122 



NATURE 



[March 25, 1920 



in either direction from that giving the maximum 

 wave-length gives also a decreased amplitude. — H. A. 

 Daynes : The theory of the katharometer. A his- 

 torical introductory note by Dr. G. A. Shakespear 

 gives a description of the katharometer and an 

 account of its development by him for hydrogen purity 

 measurements and similar work in connection with 

 lighter-than-air craft. The paper discusses the condi- 

 tions which determine the temperature of the hot 

 wire in the katharometer cell, and shows that loss of 

 heat by conduction through .the gas is the most im- 

 portant factor, convection and radiation being quite 

 unimportant. Equations are given expressing the 

 experimental law of heat loss in a single katharometer 

 wire, and these are applied to the case of two wires 

 in parallel in the arms of a Wheatstone bridge. 

 These equations are then used to show what are the 

 conditions for greatest sensitiveness and precision in 

 various cases arising in practice. — H. A. Daynes : The 

 process of diffusion through a rubber membrane. 

 The nature of diffusion of gases through rubber 

 membranes is discussed in the light of some recent 

 work. This all points to a simple process, deter- 

 mined by the case of diffusion through the rubber, 

 and by the absorption of the gas by the rubber. This 

 is introduced mathematically into the problem of dif- 

 fusion through a membrane. The unsteady state is 

 considered, in which the membrane, after being ex- 

 posed to air, is suddenly exposed on one side to, 

 say, hydrogen, and the rate of emission of hydrogen 

 from the other side calculated. The passage of gas 

 through the material is treated purely as a diffusion 

 problem, the boundary conditions only being deter- 

 mined by absorption. It is shown that measurements 

 of the permeability of a membrane and of the lag on 

 reaching a steady state are sufficient for the deter- 

 mination of both absorption and diffusion constants. 

 Experiments are described in which these conditions 

 are fulfilled. The measurements of the diffusion are 

 made by means of a katharometer. From tVese 

 experiments the constants of diffusion and absorption 

 for hydrogen, nitrogen, oxygen, carbon dioxide, 

 nitrous oxide, and ammonia are determined. Tem- 

 perature coefficients for the constants are given for 

 hydrogen, and the high temperature coefficient of 

 permeability of rubber is shown to be due chiefly to 

 the high temperature coefficient of the diffusion con- 

 stant. The extraordinarily high permeability of rubber 

 to carbon dioxide, ammonia, etc., is shown to be due 

 entirely to the high absorption. A relation is also 

 suggested between absorption and critical temperature 

 of the gas. 



Physical Society, February 27.— Prof. W. H. Bragg, 

 president, in the chair. — ^T. Smith : The balancing of 

 errors. In calculating functions from Taylor expan- 

 sions or otherwise, the results obtained by summing 

 any finite number of terms will differ to a greater or 

 less extent from the true results. It is shown in the 

 paper that by suitable modifications of the coefficients 

 the results obtained, even when comparatively few 

 terms of the expansion are taken, can be made to 

 approximate very closely to the true results for all 

 values of the variable between selected limits.- — Dr. 

 N. W. MacLachlan : Notes on the testing of bars of 

 magnet steel. The paper describes the results of 

 experiments with the Ewing double permeameter. It 

 is shown that the assumption underlying the theory of 

 the method, viz. that the end effects are the same 

 with the long and short bars, is not justified, and 

 that the value of H, as found by calculation on this 

 assumption, is in error. The error did not, however, 

 exceed i per cent, for any of the bars tested, but 

 the author concludes that the method is inferior as 

 regards accuracy and convenience to the differential- 



NO. 2630, VOL. 105] 



coil method. — G. D. West : The forces acting on 

 heated metal-foil surfaces in rarefied gases. The 

 present paper arises out of two previous papers by the 

 author on the pressure of light (Proc. Phys. Soc, 

 XXV., p. 324, 1913, and jycviii., p. 259, 1916), and 

 consists of an experimental investigation of the nature 

 of certain peculiar movements of strips of thin metal 

 foil surrounded by rarefied gases and exposed to radia- 

 tion. The experiments deal chiefly with phenomena 

 at gas pressures below i cm. of mercury, and it is 

 shown that the apparently diverse results obtained 

 can be connected by a theory based on the work of a 

 previous paper (Proc. Phys. Soc, xxxi., p. 278, 

 19 19). The author concludes that at the highest 

 rarefactions the pressures on the strips arise from 

 the fact that, if differences of temperature exist in 

 an enclosure, the pressure of the gas is not uniform, 

 but varies approximately as the square root of the 

 latter's absolute temperature. The simple conditions 

 that exist at low gas pressures are complicated at the 

 higher pressures by gas currents which differ 

 fundamentally from convection currents, but are 

 closely connected with the phenomena of thermal 

 transpiration. 



March 12. — Prof. W. H. Bragg, president, 

 in the chair.— F. H. Newman; Absorption of 

 gases in the electric discharge tube. — J. S. G. 

 Thomas : A directional hot-wire anemometer. The 

 instrument consists of two fine platinum wires 

 mounted close together, and forming two of the arms 

 of a Wheatstone bridge. These are heated by the 

 current in the bridge. When a stream of gas moves 

 in a direction perpendicular to the wires, but parallel 

 to the plane containing them, the leading wire is 

 cooled, while the second wire, being shielded by the 

 first, is not cooled so much, and may actually _ be 

 heated on account of the air flowing past it being 

 warmed by the first wire. A deflection of the galvano- 

 meter is obtained, therefore, which is reversed if the 

 flow of gas is in the reverse direction. The instru- 

 ment is much more sensitive than the non-directional 

 .hot-wire anemometer. 



Linnean Society, March 4. — ^Dr. A. Smith Wood- 

 ward, president, in the chair. — R. H. Compton : A 

 contribution to our knowledge of the botany of New 

 Caledonia. The subject of this communication is the 

 collection made by Mr. Compton in New Caledonia 

 and the Isle of Pines during 19 14 with the aid of 

 money grants from the Royal Society, the Percy 

 Sladen Trust, and the Wort's Travelling Fund of 

 Cambridge University. The specimens collected have 

 been presented to the British Museum, and the 

 greater part have been worked out in the department 

 of botany at that institution. Dr. Rendle gave a short 

 account of the position and physical character of the 

 island, and referred to previous work on its flora 

 and its general characters. Important features arc 

 the igneous rocks which form a mountain chain of 

 gneiss in the north-east, and the serpentine forma- 

 tion which covers the southern portion and occurs in 

 larger or smaller areas throughout the island. The 

 flora is rich, and the proportion of endemic forms 

 exceptionally high. The main affinities of the flora 

 are with Indo-Malava and South-East Australia, the 

 former represented chiefly in the forest regions and 

 the latter "in the scrub and savannah regions; and a 

 study of it suggests that New Caledonia is a very 

 ancient land mass which has been isolated for a very 

 long period. Dr. Rendle also gave a resume of Mr. 

 Compton 's account of the ferns and gymnosperms. 

 The latter are of great interest; they number jibout 

 twenty-seven, and are all endemic. Mr. Baker 

 referred to a number of interesting specimens among 

 the dicotvledonous flowering plants, which included 



