Sept. 15, 1 881] 



NATURE 



465 



previous observations, and showed that part of the light of 

 the comet wa^ reflected sunlight and part original light ; and 

 further, that carbon was present lii the cojietary matter, with 

 strong evidence al.-.o of the prese .ce of nitrogen, in addition to 

 carbjn and hydrogen. 



The EUctric Discharge through Colza Oil, by A. Macfarlane, 

 D.Sc., F. R.S.E. — The electrical properties of colza oil which I 

 have examined are its dielectric strength and some phenomena 

 which accompany the passage of the spark. By the dielectric 

 strength of a substance I mean the ratio of the difference of 

 potential required to pass a spark through air under the same 

 conditions. The electrodes used were two parallel brass plates 

 each 4 inches in diameler. When comparing the gases the 

 standard distance of the plate chosen was 5 mm. In the case of 

 liquids it is convenient to observe for a shorter distance, and 

 reduce the result Isy the liw which previous experiments of 

 mine have established, namely, that in the case of the discharge 

 between parallel plates through a liquid dielectric the difference 

 of potential required is proportional to the distance between the 

 plates {Trails. R.S.E., vol. xxix. p. 563). One set of obser- 

 vations gave the ratio for colza oil tj be 27, another gave z"5. 

 Hence 2'6 may be taken. I have now obtained the following 

 table of dielectric strengths for liquids (l being unity). 



Substance. Dielectric Strength. 



Paraffin oil 37 



Oil of turpentine ... ... ... ... 4"o 



Paraffin liquefied ... 2'4 



Olive oil '... 3'5 



Colza oil ... ... ... ... ... 2 '6 



The specific gravity of the colza oil is '91. The passage of the 

 spark was acco.npanied by the formation of gas bubbles, but there 

 was no deposition of solid particles. As the 4-inch plates 

 were placed horizontally in the oil a bubble produced by the 

 discharge was prevented from escaping by the upper plate. 

 When the upper plate is again electrified such a bubble behaves 

 in the following manner. If it is large enough it will exiend 

 itself somewhat like an hour-glass between the plates, but if it is 

 smaller it takes the form of an acorn with a flat base, the base 

 resting on one or other of the plates. When tlie upper plate is 

 charged positively the bubble is re,ielled so as to place its base 

 on the lower plate ; when the electriciiy is charged to negative 

 the bubble remains with its base on the upper plate. A reversal 

 of the order of charging did not change the effect. After a few 

 electrifications a sufficient number of solid particles collect to 

 form a chain, and thus interferes with the pheno.nenon, the 

 bubbles then being lengthened out in a remarkable manner, 

 but never repelled to the lower plate. When the u,iper plate 

 was charged negatively, gas bubbles appeared to me to rise 

 from the lower plate, as if they had been formed there. To 

 test this point further I took some sparks between two smaller 

 disks placed vertically in the oil. The gas-bubbles were ob- 

 served to rise up at the negative surface as if they had been 

 formed at the positive surface, and had been repelled or carried 

 straight across, and then rose up at the negative surface. When 

 the spark was taken between two points bent at right angles to 

 two rods dippi ig into the oil, the bubbles were observed to 

 sho3t out in the direction from the positively charged point, and 

 to circulate round the earth-rod some ti ne before rising to the 

 .surface. These phenomena indicate that the bubble is positively 

 electrified. 



On the Electric Conductivity and Dichroic Absorption of 

 Tourmaline, by Prof. Silvanus P. Thompson. — The electric 

 conductivi'y of tourmaline differs in different directions ; being, 

 accordnig to the author's experiments, a minimum alo;)g the 

 optic axis. Tourmaline also possesses the optical property of 

 dichroism, its absorption being a miximum fjr rays parallel to 

 the axis, and greater for blue rays than for red, equal thicknesses 

 c f cryst.al being cnisMered. According to the electromagnetic 

 theory of light, bodies which are good conductors of electricity 

 should be opaque 1 1 light. The author has i.i the August nu nber 

 of the Phi'osophical Magazine rewritten the equations of Max v\ ell's 

 electromagnetic theory for the case of ci-ystalline media possessing 

 different conductivities in different directions. From these equa- 

 tions it appears that in tourmaline and negative uniaxial crystals 

 e'ectric di^placements at right angles to the axis will be more 

 absorbed than electric displacements parallel to the axis. This 

 accounts for the well-known greater absorption of the ordinary 

 ray, provided the views of Stokes and Fre>nel are correct, that 

 these displacements are at right angles to the so-called plane of 

 polari ation. The difference of velocity between rays of different 



colour ace junts for the difference of absorption being greater in 

 that direction in \\hich the conductivity is a minimum. It was 

 al.Q pjinted out that in poritive uniaxial cry.-tals, in which the 

 electric conductivity is a maximu n along tlie axis, there will be 

 maximum absorption of the extraordinary ray, and there will 

 be leabt opacity along the axis. Smoky quartz and magnesic 

 platinacyanide fulfil the latter condition. Specimens of tourma- 

 line cut into cubes to show tlie colom-s in different directions 

 were shown, and also specimens of magnesic platinocyanide and 

 of herapathite. Mechanico-optical models wereal^o shown illus- 

 trating the theory ; a tourmaline being represented by a cube 

 built up of layers of glass and wire-gauze. In conclusion it was 

 shown that crystals in which the electric conductivity differs in 

 three different directions will exhiliit trichroism ; and that di- or 

 tri-chroic absorption is a general property of all coloured ci"ystals 

 other than those of the cubical system. 



On the Amplication of Electricity to the Localisation of a Bullet 

 in a Wound, by W. H. Preece. — The author showed how an 

 electric current could be made an invi^ible and immaterial probe 

 Ijcalising the position of a bullet in the human body without 

 touching or giving the slightest sensation of pain. The concep- 

 tion of using elect icity alone as the tool occurred to Prof. 

 Graham Bell in Washington, who at once telegiaphed to the 

 author to consult him in reference to the use of Hughes' induc- 

 tion balance. In order to apply this apparatus to the localisa- 

 tion of a bullet in a wound. Prof. Hughes recommended that a 

 pair of exploring coils should be made movable and portable, 

 in order that they might be moved over the body of the wounded 

 man. If the coils were biuught within three inches of the bullet 

 its presence could be detected, tlie direction in which the bullet 

 was situated could be determined by ob erving the ])osition of 

 maximum sound, for in that po ition the bullet would be in a 

 line witli the axis of the coil. In order to ascertain the depth of 

 the bullet a similar bullet is moved along in t'le direction of the 

 axis of the other coil until neutrality is obtained ; the depth of 

 the trial bullet then w ill be equal to the depth of the buried one. 



On the General Coincidence iittoe-sn Sun spit Activity and Ter- 

 restrial Magnetic Disturbance, by the Rev. F. Howlett, F. U. A.S. 

 — The object of this piper was to inquire how far solar activity, 

 more especially as regards sun-spots, is wont to be accompanied 

 by terrestrial magnetic disturbances, as recorded by the auto- 

 mitic magnetic declination curves at Kew and Greenwich. The 

 data for such an investigation were furni-hed by comparisons 

 instituted between the most striking instances of sun-spots 

 gathered out of a long series of solar observations carried on by 

 Mr. Howlett from the year 1859 to the present epoch, and the 

 synchronous conditions of the magnetic curves at the observa- 

 tories above mentioned. The telescopic drawings of the spots 

 were obtained with an achromatic of three inches aperture by 

 Dollond, of forty-eight inches focal distance, projecting the sun's 

 image on a large white screen in a darkened chamber. By em- 

 ploying a Huygenian eyepiece magnifying 120 linear, and 

 placing the screen at the distance of five feet two inches from 

 the eyepiece, a very distinct image of the sun was obtained 

 of about five feet four inches in diame'er, and of which every 

 inch corresponded to just 30" of the celestial arc. Not only 

 were the measurements of all the solar phenomena rendered 

 thereby exceedingly easy, but the condition; of amplification, 

 illamination, and definiticn of details were combined in about 

 the be>t pis ibie manner for the observer's purpose, which was 

 to maintain an accurate record of the solar spots, and very fre- 

 quently cf the faculK aUo, on a large scale, and which have 

 been collected into five voluvues and presented to the Royal 

 Astronomical Socidy. The comparisons conmence with the 

 very remarkable and cyclonic group of August, 1S59, which was 

 uniquely distiuguis'jed by the remarkable outburst of intense 

 w hite light, far brighter than the photo phere itself, w hich for- 

 tunately was witnessed by the late Messrs. Carrington and 

 Hodgson on the forenoon of September 1, but which Mr, 

 Howlett missed seeing by only a few minute^, having completed 

 his dravin^s, and left the telescope. Other striking and, if 

 they may be so termed, crucial groups were compared with the 

 magnetic records — very notably the great spot of October, 1865, 

 engravings of which may be found in the volume of the Pro- 

 ceedings of the R'lyal Astronomical Society for the year last 

 mentioned, as alo the large groups of February, 1870, which 

 were observed and drawn on the occa ion of the recurrences by 

 rev<:ilution of the same groups in the three consecutive months of 

 February, March, and Apri' of that year, and on the last of 

 which months the total disp'acement, at one and the same time. 



