May 26, 1916] 



SCIENCE 



759 



F = 



F = ' 



where p is the vapor pressure ; M, molecular 

 weight; N, number of molecules per gram mole- 

 cule; d, density of the vapor; V, density of the 

 liquid; K, cohesion force between a vapor mole- 

 cule and the liquid surface. 



From this equation another constant, F, repre- 

 senting the force between two single molecules was 

 found to be given by the equation 



dD ' 



where the letters have the same meanings as be- 

 fore. At the critical point this becomes 



This constant is directly proportional to the 

 constant "a" in Van der Waals's equation and 

 nearly proportional to the Eankine, Heydweiller 

 and Kleeman constants. 



A New Automatio Mercury Pump: W. P. Eoop 



and L. T. Jones. 

 Two Small Communications on Galvanometers: W. 



P. Roop. 



(a) The reduction factor and resistance of a 

 sensitive galvanometer may be quickly and con- 

 veniently determined by means of a decade bridge. 

 Galvanometer and battery are connected to the 

 appropriate binding posts, as in the ordinary use 

 of the bridge. The binding posts to which the un- 

 known resistance is ordinarily . connected are left 

 free. Two settings of rheostat resistance and the 

 corresponding galvanometer readings yield the re- 

 quired result. 



Approximations are as follows: One ratio arm is 

 neglected in comparison with the other. Battery 

 resistance is neglected in comparison with that of 

 the high ratio arm. Battery E.M.P. is taken as 

 constant. The deflections of the galvanometer are 

 supposed proportional to current. Failure to meet 

 this last condition can be remedied by applying a 

 process of successive approximation to the calcu- 

 lations. 



(&) The sensitiveness of a galvanometer may be 

 doubled by doubling the scale distance and plac- 

 ing the telescope close to the mirror. As com- 

 pared with other means of increasing the sensi- 

 tiveness''' this scheme has the following advan- 

 tages: It requires no change in the galvanometer. 

 It brings the observer close to the instrument. It 

 enlarges, rather than diminishes, the field of view, 

 or, if desired, permits the use of a smaller mirror. 

 " Geiger, Fhysihalische Zeitschrift, January, 

 1911. 



It introduces none of the difficulties of the other 

 method which arise in multiple reflection. 



Some Properties of Thin Films: W. P. RoOP. 



An attempt to observe a change in ohmic resist- 

 ance of a thin film on illumination with ultra- 

 violet light resulted negatively. Such a change 

 might be anticipated as a result of photoelectric 

 emission. Diminution in density of free electrons 

 might increase resistance, or liberation of bound 

 electrons might result in decrease. 



Other phenomena were observed, however, for 

 which no simple explanation has been found. 

 These include: (a) Spontaneous change in resist- 

 ance of the film. (&) Negative temperature co- 

 efficient, (c) A quasi-polarization effect. The 

 film was connected like a condenser, first with a 

 dry cell, and then with a galvanometer. The gal- 

 vanometer showed ballistic deflections which di- 

 minished approximately logarithmically with in- 

 creasing time interval between disconnection from 

 cell and discharge. The half-value period was 

 about two seconds, (d) Fluctuations in the re- 

 sistance of the film. In a typical case, there was 

 a variation of the order of 1 per cent, in the re- 

 sistance of a film of 2 X lOe ohms. The fluctua- 

 tion was entirely spontaneous, uninfluenced by 

 small temperature changes and by the nature of 

 the gas in contact with the film. It covered a 

 wider range at higher resistances. 



Oxidation has an undoubted effect on the phe- 

 nomena observed. All were present, however, in 

 high vacuum and hydrogen as well as in air. 

 Substances most used were nickel and zinc. Ob- 

 servable tarnish of the surface was in no case ap- 

 parent. 



A recess was declared at 6 o'clock in order to 

 enjoy a delightful dinner, served in the Faculty 

 Club house on the campus. After the dinner mem- 

 bers listened to the paper by Professor Sanford. 



The Specific Inductive Capacity of Certain Metals: 



Feenando Sanfoed. 



It is well known that in the spectra of a number 

 of metals series of lines have been found whose 

 wave-lengths may all be computed from a simple 

 formula. In all these eases the wave-lengths con- 

 verge towards a shortest possible wave-length for 

 the series. In the group of the alkali metals a 

 spectral series whose convergence wave-length is 

 shorter than any known wave-length in the spec- 

 trum of the respective metal is known for each 

 element. It is assumed in the paper that these 

 convergence wave-lengths represent the shortest 

 wave-lengths that can exist in their respective 



