August 5, 1920] 



NATURE 



709 



interfering beams of 33 mm. was set up so as to 

 produce achromatic fringes from the light of a 

 inercury-vapour lamp. 



Two tubes each about 100 cm. long, with worked 

 glass plates cemented on the ends, were placed 

 piiralk-1, one in each of the interfering beams. Each 

 tube was evacuated, but one contained a glowing 

 cathode and a cylindrical anode which were connected 

 to an alternating source of potential capable of 

 delivering about 20 milliamperes at 100,000 volts, or 

 alternatively 200 milliamperes at 5000 volts, these 

 two potentials corresponding to electron velocities 

 of about 2x10"' cm. /sec. and 4x10' cm. /sec. respec- 

 tively. . 



The method of experiment was to produce wide 

 fringes in the observing telescope, and, by means of 

 two tapping switches, to turn on first the filament- 

 heating current and then the applied potential. In 

 no case was any shift or certain broadening of the 

 fringes observed. 



The main difficulty in the experiment was the fre- 

 quent fracture of the necessarily small glass tube 

 employed as the result of the great heat dissipated 

 inside' R. Whiddington. 



The Physics Laboratories, The University, 

 Leeds. 



Plant-life in Cheddar Caves. 



While recently visiting the famous caves at Cheddar 

 I noticed small patches of moss-like vegetation , grow- 

 ing near the electric lamps used to illuminate the 

 caves. The caves extend a long way into the hill- 

 side, and, as the entrances are but small, daylight 

 penetrates into them to a very short distance only. 

 They are lighted by wire filament electric lamps, of 

 which some are hung from the roof, but many are 

 laid upon their sides in the deep natural recesses, 

 and, in order better to illuminate the formation and 

 bring up the beautiful colouring and folding of the 

 stalactites, are provided with reflectors. It was close 

 against some of these lamps that I noticed the patches 

 of vegetation, and they looked so strange that I asked 

 the attendant if they had been placed there as an 

 experiment His answer was that they had not, and 

 that he himself had noticed them growing near the 

 lamps. 



It seems to me to be curious that this vegetation 

 should be flourishing under such absolutely artificial 

 conditions where there is no trace of daylight. How 

 the spores got so far in is also an interesting point, 

 but possibly they were introduced by dirty spades used 

 when the workmen were digging out the latest exten- 

 sion of the caves. 



I should be glad to know if this curious phenomenon 

 has been observed before, and what kinds of plant- 

 life succeed in these unnatural conditions. .As one of 

 the excursions during the forthcoming meeting of the 

 British Association at CardifT will be to the Cheddar 

 Caves, perhaps a botanical visitor will identify the 

 growth and communicate his conclusions to Nature. 



Lough. Pendred. 



The Diamagnetism of Hydrogen. 



In a lett*-r to Nature of July 22 (p. 645) Dr. .\sh- 

 worth discusses the atomic diamagnetism of liquid 

 and gaseous hydrogen on the hypothesis that diamag- 

 netism originates from rotations or oscillations of the 

 paramagnetic atom or molecule. He ignores, how- 

 ever, the case of atomic hydrogen in normally 

 saturated hydrocarbons given in my letter of July 8 

 (p. 581). The atomic susceptibility of hydrogen in 

 these comi>ounds is constatU and equal to —30-5x10-' 

 at room-temperature. Onnes and Perrier (Proc. 

 NO. 2649, VOL. 105] 



Amsterdam Acad., vol. xiv., p. 115, 191 1) have shown 

 that the specific susceptibility of liquid hydrogen is 

 — 27x10-', with a probable error of 10 per cent., so 

 that there is little difference between this value for 

 hydrogen at a temperature less than —253° C. and 

 that derived from the hydrocarbons at room-tempera- 

 ture. According to the kinetic hypothesis of Dr. 

 Ashworth, the paramagnetic atom will appear dia- 

 magnetic only if its oscillations exceed 130° on either 

 side of the position of rest, and oscillations of this 

 nature (or complete rotations) must be common to all 

 the hydrogen atoms in any normally saturated com- 

 pound. Ihis, I think, Dr. Ashworth will scarcely 

 admit is plausible. 



Moreover, consider the general case of crystallisa- 

 tion of a diamagnetic substance. The specific sus- 

 ceptibility of the liquid may be less than or greater 

 than that of the crystals, but each is diamagnetic 

 (Ishiwara, .Science Reports, T6hoku, vol. Hi., p. 303, 

 1914; Oxley, Phil. Trans. Roy. Soc, vol. ccxiv., A, 

 p. 109, 1914). Therefore the oscillations of the atoms 

 which appear diamagnetic must be at least 130° on 

 either side of the position of rest, even in crystals — a 

 conclusion which is scarcely consistent with the view 

 that crystalline symmetry is in part determined by the 

 electronic configuration of the atom. 



A. E. OXIF.Y. 



The British Cotton Industry Research 

 Association, 108 Deansgate, Man- 

 chester, July 29, 



Loss of Fragrance of Musk Plants. 



It is important to ascertain whether the loss of 

 scent which has Ijeen noticed lately in the musk plant 

 {Mimmulus moschatus) in certain areas is of general 

 occurrence throughout the country. 



There is no doubt that in many cases the descen- 

 dants of musk plants which used to form such 

 fragrant inhabitants of our cottagt- windows have lost 

 the power of producing the peculiar musk-like scent. 

 .An important character has dropf>ed out of the musk 

 plant's hereditary equipment, and it becomes a matter 

 of interest to know to what extent and in what 

 manner this has come about. 



If any plants can be found which still retain the 

 old scent, intercrossing between these and the scent- 

 less variety would probably give genetic results of 

 interest. ' C. J. Bond. 



Fernshaw, Springfield Road, Leicester, 

 July 26. 



Meteorological Conditions of an ice-Cap. 



In Nature of July 29 Mr. R. M. Deeley criticises 

 Prof. Hobbs's terminology in describing the meteoro- 

 logical conditions of an ice-cap as anticyclonic. He 

 arrives at the conclusion from Prof. Hobbs's state- 

 ments that low pressure exists at the centre. This is 

 scarcely necessary. 



The high pressure of an anticyclone in temperate 

 regions is maintained by the descent of air in the 

 centre drawn from the upper atmosphere; this com- 

 pensates for the surface outflow due to the disturbing 

 of the geocyclostrophic equilibrium by surface friction. 

 The same conditions, i.e. the surface outflow and the 

 central descent of air, exist in Prof. Hobbs's polar 

 ice-cap anticyclone ; the only difference is the physical 

 origin. 



In stating that the outflow of air over an ice-cap 

 produced a vacuum which was filled by inflowing air 

 from above. Prof. Hobbs was only describing in 

 separate detail what is really a continuous process, 

 no vacuum ever actually existing. 



R. F. T. Granger. 



I.enton Fields, Nottingham, July 30. 



