July i, 1922] 



NA TURE 



1 1 



iron and nickel, and the reduction of the observations 

 has now been completed, with the result that the 

 Curie law, with certain limitations, is found to 

 apply to the ferromagnetic state, and the relation 

 k . T = a constant is approximately satisfied, but 

 the constant now is of a very different magnitude 

 from the former one. There is, however, this simple 

 and important relation between the constants in 

 the two states — their ratio is the kinetic energy per 

 unit of temperature per gram of two degrees of ft; edotn, 

 and is thus immediately connected with the gas 

 constant R. This result is of importance because it 

 shows that the change from the ferromagnetic to 

 the paramagnetic state is quantitatively explicable 

 as due to the acquisition of the kinetic energy per 

 unit temperature required for the two degrees of 

 rotational freedom which are effective in controlling 

 magnetic susceptibilitv. 



Thus there is proof from magnetic data alone, 

 independently of thermal data, that the change at 

 the critical temperature from ferro- to paramagnetism 

 is due to the gain of energy associated with two 

 degrees of freedom. 



This acquisition of energy-content makes itself 

 evident in the increase of specific heat which ferro- 

 magnetics show at and above the critical temperature, 

 and is quantitatively in agreement with the magnetic 

 result. 



It is no longer necessary now to assume, as has 

 been done, that an immense intrinsic magnetic field 

 is the cause of ferromagnetism, although it may 

 be convenient to introduce a fictitious magnetic 

 field such that it will give rise to energy effects 

 equivalent to the energy of two degrees of freedom. 



The results which have been discussed above are 

 also a confirmation of the simple view advanced by 

 Ewing in his earlier papers on the molecular theory 

 of magnetism, in which he suggests that the loss of 

 ferromagnetic qualities may be caused by the 

 oscillations of the molecular magnets which become 

 wirier and wider up to the critical temperature, at 

 which point they pass from vibration to rotation. 

 J. R. Ashworth. 



May 30. 



Molecular /Elotropy in Liquids. 



A very remarkable feature shown by many liquids 

 in experiments on the molecular scattering of light 

 is that the scattered beam in a direction transverse 

 to the primary rays shows a large admixture of 

 unpolarised light, the proportion of this to polarised 

 light in the scattered beam being several times 

 greater than in the case of the same substance in the 

 condition of vapour at atmospheric pressure. This 

 fad seemed at first very puzzling ; an explanation 

 is, however, now forthcoming. A theory of the 

 phenomenon has been worked out by the writer 

 which not only explains the facts in a simple and 

 quantitative manner, but has also pointed out the 

 way to further fruitful research. It may be briefly 

 indicated as follows : 



The polarised and unpolarised parts of molecularly 

 scattered light may be conceived as arising in two 

 distinct wavs ; the former is a mass-effect arising from 

 the thermal fluctuations of density in the fluid, and 

 its magnitude is given by the Einstein-Smoluchowski 

 formula 



IS 



RT.i 



Nxi ■ (m 2 -i)V+2) 2 . 



and as we pass from the condition of vapour to that 

 of liquid in which the molecules are more closely 



NO. 2748, VOL. I 10J 



packed together, it increases much less than in 

 proportion to the increased density. The unpolarised 

 part of the scattered light is, on the other hand, a 

 molecular effect, and its magnitude increases simply 

 in proportion to the number of molecules per unit 

 volume. The ratio of unpolarised to polarised part 

 of the scattered light should therefore be considerably 

 enhanced. This is exactly what is observed. If I, 

 and 2 1, are respectively the polarised and unpolarised 

 parts of the transversely scattered light, the ratio 

 I 2 /(I 1 + I 2 ) may be determined experimentally by 

 analysis with the aid of a double-image prism and a 

 nicol. The Table below shows in the second column 

 the value of this ratio as determined by Lord 

 Rayleigh for certain substances in the state of 

 vapour, in the third column the value of the ratio for 

 the liquid state at ordinary temperature as calculated 

 from the writer's theory, and in the fourth column 

 the value as determined by Mr. K. Seshagiri Rao in 

 the present writer's laboratory. The agreement is 

 significant. 



Ratio of Components of Polarisation 



We may also view the matter in another way. 

 When a substance is in the state of vapour under 

 small pressures, both the positions and orientations 

 of its molecules are absolutely at random, and 

 assuming the molecules to be aelotropic, the degree 

 of imperfection of polarisation of the light scattered 

 by it may easily be calculated, as has been done by the 

 late Lord Rayleigh. On the other hand, in the liquid 

 state, the packing of the molecules is so close that 

 their ordering in space is no longer at random ; but we 

 may still, at least in the case of ordinary liquids, 

 consider the orientations to be arbitrary without 

 serious error. If we take this into account in deter- 

 mining the resultant effect of the waves scattered by 

 the individual molecules, we should be led to the same 

 result as has been indicated above. 



The theory put forward has other notable successes 

 to its credit. The Einstein-Smoluchowski formula 

 indicates that though the density of a liquid 

 diminishes with rise of temperature, its scattering 

 power should increase and become very large as the 

 critical temperature is approached. Similarly, as 

 the temperature is increased, the scattering power 

 of the saturated vapour should increase much more, 

 rapidly than in proportion to its density. Accord- 

 ingly, In both cases, we should expect the polarisation 

 of the scattered light to improve steadily with rise of 

 temperature and become practically complete as the 

 critical temperature of the liquid is approached. 

 Experiments with benzene liquid and vapour made 

 by Mr. K. R. Ramanathan have quantitatively 

 confirmed this prediction. A similar improvement 

 in polarisation has also been observed by Mr. V. S. 

 Tamma in experiments on the scattering of light in 

 binary liquid mixtures as the critical temperature 

 for separation into two phases is approached. 



C. V. Raman. 



210 Bowbazar Street, Calcutta, 

 May 11, 1922. 



