Chemistry and Physics. 481 



recess which are sliot out tangentially forwards from the cylinder 

 will have a velocity directed away from the trailing or inlet 

 tube and towards the leading or oullet pipe. Their speed will 

 be more than twice the mean molecular speed. Theoretically, an 

 extremely small percentage of molecules can pass in the opposite 

 direction. 



From the practical standpoint, as well as from the theoretical, 

 the molecular air-pump is almost perfect. When run at a speed 

 of 12000 R.P.M. the pressures at the "suck-nozzle" were 0-0003, 

 0-00003, 0-000005, and 0-0000002 mm when the pressures in the 

 housing were 20, 10, 1 and 0-05 ,nm respectively. The commercial 

 pumps are designed to run at 8000 R.P.M. A conception of the 

 rapidity of working of the new pump may be formed from the 

 following test. An X-ray tube of about one liter capacity was 

 evacuated, in 10 sees., from about 5 mm to a pressure so low that 

 sparks passed in a parallel gap 15 cms long. Gaede's well-known 

 mercury pump required 100 sees, to bring about the same result. 

 Another great advantage of the molecular air-pump is that it 

 works just as well for vapors as for dry gases. In particular, 

 therefore, the usual drying agents may be dispensed with and 

 much time saved in this connection. Of course, the new pump 

 alone is useless at atmospheric pressure. — Physikal. Ztschr., No. 

 18, Sept. 1912, p. 864. h. s. u. 



8. On the Emission Velocities of Photo- Electrons. — The 

 results obtained by various investigators in this field have been 

 discordant, and hence the present paper by A. Ll. Hughes is 

 rather important because in it he discusses the sources of error 

 which may have vitiated, in a greater or lesser degree, the work of 

 his predecessors, and also describes his method of experimenting, 

 which seems to overcome most of the difficulties. The two sources 

 of error which have not been taken into account in any previous 

 work are : (a) the effect on the velocity-distribution curves of a 

 magnetic field, such as, for example, that of the earth, and (b) the 

 effect on such curves of an electron approaching the boundary 

 obliquely. By distilling the metal to be investigated and by 

 allowing it to deposit on a disc of nickel, which could be raised 

 and lowered at will inside of the glass apparatus itself, Hughes 

 avoided both the production of surface films of gas and the polar- 

 ization of the metallic surface which arises when the method of 

 anode sputtering is employed. The whole process of distillation 

 and adjustment was carried out in a liquid air vacuum, so-called. 

 A control experiment was performed by using the surface of flow- 

 ing mercury in place of the solid metal deposited on the nickel 

 disc. 



A portion of the summary given by Hughes will now be 

 quoted. " (1) The maximum velocity of photo-electrons from 

 the surfaces of a number of elements prepared by distillation in 

 vacuo has been measured. (2) It has been shown that the energy 

 of the fastest electrons emitted when monochromatic light falls 

 on the surfaces is proportional to the frequency of the light. The 



