SECT. XXIX. VOLTAIC AEG. 303 



state, as diamond, is as perfect a non-conductor as we know, while 

 in an opaque amorphous state, as graphite or charcoal, it is one 

 of the best conductors : thus in one state it transmits light and 

 stops electricity, in the other it transmits electricity and stops 

 light. Jt is a circumstance worthy of remark, that the arrange- 

 ment of molecules which renders a solid body capable of trans- 

 mitting light is most unfavourable to the transmission of elec- 

 tricity, transparent solids being very imperfect conductors of 

 electricity ; so all gases readily transmit light, but are amongst 

 the worst conductors of electricity, if indeed they can be said to 

 conduct it at all. The fact that the molecular structure or 

 arrangement of a body influences, indeed I may say determines, 

 its conducting power, is by no means explained by the theory of 

 a fluid ; but if electricity be only a transmission of force or 

 motion, the influence of the molecular state is just what would 

 be expected." 



Professor AVheatstone, by fixing metallic points at the extremi- 

 ties of the wires or poles, has found that the appearance of the 

 spectrum of the voltaic arc or vivid flame that is seen between 

 the terminals of a battery, depends, as in static electricity, upon 

 the metal from whence it is taken. The spectrum of that from 

 mercury consists of seven definite rays, separated from each other 

 by dark intervals ; these visible rays are two orange lines close 

 together, a bright green line, two blueish-green lines near each 

 other, a very bright purple line, and, lastly, a blue line. It is 

 the same when it passes through carbonic acid gas, oxygen gas, 

 air, or vacuum. The light from zinc, cadmium, tin, bismuth, 

 and lead, in a melted state, gives similar results ; but the number, 

 position, and colour of the lines vary so much in each case, and the 

 appearances are so different, that the metals may easily be distin- 

 guished from one another by this mode of investigation. The 

 electric spark is considered by M. Angstrom to be the overlapping 

 of two spectra, one of which belongs to the metal, and the other 

 to the gas through which the spark passes, and that the bright 

 lines vary with the gas as well as with the metal. In an oxygen 

 spectrum the greatest number of bright lines occur in the blue 

 and violet, in nitrogen in the green and yellow, and in hydrogen 

 in the red. These effects must necessarily be connected with 

 the chemical and thermal properties of the gases. 



Mr. Grove considers that the colour of the voltaic arc, or flame, 



