104 



NATURE 



\ynnc 3, i8So 



vacuum is so high that the spark has difficulty in 

 passing, the penumbra (which becomes visible on insu- 

 lating the idle pole) is much wider than before, and 

 apparently eight or ten times as wide as it was at the 

 lowest exhaustion at which observations were taken. 



If the object whose shadow is cast on the screen is a 

 non-conductor (such as a piece of glass rod), its shadow 

 remains constant at all exhaustions, no penumbra being 

 visible, as it cannot be uninsulated. 



Prof. Stokes, whose suggestions throughout the course 

 of this research have been most valuable, considered that 

 much information might be gained by experimenting with 

 an apparatus constructed in the following manner : the 

 two poles of the tube (Fig. 9) are at a and b. At c is a 



.f 



1 ^^ 



fluorescent screen ; rf is a fixed bar of aluminium, and v 

 is another aluminium bar hanging from a platinum pole/, 

 by a metal chain. The bar and pendulum are on opposite 

 sides of the horizontal axis of the tube, as shown in the 

 plan, so that when properly exhausted and the pole a 

 made negative, the shadows of bar and pendulum shall 

 fall side by side on the screen, as shown in Fig. 9.\. 

 On swinging the pendulum, the shadow alternately over- 

 laps and recedes from the shadow of the bar (Figs. 91; 

 and 9c). 



This apparatus was tried many times with an induction 

 coil, and also with a Holtz machine ; but the results 

 were not sufficiently definite to render it safe to draw any 

 inference from them. By the kindness of Mr. De La Rue 

 I have lately had the opportunity of experimenting with 

 his large chloride of silver battery, and the results now 

 come out with great sharpness and with none of the 

 flickering and indecision met with when working with an 

 induction-coil. 



The tube was so adjusted that the pendulum hung free, 

 and a narrow line of molecular discharge passed between 

 the edges of the bar and the pendulum, forming a line of 

 light between the two shadows on the screen (Fig. 9A). 

 When the pendulum was set swinging, and the idle pole 

 _/connected with it was kept insulated, the regular appear- 

 ance of the moving and fixed shadows was very slightly 



interfered with. That is to say, the shadows followed the 

 successive positions between those shown in Figs. 9B 

 and 9c almost as if they had been cast by a luminous 

 point in place of the negative pole. As the shadow of 

 the swinging pendulum came very near that of the bar, 

 the latter shadow seemed to shrink away, showing that 

 the pendulum itself exerted slight rep'ulsion on the 

 molecules which passed close to its edge. 



The pendulum was again set stationary, as shown on 

 the plan (Fig 10), the line of light separating the two 



-----d' 



being atyj so that the appearance on the screen was as 

 shown at Fig. 9A. The pendulum pole was then con- 

 nected with earth, and instantly the line of light which 

 separated the poles moved from /to o through an angle, 

 measured from <•, of about 30", the shadow widening out 

 and getting indistinct at the same time. 



When the pole a was negative and positive, the bar d 

 and pendulum c were each found to be positively electri- 

 fied. The outside of the glass tube, both near the 

 negative pole and near the positive p^le, was also 

 positively electrified. 



The above experiments were tried with 6300 cells, a 

 resistance equal to 800,000 ohms being interposed. The 

 current through the tube was o'oo383 weber. These 

 measurements were taken by Mr. De La Rue, to whom I 

 am greatly indebted for permission to experiment with 

 his magnificent battery, and who himself kindly assisted 

 me in making the arrangements. William Crookes 

 (To be continued.) 



ROCK-WEATHERING, AS ILLUSTRATED IN 

 CHURCHYARDS' 



COISIPARATIVELY little has yet been done in the 

 way of precise measurement of the rate at which the 

 exposed surfaces of difterent kinds of rock are removed 

 in the processes of weathering. A few years ago some 

 experiments were instituted by Prof. Pfaft' of Erlangen 

 to obtain more definite information on this subject. He 

 exposed to ordinary atmospheric influences carefully mea- 

 sured and weighed pieces of Solenhofen limestone, syenite, 

 gr.anite (both rough and polished), and bone. At the end 

 of three years he found that the loss from the limestone 

 was equivalent to the removal of a uniform layer 0*04 mm 

 in thickness from its general surface. The stone had be- 

 come quite dull and earthy, while on parts of its surface 

 fine cracks and incipient exfoliation had appeared.- The 

 time during which the observations were continued is 

 however too brief to allow any general deductions to be 

 drawn from them as to the real average rate of disinte- 

 gration. Prof Pfatl relates that during the period a 

 severe hailstorm broke one of the plates of stone. An 

 exceptionally powerful cause of this nature might make 

 the loss during a short interval considerably greater than 

 the true average of a longer period. 



It occurred to me recently that data of at least a pro- 

 visional value might be obtained from an examination of 

 tombstones freely exposed to the air in graveyards in 

 cases where their dates remained still legible or might be 

 otherwise ascertained. I have accordingly paid attention 

 to the older burial-grounds in Edinburgh, and have 

 gathered together some facts which have perhaps suffi- 

 cient interest and novelty to be communicated to the 

 Society. 



At the outset it is of course obvious that in seeking for 



' A P.iper re,id to the Royal Society of Edinburgh, on April 19, by 

 Prnf. Geikie, F.R.S. 

 - "Allgemeine Geologic als exacte Wissenschaft," p. 317. 



