250 Dr. Gladstone on the Electric Light of Mercury. 



were generally altered in colour ; but the following substances of 

 known composition are more instructive in the contrast between 

 the colours they then exhibited and those they display by day- 

 light. 



Crystals of protosulphate of iron appeared absolutely colour- 

 less instead of pale bluish green. 



The blue sulphate of copper, and the yellow chromate of pot- 

 ash, were only enhanced in the brilliancy of their colours. 



Bichromate of potash became of a yellow instead of a red- 

 orange, and dull instead of bright. 



Red prussiate of potash showed a yellow powder, and dingy- 

 orange crystals. 



Chloride of cobalt gave a dirty-brown instead of a pale-red 

 solution. 



Nitrate of chromium, though in such strong solution as to 

 appear red by sunlight, was only dark dull green. 



Amorphous phosphorus showed scarcely any red, or other 

 colour, but presented a dark metallic appearance. 



Iodide of mercury was not scarlet, but of a brown colour, very 

 glistening, suggesting the idea of scales of tarnished silver. 



Gold looked like brass. 



Cochineal in aqueous solution appeared violet in a thin stra- 

 tum, red brown in a deep one. 



A decoction of coffee with milk, as usually drunk, assumed a 

 dirty green hue. 



A solution of bisulphate of quinine, uranium glass, and cer- 

 tain diamonds, exhibited the phenomena of fluorescence more 

 beautifully even than by sunlight. 



Among substances that had much the same colour by daylight 

 and by the mercury flame, may be mentioned blue cobalt salts, 

 yellow nitrate of uranium green chlorophyll, and purple aniline- 

 dye, permanganate of potash and murexide. 



On analysing this light by means of Prof. PowelPs refraction 

 goniometer, I found it to consist of a number of separate and 

 variously coloured rays. This appearance is represented in 

 Plate III. fig. 1, the angle of the prism being 45°. None of the 

 lines are of any appreciable breadth ; that is, they are only as 

 broad as the slit itself; but the more intense ones appeared 

 broader on account of irradiation, and of greater extent because 

 the slightly luminous environment of the spark gives a percept- 

 ible amount of these rays. These peculiarities of the pheno- 

 menon as actually observed are retained in the figure, because 

 they serve as a rough representation of the relative intensity of 

 the different rays ; but this is more correctly indicated by the 

 numbers affixed to the different lines : 1 denoting the brightest 

 lines, or those of the first magnitude ; 2 the next brightest, and 



