5^4 



NATURE 



[August 28, 19 19 



OUR ASTRONOMICAL COLUMN. 



Kopff's Comet 1906 IV. = 19190. — Observations of 

 this comet made at Nice and elsewhere early in the 

 month have been published in the Comptes' rendus, 

 which show the correction of -7' to M. Ebell's 

 ephemeris already noted. This correction is included 

 in the following positions of the comet : — 



For Greenwich Midnight. 



A New Comet. — A telegram received from Harvard 

 through Paris, which is now the channel for astro- 

 nomical information, states th^t a comet of the 

 8th magnitude was discovered in the constellation 

 Pegasus by Mr. Metcalf on the night of August 20. 

 A second message gives particulars of an observation 

 made by M. Giacobini at Nice on August 22 at 

 iih. 45m., Paris time. The observed right ascension 

 was then 22h. 46m. ^g-ss., declination 28° 22' 53" N, 

 Comparison of this with the approximate place given 

 in the first message shows that the R.A. is decreasing 

 39s., and that the comet is going northward at the 

 rate of no', per day. M. Giacobini considers the 

 comet to be of 9th magnitude. 



Distribution of Globular Clusters and Spiral 

 NEBUL.ffi:. — Dr. Harlow Shapley continues his in- 

 genious researches into the structure of the universe 

 in Contributions from the Mount Wilson ' Observa- 

 tory Nos. 160 and 161. He here concerns himself 

 largely with the distance of the clusters, and makes 

 the point that if there is obscuring matter in the 

 equatorial segment of our system which blocks out 

 these clusters, all the clusters within a conical space 

 should be so blocked out. But his diagram of the 

 positions of these clusters drawn to scale shows that 

 many of them are seen which are within this cone, 

 so that the argument for the obscuring matter fails. 



A Planet beyond Neptune. — Prof. W. H. Pickering 

 points out (Harvard Circular, No. 215) that, accord- 

 ing to measures of Harvard plates made by Prof. 

 Russell, Neptune has recently begun to deviate from 

 its computed position, as it would if perturbed by 

 an unknown outer body, as shown by him in Harvard 

 Annals (vol. Ixi.) in 1909. The deviation is slight, 

 "being at present only 2", and this, according to the 

 investigation cited, was not expected until 1924. At 

 its next opposition, December 30, 1919, this hypo- 

 thetical planet should be located in R.A. 6h. 35m., 

 •dec. 23° N., and it is suggested that not only should 

 search be made for this — though it will be difficult, 

 for the place is in the Milky Way, where there are 

 countless stars brighter than the object looked for — 

 but rather that the position of Neptune should be 

 observed as completely as possible. 



T 



PHOTOPHORESIS. 



HE ratio of the surface area to the volume of a 

 spherical bodv varies inversely as its radius. 

 This fact led F. Ehrenhaft {Ann. d. Phys., lvi.,81, 1918; 

 cf. Phys. Zeit., xv., 608, 1914; and xviii., 352, 1917) 

 to use small spherical particles of various substances, 

 produced either by volatilisation or by burning an 

 electric arc between electrodes of the substance in an 

 inert atmosphere, in his examination of the forces 

 exerted by stationary light radiation on matter. 

 Such particles, suspended in argon or nitrogen, were 



NO. 2600, VOL. 103] 



introduced into a small observation chamber, which 

 was strongly illuminated from the side, the observa- 

 tions being made with a microscope placed per- 

 I>endicular to the illuminating pencil of light. Ehren- 

 haft concentrated the light from an arc — with 

 exclusion of the ultra-violet and infra-red rays — to a 

 small conical pencil, with a diameter of only 

 i/ia mm. at its narrowest part. The behaviour of 

 the small spherical particles in and near the region 

 of most intense illumination was studied. 



Outside the illuminating pencil all particles fall 

 under gravity, but within it some travel in the direc- 

 tion of the light ("light-positive"), and others 

 in the opposite ("light-negative") direction. To this 

 phenomenon of the movement of material particles 

 produced by light radiation Ehrenhaft has given the 

 name " photophoresis." Particles of some substances 

 (e.g. water) remain practically uniniluenced bv the 

 light; they are "light-neutral." 



By the use of an electric field between the plates 

 of the condenser in which a charged particle is 

 obser\ed, it can be kept suspended, and a second 

 symmetrical illuminating system enables one to pro- 

 ject light into the chamber from the opposite side. 

 In this way, and with suitable light stops, long-con- 

 tinued observations of a single particle can be made. 



The movement of any one particle is quite uniform 

 in a homogeneous field of light; the force exerted on 

 it is thus proportional to its velocity, and inversely 

 proportional to its mobility. Since the velocity of the 

 particle can be determined experimentally, and the 

 mobility by use of the law of Stokes-Cunningham, 

 it is a simple matter to calculate the force of photo- 

 phoresis on the particle. In Ehrenhaft's experiments 

 these forces were of the order of magnitude ro-" to 

 10-* dyne. 



The force of photophoresis on particles of the 

 same colour^ and hence of the same size is inde- 

 pendent of the pressure of the gas in which they are 

 suspended, even when the mean free-path of the gas 

 molecules is large compared with the size of the 

 particles. Irene Parankiewicz (Ann. d. Phys., Ivii., 

 489, 1918) made observations on a single particle in 

 an inert gas at different pressures, and found the force 

 exerted on the particle to be independent of the 

 pressure. 



Particles of any one substance (e.g. selenium) of 

 the same size attain different velocities in the gases 

 hydrogen, nitrogen, and argon, but^ the photo- 



phoretical force f = — hTt ) ^^ ^°""d ^o be completely 

 independent of the nature of the gas, though it 

 increases with the intensity of the incident radiation. 

 Photophoresis thus depends solely on the nature of 

 the substance of which the particles are composed. 



Of the elements so far examined, Na, K, Cu, Ag, 

 Au, Mg, Zn, Cd, and Hg have been found to be 

 light-positive, whilst Tl, Sn, Pb, P, Bi, S, and I are 

 light-negative. The behaviour of the elements As. 

 Sb, Se, and Te is noteworthy. When they are 

 volatilised in pure, dry, inert gases, two kinds of 

 particles result, one kind moving in the direction of 

 the light and the other in the opposite direction. 

 They are thus separable by light in exactly the same 

 way as a mixture of light-negative sulphur and 

 light-positive silver particles would be. 



The photophoresis of light-negative selenium par- 

 ticles is constant and independent of time. Light- 

 positive selenium particles are remarkable, however, in 



1 When the dimensions of the particle are less than the average^ wave- 

 length of visible light, the natural colour of the substance examined is 

 replaced by the colour of the light scattered by the particle. Particles of a 

 blue colour are smaller than those of a colonr corresponding with the less 

 refrangible rays of the spectrum, and Ehrenhaft hai been able to use the 

 colour of such particles as a means of estimating their size. 



