EXPERIMENTS ON LIGHT-SPECTRA. 495 



background. The sheet may be blackened with lampblack ; this is 

 mixed with a thin size made of water and glue, bnt there should 

 only be sufficient glue to produce a dull black. As the lamp- 

 black is not easily moistened by water, it should be first wetted by 

 a few drops of spirits of wine, rubbed with the latter into a stiff 

 paste, which is afterwards made thinner with a little water, and 

 then mixed with the size, in which the glue must be dissolved while 

 warm. 



Common salt and lithium salts volatilise without residue when 

 pure ; calcium chloride leaves a residue of lime behind which may 

 be removed by dissolving it in dilute hydrochloric acid. It is best 

 to keep the platinum wire in a little bottle filled with hydrochloric 

 acid ; a hole is bored through the cork, in which the glass handle 

 fits tightly, and when the wire tube is required it is simply drawn 

 out while the cork is left in the neck of the bottle. Before using it 

 the wire is each time well washed with a jet of water from the 

 washing bottle. Compounds of sodium are contained in small 

 quantities in a great many substances, and the smallest trace of 

 sodium is sufficient to give a yellow colour to the flame. The cal- 

 cium chloride prepared in the manner above described contains also 

 an admixture of sodium, and without the addition of common salt 

 the yellow sodium flame will make its appearance between the green 

 and orange calcium flame. But if only a small quantity of the 

 calcium chloride is placed on the wire, and it is left for some time 

 in the flame, the sodium salts evaporate, and the residue of lime, 

 although it produces a fainter coloration than the calcium chloride, 

 gives two images, one green and the other orange, due to the cal- 

 cium flame, while the yellow sodium flame is now absent. 



Potassium carbonate may be employed instead of lithium car- 

 bonate ; but the image of the flame is neither so beautifully red nor 

 so distinctly visible as the lithium flame, while its deviation to the 

 left is somewhat less than that of the latter. Cigar ashes contain 

 considerable quantities of compounds of potassium, calcium, and 

 sodium ; if a small quantity of ash, moistened with a few drops of 

 hydrochloric acid, be held in the hydrogen flame, and the images 

 observed through the prism, the two calcium flames will be seen, 

 between them the sodium flame, and to the left of the orange cal- 

 cium flame, but somewhat further from it than the lithium flame in 

 our figure, the red potassium flame will appear. Sometimes cigar 

 ashes contain traces of lithium, and in that case five distinct images 

 are seen when the ash is placed in the flame. This must, however, 

 be done by an assistant while the eye of the observer is applied to 

 the prism ; otherwise the small quantity of lithium present will be 



