090 



10. Testbu) of the obtained results by means of a mercury wirror 

 got by (listiUation of pure mercury. According to ^ 6 (he phase- 

 difference was measured with tlie compensator by annulling this 

 phase-difference, in oi'der to determine the influence exerted by the 

 adsorbed lajer of air. From the values given in §4 it appears, that 

 the lowest compensator-reading corresponds with the smallest phase- 

 difference. As according to Dkudk every surface layer diminishes the 

 angle of principal incidence, hence increases the phase-difference for 

 every angle of incidence, the mercury surface is the better, i.e. less 

 contaminated by suiface layers, as the compensator-reading is smaller. 

 For an angle, somewhat greater than 7, this smallest reading was 

 56.84 '). Mercury, distilled in vacuum, which was conveyed into a 

 dish after being filtered through a paper funnel, yields the reading 

 56.84. This dish had been placed free in the air and was not sui-- 

 rounded by a case (see § 6), so that the rays of light need not pass 

 through glass windows. Mercury purified by being shaken with 

 potassium hydroxide and nitric acid, but not distille<l in vacuum, 

 yields the reading 56.85. The difference with the preceding value 

 falls within the errors of observation. Mercury, conveyed into the 

 dish through a drawn-o«it glass tube, yields 57.00, if the rube has 

 not been very well cleaned. For a thorough cleaning heating to a 

 dull red glow is gener-ally sufficient, 'i'ouched by a piece of cloth, 

 which is not clean, the mercury gives the adjustment 57.20. When 

 breathed upon, the mercury yields a reading increased by 0.04 or 0.05. 



In order to prove by another way, that 56.84 is the reading for 

 |)ure mercury without surface layer, pure mercury was distilled 

 in vacuum into the iron dish, which is situated in the bronze cylinder. 

 It was previously ascertained, that the glass windows — carefully 

 cooled glass plates of a thickness of 3 mm. — did not modify the 

 compensator-reading, e\en though the cylinder was exhausted 

 of air. For this purpose an iron mirror was placed in the dish and 

 the compensator-reading was observed before and after the ex- 

 haustion. The pure mercury was heated in a glass globe. The vapour 

 was condensed in the spiral windings of a glass cooler and received 

 in a glass bottle, from which it flows out into the iron vessel through 

 a glass tube with drawn-out point. k\\ the junctions of this apparatus 

 consist of sealed glass. The aii'-tight connection of the glass tube with 



great a force these adsorbed layers of air are attached to the surfaces. Clf. among 

 others Voigt, Wied. Ann., 19, 39, 1884. Likewise it is in agreement with the 

 fact, that the layer of air cannot be removed by means of carbon powder, which has 

 been heated just before to ared glow. Cf. among others Sissingh, Thesis for the doctorate, 

 p. 162; Arch. Néerl. 20, 228, 1886. 



1) The values recorded here are again the irleans of four readings. 



