42 



NATURE 



[January 12, 1922 



non-adaptive characters show no relation to differ- 

 ences of habit or environment, and, as a rule, develop 

 directly without recapitulation. I have instanced the 

 adaptive characters of Pleuronectidae (flat-fishes) on 

 one hand and their specific and generic characters 

 on the other. The adaptive characters of flat-fishes 

 exhibit one of the most remarkable cases of meta- 

 morphosis and recapitulation in the whole field of 

 zoology, while the various peculiarities of the scales, 

 as examples among specific and generic characters, 

 show neither recapitulation nor any relation to habits 

 and conditions of life. Thus, instead of agreeing tliat 

 there is only one kind of character, I find it necessary 

 to distinguish three kinds, one due to the effect of 

 an external stimulus on the individual, and not ap- 

 parently inherited, and two kinds which are here- 

 ditary. J. T. Cunningham. 

 Chiswick, December 31. 



Optical Observation of the Thermal Agitation of the 

 Atoms in Crystals. 



According to the theory of specific heats developed 

 by Debye, Nernst, and others, the thermal energy of 

 a solid is made up of the energy of elastic vibrations 

 in its material, the frequencies of such vibrations 

 ranging from very small values up to a maximum limit 

 determined by the ultimate molecular or atomic struc- 

 ture. On this view it is clear that at ordinary tem- 

 l>eratures the density of a solid, and therefore also its 

 refractive index if it be of transparent material, would 

 vary arbitrarily from point to point about its mean 

 value. In other words, a transparent crystal cannot 

 be regarded as optically homogeneous even with refer- 

 ence to the comparatively long waves which constitute 

 ordinary light. It follows that a certain proportion of 

 the energy of a beam of light traversing the medium 

 \yould be deviated laterally and appear as scattered 

 light, the intensity of such scattering being a measure 

 of the thermal agitation within the crystal. That some 

 such effect must occur has alreadv been pointed out by 

 Sir Joseph Larmor (P/n7. Mag., vol. 37, p. 163, 1919), 

 but no theoretical discussion of its magnitude appears 

 so far to have been put forward. It has occurred to 

 the present writer that the efl"ect to be expected may 

 be found in the following way :— If the principles of 

 statistical mechanics and the eauipartition of energy 

 were applicable in the case of solids, precisely the same 

 considerations which determine the molecular scatter- 

 ing of light in fluid media would enter here as well, 

 and the scattering coefficient would be given by the 

 Einstein-Smoluchowski formula 



7r« RT/3 

 18 • NX< 



(m2-i)V + 2)2, 



where ^ is the compressibility of the solid, /x is its 

 refractive index, A is the wave-length of the light, and 

 R, T, N are the constants of the kinetic theory. It is 

 known, however, that the heat-content of solids at the 

 ordinary temperature is much less than that indicated 

 by the equipartition principle, the deficiency being most 

 marked for substances, such as diamond, having a 

 high "characteristic temperature." The scattering 

 coefilcient given bv the preceding formula must there- 

 fore be diminished in the ratio which the actual heat- 

 content at the temperature of observation bears to the 

 heat-content indicated bv the equipartition principle 

 This correction-factor mav be found from the experi- 

 mental data for thp soecific heats at low temperatures 

 given bv Nernst, Lindemann, and others. 



Calculations made in the wnv indicated above show 



that transparent quartz should scatter light qi times 



as strongly as dust-free air at normal. temperature and 



pressure. A scattering of approximately this magni- 



NO. 2724, VOL. 109] 



tude in clear quartz was detected photographically by 

 R. J. Strutt (now Lord Rayleigh) (Proc. Roy. Soc.^ 

 vol. 95, p. 495, 19 19), but was ascribed by him ta 

 inclusions which he assumed were present in the 

 crystal. It is clear from what has been said above 

 that the effect observed by him was actually due tc 

 the thermal agitation of the atoms in the crystal. The 

 present writer has succeeded in demonstrating the 

 scattering of light in clear quartz by direct visual 

 observation. For this purpose a block of the crystal 

 with smooth polished faces is immersed in a tank of 

 clean distilled water to minimise surface-reflections 

 and a converging lens is used to bring a beam of 

 sunlight to a focus within the crystal. The blue track 

 of the beam within the crystal may then be readily 

 observed, and its intensitv can be judged by com- 

 parison with the scattering of the beam in saturated 

 ether vapour. The writer has had the pleasure of 

 exhibiting the phenomenon to Sir W. J. Pope and 

 other dislintjuished callers at his laboratory. 



Transparent rock-salt which has a low characteristic 

 temperature and shows a marked " Debye-effect " in 

 experiments on X-ray reflection exhibits a very strong 

 scattering of ordinary light. The increase of the scat- 

 tering with rise of temperature may readily be ob- 

 served with it. C. V. Raman. 



210 Bowbazaar Street, Calcutta, November 19. 



A Fossil Buttercup. 



When we examine a catalogue of fossil plants, such 

 as that for North America recently published by 

 Knowlton, we are struck by the enormous number of 

 recorded species, and readily receive the impression 

 that the flora of former ages is quite well known. It 

 is only when we make a more critical investigation 

 that we perceive the great gap in our present know- 

 ledge. We do, perhaps, know a fair proportion of 

 the trees and deciduous-leafed shrubs of a number of 

 geological periods, but when we look for the 

 herbaceous flora the limitations of our knowledge at 

 once appear. Thus the Ranunculaceae, an extensive 

 family in the present North American flora, do not 

 furnish a single definitely recorded fossil in the same 

 area. Dawson in 1875 vaguely referred to a Thalic- 

 trum, without specific name, supposedly from the 

 Eocene, but it is not to be taken seriously. Schenk 

 thought the fossil genus Dewalquea presented a cer- 

 tain analogy with Helleborus, but it is now referred 

 to quite another family. It is, of course, impossible 

 to suppose that the Ranun- 

 culaceae were absent from % ^ "•*• 

 North America during Tertiary 

 times ; thev simply must have 

 escaped preservation or ob- 

 servation. To those who would 

 see in the geological record a 

 proof that herbaceous plants 

 did not exist in the past, or 

 were extremely rare, we can 

 only reply that the record as it 

 stands proves too much. To 

 accept it at its face-value pos- _ , -• t> * 



^ t I ^. • ., , Vu '^"^- !•— Achenes of Sanun- 



tulates the irnpossible. The cuius florissanUnsis. 



general proposition that the 



herbaceous flora is, on the whole, more recent than 

 the woody may be valid, and has much to 

 recommend it. 



With regard to the Tertiary Ranunculaceae of North 

 America, we can fortunately rescue them from utter 

 oblivion. Several years ago I found some slabs of 

 Miocene shale at Florissant, Colorado, plentifully be- 

 sprinkled with small dark fossil ceeds. The exact 

 locality is the railroad cut just east of the town. 



