On the Photographic Transparency of various Bodies. 309 



In many cases metals which are allied in chemical properties ex- 

 hibit a certain similarity in their spectra. This occurs, for example, 

 with the magnetic metals, iron, cobalt, and nickel, and with the 

 group embracing bismuth, antimony, and arsenic. The more vola- 

 tile metals exhibit generally the most strongly marked lines. Cad- 

 mium, for instance, gives two intense groups. Zinc, two very strong 

 lines near the less refrangible extremity, three near the middle, and 

 four nearly equidistant lines towards the termination of the more 

 refrangible portion, whilst in the spectrum of magnesium the che- 

 mical action is almost suddenly terminated near the middle by a 

 triple group of very broad and strong lines. 



It will be observed, on examining the photographs of these spectra 

 of the various metals, that the impressions, particularly in the more 

 refrangible portions, consist of a double row of dots, running parallel 

 with the length of the spectrum, and forming the terminations of 

 lines rather than lines themselves, as though the intense ignition of 

 the detached particles of metal, necessary to furnish rays capable of 

 exciting chemical action, had ceased before the transfer of these par- 

 ticles to the opposite electrode had been completed. 



If each electrode be composed of a different metal, the spectrum 

 of each metal is impressed separately upon the plate, as is evident 

 on examining the photographs. 



When alloys are employed as electrodes, the spectrum exhibited 

 , is that due to both the metals ; but if the metals made use of are 

 approximatively pure, the spectrum is hardly to be distinguished from 

 that of the pure metal. In the case when alloys are used as 

 electrodes, it is not always the more volatile metal which impresses 

 its spectrum most strongly. A specimen of brass, for example, 

 containing 38 per cent, of zinc, gave a spectrum which could not be 

 distinguished from that of pure copper, though an alloy of three 

 parts of gold and one of silver gave a spectrum in which the lines 

 due to silver predominated. 



The author then proceeds to describe a number of experiments 

 upon the transmission of sparks between electrodes of different 

 metals in a current of several different gases. The apparatus em- 

 ployed consisted of a glass tube ; into the side an aperture was drilled, 

 which could be closed by a plate of quartz ; the ends of the tube 

 were closed by ground brass plates, each supporting a pair of brass 

 forceps, into which the electrodes were fitted ; through the axis of the 

 tube a current of each gas was transmitted at the ordinary atmo- 

 spheric pressure. 



Among the gases thus tried were hydrogen, protoxide of nitrogen, 

 carbonic acid, carbonic oxide, olefiant gas, marsh-gas, cyanogen, 

 sulphuretted hydrogen, sulphurous acid, nitrogen, and oxygen. The 

 spectrum obtained from the same metal varied considerably in these 

 different media. In hydrogen the intensity of the spectrum was 

 greatly reduced, and the more refrangible rays were wanting, but 

 no new rays made their appearance. In carbonic acid, carbonic 

 oxide, olefiant gas, marsh-gas, and cyanogen, the special lines due to 

 the metal were produced, but in each a series of identical lines 



