Oct. 



889] 



NATURE 



545 



near the sea-coast of Georgia (U.S.A.). In these three-days 

 cyclonic storms, before the rain begins and usually before the 

 lowest strata of air are sensibly disturbed by the upper currents, 

 the low-lying dense masses of clouds coming from the north-east 

 scud across the skies with great rapidity. Under these cir- 

 cumstances, during moonless nights, the general illumination is 

 sufficient to plainly indicate the road to the traveller, even when 

 it is bordered by tall overhanging trees. Under ordinary cir- 

 cumstances, an equivalent degree of cloudiness would have 

 compelled the traveller to abandon all attempts at guiding the 

 horse, and to rely entirely >ipon the superior acuteness of the 

 nocturnal vision of his equine companion. In such storms, the 

 ■cloudiness involves the entire firmament, and there are no 

 electrical manifestations. To the traveller, the general illumina- 

 tion apparently surpasses that of a starlight cloudless night. 



Now the question is. What is the origin and source of this 

 general nocturnal illumination on cloudy moonless nights ? No 

 degree of cloudiness seems to completely obliterate the faint 

 illumination. For, as Arago intimates, even when the heavens 

 are overcast, during moonless nights, and the stars are hidden by 

 an unbroken mass of the most dense clouds, there is a sufficiency 

 of diffiised light in the open country to prevent the difficulty and 

 inconvenience which would attend any attempt to walk in the 

 Cimmerian darkness of a cavern. 



It is a popular opinion that the clouds act like ground-glass 

 lamp-shades in diffusing the aggregate starlight, so as to produce 

 a faint illumination from all parts of the sky, and thus obliterating 

 shadows on the surface of the earth due to the greater amount of 

 light radiated from the more luminous regions of the celestial 

 vault. But Arago justly maintains that when we consider the 

 immense effect of clouds in weakening the dazzling light of the 

 sun on particular days in winter, it is scarcely possible to admit 

 that the faint diffused light, which, on a cloudy night, guides the 

 steps of the traveller, comes from the stars. In other terms, in 

 view of the loss from reflection and absorption, the amount of 

 starlight penetrating the cloud-canopy seems to be quite 

 inadequate to account for the degree of general illumination 

 observed at the surface of the earth. If we exclude the stellar 

 origin, there remains no other explanation of the nocturnal light 

 of a clouded sky, except the admission that the clouds themselves 

 Tiave a luminosity of their own. This is the view taken by 

 Arago, 



But since, for all degrees of obscuration, more or less of the 

 starlight incident upon the canopy of clouds must penetrate it, 

 and be diffused at the surface of the earth, this source of lumin- 

 osity must be looked upan as a vi:ra causa. Its adequacy to 

 explain the observed illumination in any given case will depend 

 upon the density of the overcasting cloudy masses and upon the 

 sensitiveness of the human organ of vision. Hence it seems to 

 be more rational to conclude that some portion of the nocturnal 

 luminosity of clouds may be due to the faint diffused starlight ; 

 but that, when the amount of illumination from comparatively 

 dense noctilucous clouds surpasses that of clear moonless nights, 

 we are warranted in assigning them self-luminous properties. 

 This seems to have been the condition of the low, dense, and 

 rapidly-drifting clouds observed by me during the incipient 

 stages of north-east storms on the Atlantic coast. Moreover, 

 the fact that an equal degree of cloudiness is not always attended 

 with an equal amount of illumination points to the same con- 

 clusion. In other words, it seems to be reasonable that the 

 degree of luminosity sometimes manifested in the deep win- 

 ter nights, when the whole heavens are overcast with dense 

 clouds, is vastly too great to be due to diffused stellar light, and 

 is more probably ascribable to the greater or less self-luminous 

 properties of the clouds themselves. 



In the case of isolated clouds, augmented nocturnal brightness 

 may be due to well-known local causes, independent of self- 

 luminosity. For example, the source of the brightness of the 

 clouds observed by Prof. Piazzi Smyth in 1882 and 1883, was 

 traced by him to the reflection of the gas-lights of the city of 

 Edinburgh, from water-drops in the clouds (Nature, vol. 

 xxviii. p. 239). In like manner, the bright nocturnal clouds 

 observed by Mr. T, W. Backhouse and others in 1886 

 (Nature, vol. xxxiv. pp. 239, 312, 386) were probably due 

 to bands of lofty clouds illuminated by the lingering sunlight. 



But even in cases in which tbe noctilucous condition of the 

 clouds is general, it is more than possible that the starlight illu- 

 mination may be reinforced by the prolonged twilights due to the 

 reflection of sunlight from attenuated solid particles suspended 

 in the supra-cirrus strata of the atmosphere, such as were mani- 



fested after the Krakatab eruption in the autumn of 1883. More- 

 over, in certain cases the stellar illumination may be strongly 

 augmented by cloud-obscured auroral lights. These several 

 possible sources of extra-stellar illumination of the sky during 

 cloudy nights seem almost to preclude the necessity of the 

 assumption of the existence of the condition of self-luminosity 

 of clouds under any circumstances. 



But, admitting the occasional self-luminous condition of clouds, 

 the question is. What are the physical causes of their luminosity ? 

 It is customary to refer such obscure luminous phenomena to 

 phosphorescence or to electricity. But it must be confessed 

 that, in the absence of definite knowledge of the physical causes 

 of the phosphorescence of clouds, on the one hand, or of distinct 

 electrical manifestations in such clouds, on the other, such ex- 

 planations, so far from enlightening us, would seem to be more 

 akin to illustrating the obvious by the obscure. 



Seafaring men must have had numerous opportunities of ob- 

 serving noctiucous clouds in various latitudes under every degree 

 of obscuration ; but I do not, at present, recollect any reference 

 to such observations on the ocean. In " The Voyage of II. M.S. 

 Challenger," in the " Memorandum of Meteorological Observa- 

 tions," under head " Weather," there is a record of the " visibility 

 of distant objects " ; but I have been unable to find any night- 

 observations of visibility (" Narrative," vol. ii. p. 300 et seq.). 



Berkeley, California, August 30. John Le Conte. 



ON BOSCOVICH'S THEORY.^ 



A^riTHOUT accepting Boscovich's fundamental doc- 

 '' * trine that the ultimate atoms of matter are points 

 endowed each with inertia and with mutual attractions or 

 repulsions dependent on mutual distances, and that ail 

 the properties of matter are due to equilibrium of these 

 forces, and to motions, or changes of motion produced by 

 them when they are not balanced ; we can learn some- 

 thing towards an understanding of the real molecular struc- 

 ture of matter, and of some of its thermodynamic properties, 

 by consideration of the statical and kinetic problems 

 which it suggests. Hooke's exhibition of the forms of 

 crystals by piles of globes, Naviers's and Poisson's theory 

 of the elasticity of solids, Maxwell's and Clausius's work 

 in the kinetic theory of gases, and Tait's more recent work 

 on the same subject — all developments of Boscovich's 

 theory pure and simple — amply justify this statement. 



Boscovich made it an essential in his theory that at 

 the smallest distances there is repulsion, and at greater 

 distances attraction ; ending with infinite repulsion at 

 infinitely small distance, and with attraction according to 

 Newtonian law for all distances for which this law has 

 been proved. He suggested numerous transitions from 

 attraction to repulsion, which he illustrated graphically 

 by a curve— the celebrated Boscovich curve — to explain 

 cohesion, mutual pressure between bodies in contact, 

 chemical affinity, and all possible properties of matter — ■ 

 except heat, which he regarded as a sulphureous essence 

 or virtue. It seems now wonderful that, after so clearly 

 stating his fundamental postulate which included inertia, 

 he did not see inter-molecular motion as a necessary con- 

 sequence of it, and so discover the kinetic theory of heat 

 for solids, liquids, and gases ; and that he only used his 

 inertia of the atoms to explain the known phenomena 

 of the inertia of palpable masses, or assemblages of very 

 large numbers of atoms. 



It is also wonderful how much towards explaining 

 the crystallography and elasticity of solids, and the 

 thermo-elastic properties of solids, liquids, and gases, we 

 find without assuming more than one transition from 

 attraction to repulsion. Suppose, for instance, the mutual 

 force between two atoms to be repulsive when the distance 

 between them is < Z ; zero when it is = Z ; and attractive 

 when it is > Z : and consider the equilibrium of groups 

 of atoms under these conditions. 



* Abstract by the Author of .1 communication to Section A of the British 

 Association, on Friday, September 13, at Newcastle. 



