1882.] producing the reversal of spectral lines of metals. 261 



graph, and so are some less refrangible calcium lines. No doubt 

 many of the other lines which have disappeared are not seen 

 because absorption just balances emission. Indeed by varying the 

 current different effects may sometimes be produced, the same line 

 appearing at one time bright, at another time dark, and at another 

 time not at all. We have seen the calcium lines at H and K, and the 

 strong blue triplet just below the well-known (indigo) calcium line 

 at wave length 4226 all bright, while the indigo line itself was 

 quite invisible, it was neither bright nor reversed. This and 

 similar effects are no doubt due to differences in emissive power at 

 the different temperatures of the emitting and absorbing vapour. 

 At the higher temperature the emissive power of calcium for H 

 and K and the blue triplet is relatively greater than for the indigo 

 line, while at the lower temperature of the absorbent vapour the 

 reverse holds ; so that the emission for the indigo light is balanced 

 by absorption, while that of the other rays is not balanced. In 

 like manner we have frequently seen H when K was quite invisible, 

 sometimes K reversed when H was not reversed. 



It is not at first sight easy to explain the action of the gas. 

 Probably hydrogen is the chief agent in the case, for a mere current 

 of air has no such effect. The gas may act by helping to diffuse 

 the metallic vapours, diminishing their density in the arc, and in- 

 creasing it in the tubular part of the crucible without too much 

 lowering the temperature. Hydrogen gas also maintains a re- 

 ducing atmosphere, preventing the oxidation of the metallic 

 vapour. It also forms compounds with some metals, notably with 

 magnesium and the alkali metals, as well as with the carbon of 

 the electrodes, which are probably all endothermic, and whether 

 that be so or no, must affect the distribution of the temperature in 

 the arc and tube. In the case of expanded bright lines, when 

 they are not wholly swept out by the gas the effect is to diminish 

 the wings, and as absorption does not begin at the wings, this can 

 only be by diminishing the range of emission, probably by diluting 

 the metallic vapour. The amount of heat absorbed by such a 

 small mass of gas will not lower the temperature much. Moreover 

 a current of chlorine has usually the opposite effect to that of a 

 current of hydrogen, increasing the strength of the bright lines, 

 probably by assisting the volatility of the metals, and so increasing 

 the quantity in the arc. That the explanation here offered of the 

 action of hydrogen is correct, is borne out by the behaviour of 

 mixtures of metals. Thus an alloy of zinc with a little lead gives 

 far sharper and cleaner reversals of the lead lines than lead alone 

 does. When lead alone is put into the arc in the crucible the lead 

 lines come out very strong and diffuse, and the emission is not 

 nearly balanced by the absorption in the (comparatively) cool 

 tube ; but when the alloy with zinc is used the lead lines are 



