164 Mr. E. GL Bilham on a Comparison of the 



the pressure chamber would have been necessary, the diffi- 

 culty of maintaining a spark under pressure is by no means 

 inconsiderable. The chief source of trouble lay in the rapid 

 disintegration of the electrodes. Working with very small 

 spark gaps, of the order of about a millimetre, a frequent 

 result was that a short circuit was produced by a detached 

 fragment of nickel lodging between the electrodes. Another 

 frequent mishap was that the spark gap rapidly increased 

 in width until a very violent discharge between projecting 

 parts on the outside of the pressure chamber announced the 

 fact that an easier path had been found. Clouding of the 

 window was also a serious source of trouble. Turning to a 

 different aspect of the case, an inspection of the appearance 

 of the photographs produced under a pressure of eleven 

 atmospheres points very forcibly to the conclusion that, in 

 very many instances, measurement of the spectrum "lines" 

 was rapidly approaching the limit of possibility on account 

 of their diffuseness. 



The above considerations will perhaps serve to show that 

 the investigation of spark spectra under high pressures is 

 attended with formidable difficulties, although such data 

 would unquestionably be of great interest and value. Our 

 knowledge of the variation of displacement with pressure in 

 the case of spark spectra is thus limited to comparatively 

 low pressures, and, in attempting to draw any conclusion 

 from the published results, this must be borne in mind. 



There can be no doubt, other things being equal, that the 

 most desirable course to take in attempting to estimate the 

 comparative magnitudes of the displacements in the arc and 

 spark spectra, would be to compare measurements actually 

 made under the same pressure in both cases. This pro- 

 cedure, as Duffield has shown, leads to the conclusion that 

 in most cases the shifts in the arc are much higher than in 

 the spark. The question arises as to whether such a con- 

 clusion fairly represents the case, when we consider that 

 Duffield's measurements of the shifts in the arc at this 

 pressure are, in general, about double the mean shifts calcu- 

 lated from the measurements made at pressures of from 

 twenty to a hundred atmospheres. Although most investi- 

 gators have reached the conclusion that the increase of wave 

 length is directly proportional to the increase of pressure, 

 there is, so far as I am aware, no reason for supposing that 

 no departure from this law is possible in a particular 

 instance. Moreover, the possibility exists that the relation 

 between displacement and pressure may be different for the 

 arc and spark spectra of the same element. Duffield has 



