io6 



NATURE 



[January 26, 1922 



of the colours and on account of the ease with which 

 the surface can be given a satisfactory poHsh. 



It is clear from the observations mentioned above 

 that the colours under discussion are in the nature 

 of diffraction effetits arising from a film which is not 

 continuous, but has a close-grained structure. In- 

 teresting effects are observed when the surface of the 

 illuminated plate is viewed through a nicol, the 

 colour and intensity of the scattered, as well as of 

 the regularly reflected, beams varying as the nicol is 

 rotated about its axis. The most striking effect is 

 obtained when the direction of observation is nearly 

 parallel to the surface of the plate. The scattered 

 light in this case is nearly completely polarised, and 

 the colour of the regularly reflected light changes 

 nearly to its complementary when the nicol is turned 

 through 90°. The phenomena strongly recall to mind 

 the observations of R. W. Wood on the colours of a 

 frilled collodion film on a silvered surface, which have 

 been discussed by the late Lord Rayleigh {Phil. 

 Mag., November, " 1917), and it seems probable that 

 the explanation of the phenomena will ultimately be 

 found to be somewhat similar in the two cases. 



C. V. Raman. 



210 Bowbazaar Street, Calcutta, India, 

 October 11. 



Mr. Mallock has shown that the colour of the 

 oxide film is an intrinsic property of the material of 

 which it is composed and the material retains this pro- 

 perty as it is gradually ground down from its original 

 thickness to the vanishing point. Sir George Beilby's 

 observations have confirmed this, and have further 

 shown that the film is an aggregate in open formation 

 through which oxygen molecules can penetrate to the 

 metallic surface. For each temperature above the 

 temj>ering range the thickness of the film is deter- 

 mined by the porosity of the aggregate to the 

 oxygen molecules at that temperature. Direct experi- 

 mental observations have shown the part played by 

 time of heating at any given temperature. For 

 example, at 275° C. a deep purple was reached in 

 ten minutes, and this changed to blue from the 

 margin inwards during a further period of twenty 

 minutes. It was thus shown that the watchspring- 

 blue, which could immediately be produced by a tem- 

 perature of 300° C, could also be produced by heating 

 at 275° for thirty minutes. Sir George Beilby's view 

 is that the intrinsic colours of the films which are 

 produced at different temperatures result from changes 

 in molecular aggregation in relatively open forma- 

 tion of a similar nature to those which have been 

 shown to occur in thin metal films, e.g. gold. This 

 is referred to in his recently published volume en- 

 titled "Aggregation and Flow of Solids," sections 3 

 and 10.— Ed. Nature. 



Some Terrestrial Experiments on Gravitation and 

 Einstein's Theory. 



The object of this letter is to direct the attention 

 of writers on Einstein's theory of gravitation to some 

 recent experiments on the terrestrial aspects of gravita- 

 tion which seem to have been overlooked, although 

 they appear to be of great importance for the purpose 

 of forming a just estimate of the correctness of Ein- 

 stein's theory. 



-The first investigation referred to is that of Dr. 

 P. E. Shaw on the effect of temperature on gravita- 

 tive force (Phil. Trans., 1916, A, vol. 216, pp. 349-92). 

 On p. 390 Dr. Shaw writes: — "When a large mass 

 attracts a small one, the gravitative force between 

 them increases by about 1/500 as the temperature of 

 NO. 2726, VOL. 109] 



the large mass rises from, say, 15° C. to 215° C." 

 The only cause capable of producing this effect on the 

 relativity theory seems to me to be the absorption of 

 heat by the large mass (lead), amounting to 6 calories, 

 or 25x10* ergs per gram, and resulting in a frac- 

 tional increase of inertial mass of about 2-8x10-". 

 We require 7000 million times this amount in order 

 ,to account for Dr. Shaw's result on the hypothesis 

 of the proportionality of the graviiative and inertial 

 masses, which is one of the basal assumptions of 

 Einstein's theory. 



Another investigation is that of Majorana on the 

 absorption of the gravitational flux (Phil. Mag., 

 vol. 6, No. 39, pp. 488-504, 1920), in which he finds, 

 inter alia, that a lead ball weighed in vacuo loses 

 7-7x10-"' of its weight when it is surrounded sym- 

 metrically by 104 kg. of mercury. If the gravitational 

 flux be assumed to be absorbed by the mercury ac- 

 cording to an exponential law of density and thick- 

 ness, the quenching constant, or factor of absorption, 

 is found to be 673 x 10- " per unit density and length. 

 A possible interpretation is that the gravitational mass 

 of a homogeneous sphere at an outside point is only 

 a fraction of its inertial mass ; according to Majorana, 

 it is about one-third for the sun. If this interpreta- 

 tion be legitimate, the results of Majorana, like those 

 of Shaw, lead to the conclusion that the gravitational 

 and inertial masses are not proportional to one another 

 in all circumstances. G. A. Schott. 



University College of Wales, Aberystwyth, 

 January 5. 



British Scientific Instruments. 



In the timely and encouraging leader in Nature 

 of January 19, with which my experience is in entire 

 agreement, there is a point of some importance to 

 which reference is omitted. This is the practical 

 question of cost. I would ask permission to draw a 

 moral for application at the present time. Without 

 expressing any opinion as to whether this cost could 

 be reduced by improved methods of manufacture, I 

 would direct attention to the fact that in the im- 

 poverished state of the finances of universities and 

 similar bodies it is impossible adequately to equip 

 their laboratories with costly apparatus. 



The moral is this : The most effediive way in 

 which Government intervention can assist British 

 makers of scientific apparatus is to increase the 

 grants to universities and to research in general. It 

 is impossible for individual workers to purchase expen- 

 sive British instruments out of their own incomes, and 

 until the resources of the laboratories in which they 

 work are sufficiently increased it is an unjustifiable 

 and foolish restriction to prevent their obtaining from 

 abroad apparatus often admittedly inferior, but 

 capable of good use. How many laboratories can afford 

 to obtain Hilger optical apparatus or the Cambridge 

 string galvanometer? It is further to be remembered 

 that as science advances the Instrumental equipment 

 for continued pushing forward tends to become more 

 elaborate, sensitive, and accu^-ate, and necessarily of 

 greater cost ' W. M. Bayliss. W 



University College, London. 



Globular Lightning Discharge. 



The following is an account of what appears to 

 have been a genuine case of globular electrical dis- 

 charge observed by the sisters of one of my colleagues, 

 the Misses Pitman, at Eastbourne on August 17 last. 

 Authentic instances of this phenomenon are rare, and 

 as the conditions which accompanied this particular 



