294 REPOKT— 1880. 



tlie long lines are due to a comparatively cool state of tlie metallic 

 vapour. Now, according to the theory of molecular shocks, it is an open 

 question how the long lines behave when the temperature is increased ; 

 they may get stronger, they may not alter their intensity, or they may 

 get weaker. According to the hypothesis of molecular combinations, on 

 the other hand, the long lines must necessarily get weaker and finally 

 disappear. Now, in reahty, they do get weaker and finally disappear in 

 many cases. Thus in the case of calcium, Mr. Lockyer ^ has pointed out 

 that the blue line which is the strongest at the temperature of the Bunsen 

 burner, of the arc, and even of a weak electric spark, gradually weakens 

 when the intensity of the spark is increased, and finally disappears with a 

 coil of large power. Now supposing we observed the length of this 

 calcium line, as the spark is gradually increased. At first it would not 

 only be the longest but also the strongest line ; as the temperature is 

 raised, the line, while still remaining the longest, Avould decrease in 

 strength and would finally disappear. But it may not disappear at the 

 same time throiighout its length. If the temperature of the spark is 

 nearly equal throughout its length, the greater quantity of matter surround- 

 ing the electrode would increase its visibility near the pole. In that case 

 the line would shorten before it disappears. If, on the other hand, the 

 temperature of the spark is decidedly higher near the pole, the line would 

 first disappear there, and remain longest in the centre of the spark. It 

 would be interesting to decide experimentally how the line actually does 

 behave. In the mean time, we may say that, as Mr. Lockyer has pointed 

 out, a short line may not only be the first indication of a state of things 

 as they are at a higher temperature, but may also be due to the last 

 remnant of the state of things as they are at a lower temperature. 



We have, besides this calcium line, many other lines which disappear 

 when the temperatui'e is raised. Thus, of the violet rubidium lines 

 (4216, 4202), only the more refrangible one remains in the spark. The 

 two blue calcium lines both disappear when a condenser is used. The 

 disappearance of these lines is always accompanied by the appearance of 

 other strong lines. 



When indium is volatilised in a flame, two lines (4509, 4101) are 

 seen. A third line (4532) is given on Thalen's list. According to 

 Messrs. Clayden and Heycock,^ this third line appears when the spectram 

 is taken from the chloride or from the nitrate, but disappears when the 

 spark is taken from metallic indium. Other strong lines, however, in 

 different parts of the spectrum, replace it in that case. 



There are sometimes Unes appearing at low temperatures, but behaving 

 differently from proper low-temperature lines. These lines require 

 further investigation, and may in some cases, at least, be due to some 

 compounds of the metals with other elements present. We give some 

 examples : — 



Lead (5005). Mr. Brassak,' who was the first to investigate the 

 differences observed in metallic spectra, when a condenser is put in or 

 out of circuit, has noticed that in lead, without condenser, a strong line 

 appears at the point indicated. Mr. Huggins,^ who has found this line to 

 be sensibly coincident with the chief line of nebulas, has used it as a 

 reference, by means of which he might detect a proper motion of these 



J Proc. Roy. Soe. xxiv. p. 352 (1876). ^ pj^n j,/^,^_ ^ p_ 337 (1876). 



3 AhJt. Natm-f. Ges. Halle, ix. (1864). 



* BHt. Ass. Rep. (2) Bradford, 1873, p. 34. 



