374 Scientific Proceedings, Royal Dublin Society. 



On the Cause of a continuous Spectrum accompanying that of 

 Metallic Lines. — A careful examination of my series of photographs 

 has shown that certain spectra show more of a continuous spectrum 

 than others (pi. vu.). These are characteristic of those metals which 

 are readily vapourized, and which are oxidizable — in fact those 

 which yield the most copious deposits of oxide on the mica films. 

 Without entering into any elaborate explanation to account for a 

 metal like magnesium, which is both volatile and oxidizable, yield- 

 ing only a feeble continuous spectrum, it may be remarked that 

 the chemical action of the rays emitted by this element is much 

 more powerful than is the case with other metals, and therefore 

 the requisite quantity of metal volatilized and capable of oxidation 

 is less than with other elements, and consequently there is a 

 smaller quantity of oxide to render incandescent. But comparing 

 the spectra of gold and silver, which are largely volatilized, with 

 tin, lead, and cadmium, we see that the former, which yield no 

 oxide, give little or no continuous spectrum, while the latter yield 

 a rich one. Tellurium, arsenic, and antimony, are also similar in 

 this respect. Accordingly, the continuous spectrum is generally 

 to be attributed to the incandescent solid particles of oxide which 

 are formed from the vapour. 



DESCRIPTION OF PLATES VI. axd VII. 



Plate VI. 



Films deposited upon mica — Silver, brown ; Gold, blue. 



Plate VII. 



a. Continuous line in the Photographed Spectrum of Cadmium reaching from point 

 to point of metal. 



b. b. Two lines of lead occurring as an impurity in the Cadmium appearing at tbe 

 negative electrode only. 



c. One of the principal lines of Cadmium, showing a nimbus, and an extension 

 above and below the position of tbe point of metal. It will be seen that tbe extension 

 is greatest at the negative electrode, as if tbe discharge started from this point. From 

 left to right a continuous spectrum is seen. 



