May 17, 1906] 



NATURE 



55 



directly or deduced indirectly from their vapour pressures, 

 and the relation connecting the osmotic and vapour 

 pressures is quite independent of the " gas laws holding 

 for solutions." 



Leaving^ out of consideration the experiments made 

 liefore the solutions were stirred — for on Prof. Kahlcn- 

 beri^'s own showing those are not good — his conclusion that 

 the gas laws do not hold for dilute solutions in pyridine 

 is based on four experiments. If one may take No. 59 

 (p. 201 1 as a type of these, it is easy to show that the 

 exi)eriment is valueless. 



The sugar solution used is 0125 grm. mol. per litre, 

 and a pressure of 98 cm. of mercury is reached, but the 

 theoretical value is some 3 atmos. Now on p. 184 the 

 diameter of his gauge is given as 0-5 mm., and he says 

 that at the end of three days 0115 grm. sugar has come 

 through the membrane — this quantity represents 28 c.c. 

 of sohilion. If we assume that this volume of solution 

 came through the membrane at a uniform rate, a simple 

 calculation will show that the rate is equivalent to a fall 

 of 20 cm. />er hour in the gauge. No wonder the 

 theoretical pressure was never reached ! 



Berkeley. 



Foxcombe, near Oxford. E. G. J. H.\rti,ev. 



Diurnal Variation of the lonisation in Closed Vessels. 



In his letter on this subject published in Nature of 

 May 3 (p. 8) Mr. G. C. Simpson is, I venture to think, 

 under a misapprehension regarding the conditions which 

 determine the variations of the earth's electric field. His 

 statement of the problem, which I have slightly abbreviated, 

 is as follows : — " It is usual to accept that there is a 

 negative charge on the earth's surface, and the correspond- 

 ing positive charge is a volume charge distributed in the 

 atniosphere. There is very little volume charge in the air 

 close to the earth's surface, so the relation between potential 

 gradient and charge on the earth's surface is given by 

 dvU\h^ ~ ^i^-nn. Hence it follows that with a given charge 

 on the earth's surface and the corresponding charge in 

 the atmosphere above, the vertical distribution of the 

 charge and the conducting state of the upper atmosphere 

 do not in the slightest affect the potential gradient within 

 a few metres of the earth's surface." 



.'Kll this is very true, but it is equally true that •with, a 

 constant charge on the earth's surface nothing whatever 

 will affect the potential gradient close to it. Since the 

 potential gradient is a constant multiple of the surface 

 density, it is absurd to consider the variation of the one 

 whilst the other is kept constant. The only assumption 

 that it seems safe to make about the state of the earth's 

 surface is that, owing to the relatively high conductivity 

 of the earth's crust, for purposes of atmospheric electricity 

 it may be treated as an equipotential surface. The charge 

 in any particular region will be determined by the dis- 

 tribution of electrification and ionisation in the atmosphere, 

 and will readjust itself almost instantaneously when any 

 change takes place in the external conditions. It will not, 

 as an incautious reader might gather from Mr. Simpson's 

 letter, behave as if it were glued to the surface of the 

 earth. 



In my letter of April 22 I illustrated my point by con- 

 sidering the analogy with the case of ionised air between 

 two parallel plates maintained at a constant difference of 

 potential. As this comparison is inaccurate, I shall take 

 the liberty of putting the case in another way, in the hope 

 that it may prove more convincing. The earth is to be 

 regarded as a conducting sphere which is continuously 

 receiving a _ negative charge in certain areas — probably 

 those in which rain is falling — and losing it again by con- 

 duction through the atmosphere from all the rest of its 

 surface. Since the observations on the earth's field only 

 refer to fine-weather regions, we need only consider what 

 happens over them. There will be an earth-air current 

 which, under specified conditions, will have attained a 

 steady value, the charge on the earth's surface being 

 that required to give the necessary potential gradient to 

 drive the current. Suppose that by some means the 

 ionisation at some distance from the surface suffers a 

 permanent increase locally, whilst the air close to the 

 surface is unaffected. It 'is clear that, wh.'.tever view is 

 NO. 1907, VOL. 74] 



taken of the distribution of the charges producing the 

 earth's field, the increase in the ionisation will produce 

 a local increase in the earth-air current ; but by hypothesis 

 the conductivity of the air close to the earth is unaltered, 

 so that the increase in the current must be accompanied 

 by an increase in the potential gradient close to the surface. 

 This is, of course, produced by negative electricity flowing 

 from other parts of the earth. 



The above, I imagine, is an exaggerated but otherwise 

 trustworthy picture of the effect an ionising radiation from 

 outside would have on the earth's electric field. The con- 

 ductivity produced by the rays in the upper atmosphere 

 must be enormous compared with the effects close to the 

 earth. Even if the rays were homogeneous, only a mere 

 trace would remain after passing through a layer of air 

 roughly equivalent in absorbing power to 76 cm. of mercury. 

 But it is far from probable thai they are homogeneous, and 

 any want of homogeneity would exaggerate the effect. 

 Other factors conspire to this end : the presence of dust 

 near the earth loading the ions and the smaller rate of ri'- 

 combination at low pressures ; whilst the increase in the 

 mobility of the ions at low pressures would just compensate 

 for the feebler absorbing power of the upper atmosphere. 



It will be observed that the effect on the earth's field 

 of an increase in the ionisation of the atmosphere depends 

 entirely on where that increase takes place. If the con- 

 ductivity increases in a greater ratio close to the earth's 

 surface than it does further away, the result ought to be 

 a fall in the potential gradient. Mr. Simpson rightly points 

 out that such a relation between the potential gradient 

 and the leakage of electricity near the earth's surface has 

 been shown to exist. From my point of view this indicates 

 that the bulk of the ionisation near the earth's surface is 

 not caused by radiation from an external source. 



O. W. Richardson. 



Trinity College, Cambridge, May 12. 



Defects in Ostrich Feathers in South Africa 



The domestication of the ostrich on a practical basis 

 was undertaken in Cape Colony about 1867, and since 

 then ostrich farming has become one of the most important 

 industries in the eastern province. The census of 1904 

 gave 357,970 ' tame ostriches in the colony, while the ex- 

 port of feathers reached 470,381 pounds, practically the 

 whole of which came from tame birds ; the estimated value 

 of the feathers was 1.058,988;., giving about 3?. 10.'!. per 

 bird of feather-producing age. During the forty years of 

 domestication the instincts of the ostrich have apparently 

 undergone no change, though its habits are much altered. 

 The feathers cut from the tame bird are shorter, weaker, 

 and not so fluffy as those taken from wild birds, but 

 probably these differences are to be correlated with the 

 greater frequency of plucking, and not with any consti- 

 tutional change resulting from domestication. 



Within recent years much concern has arisen from the 

 prevalence of a defect in the growth of the feather, which 

 seriously reduces the value of the plumes to the farmer. 

 The imperfection, technically known as "barring," takes 

 the form of a series of narrow, chevron-shaped bars or 

 malformations across the whole feather. The genera! 

 appearance of a moderately affected plume is shown in the 

 accompanying photograph (Fig. i). Examined closely, it 

 is seen that the regularity of the individual barb is much 

 disturbed at the bars, and that the barbules are there 

 defective and only partly differentiated from the barbs. 

 The appearance is such as to suggest that the barbs have 

 been constricted at these particular regions, and that in the 

 development of the feather the barbules have failed to 

 become differentiated and open out, though \vith a needle 

 their separation can sometimes be effected. Occasionally 

 several barbs will remain joined together at the bars, they 

 also having failed to differentiate. In many cases some 

 of the barbs are shortened, the missing part having broken 



1 The statistics are taken from a paper, by the Hon. Arthur Douglass 

 read before the recent meetines of the British .Association at Cape Town. 

 Mr. Douglass is the author of a well-known work, "Ostrich Farmini; in 

 South Africa." and was one of the pioneers in the domestication of the 

 ostrich, and probably the first to hatch the chicks by artificial incubation. 

 His death, shortly after the meetings of the Association, is a great loss to 

 the agricultural and political life of Cape Colony. 



