TRANSACTIONS OF THE SECTIONS. 
11 
other in both cases. In the experiments with the mirror, he had 
used a differential thermometer containing aether instead of sulphuric 
acid, as being much more delicate in its indications of heat; and 
suggested its being made still more sensitive by the use of other 
liquids, having himself succeeded in making one containing con - 
densed sulphurous acid gas. 
On the Prismatic Decomposition of Electrical Light . By Professor 
Wheatstone. 
The following is a brief notice of the principal results stated in this 
communication: 1. The spectrum of the electro-magnetic spark 
taken from mercury consists of seven definite rays only, separated 
by dark intervals from each other ; these visible rays are two orange 
lines close together, a bright green line, two bluish green lines 
near each other, a very bright purple line, and, lastly, a violet line. 
The observations w r ere made with a telescope furnished with a mea¬ 
suring apparatus ; and to ensure the appearance of the spark inva¬ 
riably in the same place, an appropriate modification of the electro¬ 
magnet was employed. 2. The spark taken in the same manner 
from zinc, cadmium, tin, bismuth, and lead, in the melted state, 
gives similar results; but the number, position, and colours of the 
lines varies in each case; the appearances are so different, that, by 
this mode of examination, the metals may be readily distinguished 
from each other. A table accompanied the paper, showing the po¬ 
sition and colour of the lines in the various metals used. The 
spectra of zinc and cadmium are characterized by the presence of a 
red line in each, which occurs in neither of the other metals. 3. When 
the spark of a voltaic pile is taken from the same metals still in the 
melted state, precisely the same appearances are presented. 4. The 
voltaic spark from mercury was taken successively, in the ordinary 
vacuum of the air-pump, in the Torricellian vacuum, in carbonic 
acid gas, &c., and the same results were obtained as when the expe¬ 
riment was performed in the air or in oxygen gas. The light, there¬ 
fore, does not arise from the combustion of the metal. Professor 
Wheatstone also examined, by the prism, the light which accompa¬ 
nies the ordinary combustion of the metals in oxygen gas and by 
other means, and found the appearances totally dissimilar to the 
above. 5. Fraunhofer having found that the ordinary electric spark 
examined by a prism presented a spectrum crossed by numerous 
bright lines, Professor Wheatstone examined the phenomena in dif¬ 
ferent metals, and found that these bright lines differ in number and 
position in every different metal employed. When the spark is taken 
between balls of dissimilar metals, the lines appertaining to both are 
simultaneously seen. 6. The peculiar phenomena observed in the 
voltaic spark taken between different metallic wires connected- with 
a powerful battery were then described, and the paper concluded 
with a review of the various theories which have been advanced to 
