Light; a course of experimental optics, chiefly with the lantern . the colour is of course comparatively feeble. 189. Experimental Demonstration.—Professor Tyn-dall precipitates fine vapours ^ in an exhausted glass tube 1 That if5, with the card at right angles. An inclined white cardanalyses as a reflecting surface. ^ Professor Tyndall has employed vapour from nitrite of butyl and xVn.] A LUMINOUS TUBE. 347 with glass ends; but simpler apparatus will amply sufficefor us. Small particles in water show the same phenomena,and either (i) a very Utile soap, or (2) a few drops of milk,or (3) about
Light; a course of experimental optics, chiefly with the lantern . the colour is of course comparatively feeble. 189. Experimental Demonstration.—Professor Tyn-dall precipitates fine vapours ^ in an exhausted glass tube 1 That if5, with the card at right angles. An inclined white cardanalyses as a reflecting surface. ^ Professor Tyndall has employed vapour from nitrite of butyl and xVn.] A LUMINOUS TUBE. 347 with glass ends; but simpler apparatus will amply sufficefor us. Small particles in water show the same phenomena,and either (i) a very Utile soap, or (2) a few drops of milk,or (3) about six grains of resin in an ounce of alcohol, or(4) about five grains of pure mastic in the same, will answervery well. The mastic is best, and soap handiest for asudden occasion ; or a teaspoonful of the solution of coal-tar in alcohol known as Wrights Liquor Carbonis will giveexcellent effects if stirred into water. We may take a commonglass lamp-chimney, 12 inches by 2 inches, grind one end flatand cement on it a flat glass plate, and fit a vulcanised stopper. Fig. 189.—Experimental Tube. to the other. Fill carefully with filtered water, in which avery little soap is dissolved, or into which about a teaspoon-ful of the mastic in alcohol has been gradually poured whilethe water was violently stirred, either being filtered into thetube to remove dust, which mars the effect by reflectingcommon light. Mount the tube t in two semicircular notchesof a cradle stand, as in c. Fig. 189, and adjust the tubehorizontally in front of the plain optical objective— taking hydrochloric acid, nitrite of am^l, bisulphide of carbon, and manyother compounds. A friend of mine has obtained beautiful resultsfrom a whifif of tobacco smoke. 348 LIGHT. [chap. away the polariser—so as to throw the beam of Hght fromthe lantern along its axis. If a dead-black board or sheet of card is held behind thetube, it is soon seen that it appears blue. The black back-ground is not even necessary, for the
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