250 



Profs. Gr. D. Liveing and J. Dewar. [May 31, 



obtained also the triplets at P and S, the pair about X2935 and 2927, 

 all the quadruple group near \2802 and the quadruple group beyond, 

 and in one case only, in oxygen, the group near s, described below, 

 and the flame-triplet near M. When no jar is used sometimes only 

 A2852 is to be seen, sometimes A2852 and the strong pair near 

 A2802, and. sometimes also the triplet near L. We infer, therefore, 

 that this is the order of persistency of these lines under the cir- 

 cumstances. 



We have before remarked upon the necessity of avoiding all rubber 

 connexions in the construction of pumps employed in the exhaustion 

 of tubes for spectroscopic observation, and we described a modification 

 of the Sprengel pump which we had constructed for this end (' Roy. 

 Soc. Proc.,' vol. 30, p. 499). The warnings of unexpected impurities 

 .given by photographs of the ultra-violet spectra of vacuous tubes 

 have shown the necessity of preventing the contact of the mercury 

 employed with the dust and moisture of the atmosphere. Hence we 

 have used in the experiments described in this paper a mercurial 

 pump constructed wholly of glass, and in which the same mercury is 

 used over and over again without being exposed to any unfiltered air. 

 For this pump we are indebted to the ingenuity and skill in glass- 

 blowing of Mr. Lennox of the Royal Institution. The annexed 

 figure (2) represents its construction. A is a reservoir which com- 

 municates by the tube aa, which ascends vertically some distance in 

 order to prevent any mercury being driven into the exhausted tube, 

 through the spiral tube ss, with the tube to be exhausted. B is the 

 reservoir of mercury, to the bottom of which the tube gcc passes 

 through the sealed joint d. The upper part of B can be put in com- 

 munication through the three-way cock E, either with the vessel C or 

 with the outer air through the tube D which is filled with calcium 

 chloride. C forms a mercury valve, and at its upper part communi- 

 cates through the stopcock F with an exhaust pump by which the 

 pressure of the gas in C can be quickly reduced to a few millimetres 

 of mercury. When this has been done, the three-way cock E is 

 turned so as to cut off the communication between B and C and open 

 that between B and D. The pressure of the air filtered through D 

 forces the mercury in B up the tube c until it fills A and the whole 

 apparatus, as high as the bend e, driving all gas before it through the 

 tube / and through the mercury valve C, whence it is carried off by the 

 exhaust. The tube g is very narrow so as to oppose resistance to the 

 passage of the mercury whereby A is filled with mercury as quickly 

 as g. As soon as the last bubble of gas has been driven out of /, the 

 three-way cock E is turned so as to shut the communication with D 

 and open that between B and C. As the pressure of the air on the 

 surface of the mercury in B diminishes the mercury falls both in A 

 and in /, leaving a Torricellian vacuum above it, and, as soon as it 



