t6o 



NATURE 



[June i8, 189] 



disulphide, and this low surface energy may be an 

 electrical effect. 



I pointed out that the development of all the faces of 

 a form, and the similar modification of all corresponding 

 «dges and angles of a crystal, is in general necessary in 

 order to produce equilibrium under the surface-tensions. 

 But we sometimes find crystals with only half the modifi- 

 cations required for symmetry. In such cases the surface- 

 tensions must produce a stress in the interior tending to 

 deform the molecules. When the crystal was growing, 

 there must have been equilibrium, and therefore a pres- 

 sure equal and opposite to this effect of the surface- 

 tension. There are various ways in which we may 

 suppose that such a force would arise. The electric field 

 might give rise to a stress in opposition to the aggregation 

 of the molecules in the closest possible way, and then 

 the crystal would grow such faces as would produce an 

 equal and opposite stress. Inequalities of temperature, or 

 the presence of molecules of other kinds amongst those 

 of the crystal, might produce similar results. When the 

 stress due to electricity, or to temperature, was removed by 

 change of circumstances, that due to the surface-tensions 

 would persist, and the crystal would be left with an 

 internal strain. Crystals of this sort, with unsymmetric 

 faces, generally betray the internal strain, either by 

 developing electricity of opposite kinds at the two ends 

 when heated or cooled, or they affect polarized light, 

 rotating the plane of polarization. That these effects 

 are due to the internal strain is shown by the fact that 

 iiourmalines, and other crystals, which are pyro electric 

 when unsymmetrical, show no such property when sym- 

 metrically grown. Also sodium chlorate in solution, 

 quartz when fused, and so on, lose their rotatory power. 

 Substances which in solution show rotatory power, as a 

 rule develop unsymmetric crystals. This is well seen in 

 the tartrates. The constitution of the molecules must be 

 such that they will not, without some strain, form crystals ; 

 and equilibrium, when the crystal is growing, is attained 

 by means of the opposing stress due to want of symmetry 

 in the surface-tensions. In all such crystals the rotatory 

 power of the solution disappears in whole or in part. We 

 cannot test this in biaxial crystals, but, according to Des 

 Cloiseaux, sulphate of strychnine is the only substance 

 which shows rotation both in the solution and in the crys- 

 talline form, and in it the rotatory power is much increased 

 by the crystallization. Effects comparable with these 

 may be produced by mechanical means. A cube of rock 

 salt, which has no effect on plane-polarized light in its 

 •ordinary state, changes the plane of polarization when it 

 is compressed in a vice. And a cleavage slice of prussiate 

 -of potash, which is uniaxial, may by compression be dis- 

 torted so as to give in a convergent beam of polarized 

 light elliptical rings, and two eyes like a biaxial crystal. 



THE ERUPTION OF VESUVIUS OF JUNE 7, 

 1891. 



TOURING the latter part of 1890 and the early part of 

 ■*-^ the present year, the central activity of Vesuvius has 

 -very slightly varied, except about the new year, when it 

 was considerably increased, rising to the third or fourth 

 degree, simultaneous with the stoppage of the lateral 

 outflow of lava that had been going on since August 7, 

 1890. Since then, up to the present outburst, the central 

 .activity has been generally at the first degree, and the 

 •cone of eruption has slowly grown in height. 



On June i there was a crater within the central erup- 

 tive cone, of about 50 m. in diameter, near the centre of 

 Avhich was the eruptive vent, surrounded by another em- 

 bryonic eruptive cone. On that day, four small eruptive 

 mouths opened around the embryonic cone in the bottom 

 <of the central crater, the smallest being to the east. 



Thus the volcano remained till June 7, at 10 a.m., when 

 NO. I I 29, VOL. 44] 



activity stopped, only a small quantity of vapour escaping 

 from central vents. At midday a radial cleft opened at 

 the north toe of the cone of eruption (May 1889, June 

 1891) traversing towards its east end, the little sickle- 

 shaped ridge, the remnant of the 1 885-86 crater, but, as yet, 

 gave out little vapour. At 4 to 4.30 p.m., shocks of earth- 

 quake commenced, limited only to the upper slopes of 

 Vesuvius, and simultaneous with the extension of the 

 radial fissure down the side of the great Vesuvian cone 

 for nearly half its way opposite the Punta del Nasone of 

 Monte Somma, from which, at about 5.30 p.m., issued a 

 little lava, whilst from the upper extremity of the fissure 

 at the toe of the cone of eruption much vapour escaped, 

 so that from Naples the smoke-plume arose from this 

 point. From 5.30 to 7 p.m. the fissure still extended 

 lower, accompanied from time to time by local earth- 

 quakes, noises, and the elevation of columns of black 

 dusty smoke. At a few minutes to 7 the floor of the 

 Atrio del Cavallo was reached, and a remarkably black 

 column of smoke had arisen. 



My friend Dr. L. Sambon saw this column arise, and 

 came to inform me immediately, as I had left off watching 

 the mountain at 5.30. After taking a photo of the moun- 

 tain, we left Naples at 9 p.m., spent some time in in- 

 quiries at Resina and near the Observatory. Everything 

 was now dark, as the volcano had calmed down at 

 8 p.m. At 2 a.m., June 8, we were at the eastern ex- 

 tremity of the Observatory ridge, and commenced to 

 wend our way across the lava surface towards Monte 

 Somma. We were at the lowest part of the depression 

 at the west end of the Atrio del Cavallo, where it joins 

 the Fossa della Vetrana, and along which some of 

 the largest lava-streams have flowed (1855, 1872, &c.}, 

 when suddenly on our right above us (2.23 a.m.) a vast 

 quantity of bright red vapour arose from the new 

 outpour of lava. We hastened our steps as much as 

 the road and our lantern would allow us, so as to 

 reach the escarpment of Monte Somma, the foot of which 

 was followed till near the Punta del Nasone, and close to 

 the theatre of eruption. Here we clambered up some 

 distance above the level of the Atrio to watch events 

 whilst we ate our late supper or early breakfast. Along 

 the slope of the great cone in the line of fissure were a 

 few luminous points from some pieces of still uncooled 

 lava of the little that had oozed forth from the lower half 

 of the fissure. At about 60 or 80 yards from the foot of 

 the great cone two or three fountains of lava were throw- 

 ing up jets of molten rock for 2 or 3 m., and the lava 

 was slowly spreading out on the almost horizontal plain 

 of the Atrio in several tongues. The lava must have still 

 been high in the main chimney, as the vapour that issued 

 at the top of the fissure showed a slightly red illumina- 

 tion. So we remained till daylight, when we could see 

 the fissure on the side of the cone. The mouth that 

 formed at 5.30 the previous day was still smoking a httle, 

 whilst the fissure below it sent off several ramifications at 

 an acute angle like the branches of an inverted tree, from 

 several of which little streams of lava had been given out, 

 where they had soon consolidated. We now followed the 

 base of the great cone to the lower railway station, where 

 we found all the people up and dressed, frightened by the 

 strong shock and noises at 2.23 a.m., coincident with the 

 fresh outflow of lava that we had witnessed, but which 

 shocks we had not felt, although they were described as 

 the strongest that had been felt. 



Having ascended to the summit of Vesuvius, we found 

 the central crater rapidly enlarging by the falling in of its 

 edges. From the new fissure at its summit was issuing 

 much vapour under pressure, and rich in sulphurous 

 acid, which is, even in traces, intolerable ; and the hot air 

 coming from innumerable new fissures rendered approach 

 very difficult. We did, in fact, once jump across part of 

 the fissure, but returned much quicker on account of the 

 hot irritant vapours. An approach from the opposite 



