43° 



NATURE 



\_Manh 4, 1886 



capable of acting the part of independent grains whose only 

 property was to keep their shape. This was not inconsistent 

 with dilatancy in ether, for these physical properties were pos- 

 sessed by the molecules of matter in consequence of the presence 

 of the ether, and hence it was not logical that the atoms of ether 

 should possess these properties. 



If evidence of dilatancy were to be obtained frorn tangible 

 matter, it was to be sought on the most commonplace, and what 

 had hitherto been the least interesting, form, that of hard, 

 separate grains— corn, sand, shot, &c. 



That an important geometrical and mechanical property of a 

 material system should have lain hid for thousands of years, even 

 in sand and corn, was such a striking thought that it required no 

 small faith in mechanical principles to undertake the search for 

 it ; and, though finding nothing but what was in accordance 

 with previous conclusions, the evidence obtained of this long- 

 hidden property was as much a matter of surprise to the lecturer 

 as it could be to any of the audience. 



To render the dilatancy of a mass of grains evident, it was 

 necessary to accomplish two things : (l) the outside grains must 

 be controlled so that they could not rearrange, and this without 

 preventing change of shape or change of bulk ; (2) it was neces- 

 sary to adopt means of measuring the change of bulk or volume 

 of the mass or of the interstices between the grains as its shape 

 was changed. A very simple means — a thin india-rubber bag — 

 was found to answer both these purposes to perfection. 'Ihe 

 outside grains indented themselves into the india-rubber, which 

 prevented their changing their places, while the impervious 

 character of the bag allowed of a continuous measure of the 

 volume of its contents by measuring the quantity of air or water 

 necessary to fill the interstices. 



In these experiments neither the bag nor the fluid had anything 

 to do with the dilatancy of the contents considered as forming 

 part of a continuous medium, the bag merely controlling the 

 outside members as they would be controlled by the sur- 

 rounding grains, and the fluid merely measuring or limiting tlie 

 volume. 



India-rubber football cases were then .shown full of dry sand, 

 shot, corn, and glass marbles, shaken down into their densest 

 form. The bags could not be distorted, as by squeezing between 

 two plates, without enlarging the interstices between the grains, 

 and hence the volume of the bag. Such increase of bulk was 

 not, owing to the change of shape, evident to the eye ; but by 

 connecting the mouth of the bag to a pressure-gauge, it appeared 

 as the squeezing began, the pressure of the air within the inter- 

 stices began to diminish, and as the squeezing went on dimi- 

 nished as much as 6 inches of mercury, which showed that the 

 interstices had increased a third. These experiments were 

 introduced mainly to prevent the impression that the character 

 of the fluid within the interstices had anything to do with 

 dilatancy. Water affords a more definite measure of volume 

 than air. This was shown. A bag holding six pints of sand 

 full of water without air, connected by a tube with the bottom of 

 a vessel of water, drew, on being squeezed, about a pint of water 

 frnm the vessel into the bag. This was the maximum dilation ; 

 for further squeezing the water ran back into the vessel, and 

 then again, for still further squeezing, was drawn back again, 

 showing that, as the change of form proceeded, the medium 

 passed through maxinum and m.inimum dilations. 



The most striking evidence of dilatancy is obtained from the 

 fact that, since dilatant material cannot change its shape without 

 increasing in volume, by preventing change of volume all change 

 of shape is prevented. By closing the communication between 

 the bag and the vessel of water, and thus preventing further 

 increase of volume, further change of shape was instantly pre- 

 vented. Starting with the sand at its densest, and the com- 

 munication closed, a pinch of 200 lbs. was put on the planes 

 without producing the smallest apparent change in the spherical 

 shape of the bag. 



Communication with the pressure-gauge was then opened, 

 which showed that, so far from the water in the bag being at a 

 greater pressure than the atmosphere, it was less by 20 inches of 

 mercury, so that a little more pressure on the planes and a 

 vacuum would have been formed. On opening communication 

 with the water the bag instantly responded by change of shape, 

 and again instantly stopped when the supply was cut off. 



That the thickness of the envelope was of no importance so 

 long as it was impervious to air, was shown by using india-rubber 

 ba 00ns, so thin that the sand could be seen through them ; 

 one of these, which was soft and yielding when the water was in 



excess, became hard like a cannon-ball when' the excess of water 

 was drawn off, maintaining any shape it had when the bag was 

 closed, supporting 200 lbs. 



In this way a cast was taken from a mould, into which the 

 bag was shaken with water in excess till it took the form of the 

 mould ; the excess of water was then drawn olif, and the mould 

 removed, leaving an image which preserved its shape loaded 

 with 200 lbs. 



The firmness and softness of the sand by the sea was shown 

 to be due to these causes ; as the tide falls it leaves the sand 

 apparently dry, but in reality full of water, the surface of which 

 is kept up to the surface of the fine sand by capillary attraction. 

 This saturated sand cannot yield to the tread without dilating, 

 and cannot dilate until it has had time to draw more water, the 

 first effect of the foot being to draw down the capillary surface, 

 leaving the sand apparently dry round the foot. This_was 

 shown by experiment. 



The lecturer then indicated how the property of dilatancyjn a 

 continuous medium would render it capable of causing an 

 attraction between bodies at a distance, like gravitation, and 

 cohesion, and elastic forces between bodies close together ; 

 how the ability of the grains to rearrange at a free surface would 

 allow bodies to move freely in the medium which, if in a state of 

 agitation by transverse waves in all directions, would transmit 

 waves like those of light, but not like sound, and which if con- 

 sisting of grains of two different sizes or shape, would give rise 

 to phenomena resembling those of electricity. 



In conclusion, it was remarked that, promising as this dilatant 

 hypothesis of ether was, it could not be taken as proved until it 

 had been worked out in detail. This would take long, and in 

 the meantime it was put forward to add interest to the property 

 of dilatancy, to the discovery of which it had led. The property 

 of dilatancy once recognised was, however, independent of any 

 hypothesis, and seemed to have opened up a new field for 

 philosopliical and mathematical research quite independent of 

 the ether. 



SOCIETIES AND ACADEMIES 

 London 

 Royal Microscopical Society, February 10. — The Presi- 

 sident. Rev. Dr. Dallinger, F. R. S. , in the chair. — The President 

 referred to the loss sustained by the death of Mr. P. H. Lealand, 

 to whom microscopists were so largely indebted for the optica 

 productions which were so well known and appreciated. — The 

 Report of the Council was read and adopted. — Dr. Dallinger 

 then gave his annual address, in wdiich he detailed the results of 

 his later researches into the life-history of minute septic organ- 

 isms as can-ied on by means of the improved lenses constructed 

 for him by Messrs. Powell and Lealand. Four forms weie 

 selected for study. Each of these septic organisms terminate a 

 long series of fissions with what is practically a generative act 

 of fusion. The two last of a long chain of self-divided forms fuse 

 into one, become quite still, and at length the investing sac 

 bursts, and a countless host of germs is poured forth. The 

 growth of these germs into forms like the parent was continu- 

 ously watched, showing gradual enlargement, and ultimate, but 

 as to time somewhat uncertain, appearance of the nucleus, 

 and the somewhat sudden appearance of the flagella or thread- 

 like motor organs, the latter being found in each instance to 

 arise in the nucleus. Very soon after the adult stage is reached 

 the act of self-division commences, and is kept up for hours 

 in succession. The delicate plexus-like structure becomes 

 aggregated at one end of the nucleus, leaving the rest perfectly 

 clear, except that a faint beading is seen in the middle line, 

 with two or three finer thre.ads from it to the plexus. Then 

 occurs the commencement of partition of the nucleus, followed 

 by a slight indication of division of the body-substance. 

 Quickly afterwards the nucleus becomes completely cleft, and 

 the body-substance follows suit. Then the iilexus-like condition 

 is again diffused equally over the whole nucleus. When the 

 generative condition is approached by the last generation of a 

 long series of dividing forms, it is remarkable that the organism 

 becomes amceboid, showing how far-reaching is the amceboid 

 state. In this condition, when two such forms touch one another 

 they coalesce and fuse into each other almost as though two 

 globules of mercury had touched, until nucleus reaches nucleus 

 and the two melt into one, and the blended bodies become a 

 globular sac, which ultimately emits an enormous number of 

 germs. Previous to the blending it is now made out that all 



