ring _f g_. The size of these flames is regulated by the cock _b_. The inflammation, therefore, of the ignited gas will be sustained by these four subsidiary flames through which it passes, independent of any agitation of the air, or of the force with which it issues from the burner. On a projecting arm _e h_, carrying a ring _h_, I fixed a broad collar, made of coarse cotton wick, which had been previously soaked in a saturated solution of common salt. When the gas was allowed to escape at M, with such force as to produce a long and broad column of an explosive mixture of gas and atmospheric air, the bluish flame occasioned by the explosion passes through the salted collar, and is converted by it into a mass of homogeneous yellow light. This collar will last a long time without any fresh supply of salt, so that the gas lamp will yield a permanent monochromatic yellow flame, which will last as long as there is gas in the reservoir. In place of a collar of cotton wick, a hollow cylinder of sponge, with numerous projecting tufts, may be used, or a collar may be similarly constructed with asbestos cloth, and, if thought necessary, it might be supplied with a saline solution from a capillary fountain. [Illustration: _Fig. 22._]