278 



NATURE 



[Feb. 3, 1876 



them ; the scale is again read, and the length of spark obtained 

 by the difference between the first and second reading. 



Rod-chloride, 600 cells had a striking-distance of 0*0033 in. 

 ,, I2CO ,, ,, 00130 



„ 1800 ,, „ 0-0345 



2400 ,, „ 0053s 



Taking as the unit 600 cells, the spark of which was o*oo33, 

 the length of spark of 1,200, 1,800, 2,400 would, according to 

 theory, be that number multiplied by the squre of 2. 3, 4 

 respectively. 



6co cells, striking-distance 0-0033 in. 



1200 ,, 0*0033 X 4... 00132 



1800 ,, 0*0033 X 9.... 00297 



2400 ,, 00033 X 16.... 0*0528 



which numbers agree nearly with those obtained by experiment. 



The length of the spark is much influenced by the shape of 

 the terminals, those which we frequently employ consist of a point 

 as one teiminal and a plane for the other ; hitherto we have used 

 copper terminals, making the point and the plane alternately 

 positive and negative by means of a double-key discharger, or 

 by a rapidly-revolving commutator reversing up to 352 times in 

 a second. One terminal in the above determinations consisted 

 of a point of 30°, and the other of a slightly convex surface 0*46 

 inch in diameter. 



While making these measurements, we noticed in a nearly 

 dark room that when the point was negative a glow, in form like 

 a paraboloid, was seen surrounding it long before the spark 

 passed, and its appearance afforded by its increasing brilliancy 

 useful information to guide us as to the more cautious approach 

 of the terminals ; gradually the sugar-loaf-like glow extended to 

 the positive terminal. With 1,800 cells the glow was seen when 

 the terminals were 0*0545 inch apart, the spark passing at 

 0*0345 ii^ch ; with 2,400 cells the glow began at a distance of 

 0*0865 inch, the spark passmg at 0*0535 inch. Moreover, it was 

 noticed that the disc (positive) became covered all over with a 

 peach-like bloom, which became stronger in the centre as the 

 terminals were made to approach each other, giving rise to 

 Newton's iridescent rings. 



In order to study more readily the phenomena accompanying 

 the glow preceding the spark, the whole series of 5,640 cells was 

 used, the terminals being a point as before of 30°, and sometimes 

 a flat disc 1*1 inch in diameter, or a slightly convex one of o*8 

 inch in diameter for the other. In all cases a peach-like bloom 

 deposited on the disc, which was connected with the silver 

 (positive) terminal ; and when the flat disc was used the deposit 

 Avas notably greater at the periphery and the centre than in other 

 portions of it. With this number of cells, with the flat disc the 

 glow occurred at 1*073 i"-» the spark at 0*139 in. With the 

 slightly convex disc occurred at i •124 in., the spark at 0*140 in. * 



To ascertain whether a current really passed when the glow 

 appeared, various vacuum-tubes were interposed in circuit. be- 

 tween the battery and one of the terminals ; in all cases they 

 were illuminated even before a glow was perceptible on the 

 negative pole ; their interposition, as was to be expected, 

 shortened the spark and diminished the distance at which the 

 glow was perceptible. For example, with a hydrogen tube, 

 having a capillary portion between two larger tubes, such as is 

 used for spectrum experiments, and offering a resistance of 

 190,000 ohms, the glow occurred at 0*939 inch, the spark at 

 0*092 inch. 



A tube of 31 inches between the terminals, and offering a 

 resistance of 350,000 ohms, was brilliantly illuminated when 

 interposed between one terminal and the battery; when the 

 terminals were separated the extreme range of the discharge 

 was I *2 inch, and before any glow was visible at the negative 

 electrode. How much further between the electrodes it will be 

 possible to obtain a current has yet to be determined with a 

 larger discharger now in course of construction. + 



I have alluded to the resistance offered by vacuum-tubes. At 

 first I experienced considerable difRculty in measuring it. For 

 example, when in a Wheatstone's bridge the resistance of the 

 tube was balanced by inserted resistances, the galvanometer 

 could only for a short time be brought to rest, and it was then 

 found that the cause of this was that the tubes rapidly increased 

 in resistance as the current passed. After a time, however, they 



* Postscript, Jan. 7.— At the suggestion of Prof. Stokes, who saw the 

 experiment repeated, the point was made positive, when a longer spark 

 was obtained, namely, o'i54 inch and o"i64 inch. 



+ Postscript, Jan. 8th.— A current was obtained with the negative point 

 distant 5'i inches from a positive plate 6 inches in diameter. 



recovered their original resistance, sometimes rapidly, sometimes 

 only after the lapse of days. The resistances were found not 

 to be dependent on the length of tube, but to a great extent on 

 their bore, capillary tubes offering a considerable resistance. 

 Ultimately it was found that it was better to discard the indica- 

 tions of the galvanometer, and to rely solely on the appearance 

 of a luminosity in the tubes placed on one side of Wheatstone's 

 bridge as soon as the insertion of a balancing resistance was 

 made in the other. 



Later on we hope to have the honour of sending to the Society 

 a more detailed statement of our experiments in support of those 

 now quoted, and in confirmation of our former paper on the 

 cause of stratification in electric discharges in vacuo. 



In conclusion we venture to draw attention to the following 

 consequences of the law of the length of spark being dependent 

 on the ratio of the square of the number of cells of a voltaic 

 battery, in the event of its being confirmed by experiment. 

 Taking as a basis the spark with 600 cells of the rod-chloride of 

 silver battery — 00033 inch, a unit of looo such cells would 



1 r 0033 X 1000- . /■/-•, 



give a spark of — -^ = o 009100 inch, 



600* 

 one hundred units (100,000) a spark of 91*66 inches, 

 a thousand units (1,000,000) ,, 91*66 ,, =764 feet 



nearly, 



whereas a single cell would have a striking-distance of 17x7^^x7017 

 of an inch only. As far as our own experiments have gone the 

 law has been confirmed ; and although a million cells will pro- 

 bably never be made, a hundred thousand come within the range 

 of experimental possibility. 



Geological Society, Jan. 19.— John Evans, president, in 

 the chair. — ^James Buckingham Bevington, William P. Blake, 

 James Gordon Brickenden, Edward George Dyke, Henry 

 Hamilton Gunn, William Jerome Harrison, and R. G. Warton, 

 were elected Fellows of the Society. — On some unicellular algas 

 parasitic within Silurian and Tertiary corals, with a notice of their 

 presence in Calceola sandalina and other fossils, by Prof. P. 

 Martin Duncan, F.R.S. After noticing the works of Quekett, 

 Rose, Wedl, and Kolliker, which refer to the existence of 

 minute parasitic borings in recent corals, recent shells, and a few 

 fossil moUusca, the author describes the appearance presented by 

 a great system of branching canals of about 0*003 millim. la 

 diameter, in a Thamnastrsean from the Lower Cainozoic of Tas- 

 mania. He then proceeds to examine the corresponding tubes 

 in GoniophyllufH pyramidale from the Upper Silurian formation. 

 In sections of that coral one set of tubes runs far into the hard 

 structure ; these are straight, cylindrical, and contain the remains 

 of vegetable matter. Neither these tubes, nor any others of the 

 same parasite, have a proper wall : they are simply excavations, 

 the filiform alga replacing the organic and calcareous matter 

 abstracted. In some places the dark carbonaceous matter is 

 absent, and the lumen of the tube is distinguishable by the ready 

 passage of transmitted light. Other tubes run parallel to the 

 wall, and enter by openings not larger than their common calibre. 

 But there are others which have a larger diameter, and in which 

 the cytioplasm appears to have collected, in masses resembling 

 conidia ; and where fossilisation has destroyed much of the 

 continuity of a tube a series of dark and more or less spherical 

 bodies may be seen. In some places, especially in the spaces 

 between the minute curved dissepiments and tabulae, hosts of 

 globular spores, with or without tubes emanating from them, 

 may be seen. In Calceola sandalina corresponding structures exist 

 sometimes, and the method of entry of the parasite can be 

 examined. The author gave two instances, one of which was 

 seen in section. A decided flask-shaped cavity existed in the 

 wall of the shell, opening outwards and rounded and closed in- 

 wards. It was crowded with globular spores (oospores), and 

 these, where near the sides, had penetrated the hard shell, and 

 thus gave a rugged and hairy appearance to the outline of the 

 flask-shaped cavity. After noticing minute structures in a 

 brachiopod included in a Silurian coral, and in a Lower Silurian 

 foraminifer, the author asserted, from the results of his late re- 

 searches upon the algae parasitic in corals out of his own 

 aquarium, that the fossil and recent forms are analogous in 

 shape, size, and distribution. He considers that the old para- 

 site resembles Saprolegnia ferox in its habit ; and as he considers 

 that Empusina, Saprolegnia, and Achlya — members of the 

 Protista — are the same organisms, living under different physical 

 conditions, he names the old form Palaachlya penetrans ; and 

 he believes that it entered the wall by the spores fixing on to the 

 organic matter, and growing by its assimilation, and that car- 



