Spectrum of Mercury, Cadmium, and Zinc Vapours. 699 



closely related to mercury and cadmium, one would expect to 

 find absorption bands with zinc vapour in positions analogous 

 to these, where absorption was obtained with the vapours 

 of these two metals. In the zinc arc spectrum the line 

 X = 21393 A.U. is the first member of the series v=l*5, 

 S — m, P, andX=3075*99 A.U. is tiie first member of the 

 series v = 2, p 2 — ro, S. One should therefore expect to find 

 absorption \vith zinc vapour at X= 2139*3 A.U. and at 

 X = 3075*99A.U. In a paper by one of us, recently published*, 

 an account was given of experiments with zinc vapour, in 

 which it was found that there was strong and symmetrically 

 spaced absorption at X= 2139*33 A.U. No absorption, how T - 

 •ever, was found at X = 3075*99 A.U. These experiments have 

 now been repeated by the writers with a column of zinc 

 vapour about 20 cm. in length, with the result that a sharp, 

 narrow absorption band has been found at X = 3075*99 A.U. 

 In fig. 5 the upper spectrum is that of the zinc spark in air 

 taken directly, and the lower one is that of the light from 

 the same spark after it had passed through the zinc vapour. 

 The absorption corresponding to X = 2139*3 A.U. , as will be 

 seen, is extensive and clearly defined. It will also be seen 

 that the reproduction shows a narrow band at X= 3075*99 A.U. 

 In the emission spectrum of the spark taken directly there 

 are two lines close together at X = 3075*99 A.U. , while in the 

 lower photograph only one line is seen. In order to bring 

 out the absorption at this point more clearly a series of photo- 

 graphs was taken with gradually increasing vapour density. 

 One of these is shown in fig. 6. The upper spectrum shows 

 the line at X = 3075*99 A.U. to be double. In the second and 

 third spectra the line is single, and in the fourth and fifth 

 spectra a narrow dark band is seen close to and to the left of 

 the -ingle line. An enlargement was taken of this portion 

 of the absorption spectrum, and it is shown in fig. 7. This 

 photograph, it will be seen, brings out very clearly the 

 absorption band at \ =3075*99 A.U. 

 If the absorption spectrum in fig. 

 seen that absorption bands are also shown at X= 2288 A.U. 

 and X= 2536*7*2 A.U. These were no doubt due to the presence 

 of mercury and cadmium vapours in the tube containing the 

 zinc vapour. As this tube was a new one and had not been 

 used previously, it would seem that the mercury and the 

 cadmium must have been present in the zinc as impurities. 

 It is of interest to note this, for the zinc had been purchased 

 as being doubly distilled and specially pure. With regard 

 to the mercury, it is just possible that when the zinc tube 

 * McLennan, Phil. Mag. vol. xxviii. Sept. 1914, p. 360. 

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