234 



SCIENCE. 



[Vol. XIII. No. 321 



Arizona, which were exhibited by Professor Edward S. Morse and 

 Sylvester Baxter during the recent Congress of Americanists at 

 Berlin. This discovery confirms the American origin of the bean. 

 The plsLUt ca.\\ed ^/lase/os, /aseo/us, etc., in antiquity, is, according 

 to Kornicke, Dolichos chinensis, or a variety of the species D. 

 melanophihabnos. Mr. Wittmarck has found also seeds of the 

 pumpkin in ancient Peruvian burials, and concludes that the pump- 

 kin is originally an American plant. The so-called pumpkins of 

 the Bible are, according to Ascherson and Magnus, melons {Cti- 

 cutnzs Chate L), and so are those represented on ancient Egyptian 

 paintings. On the other hand. Gray and Trumbull have proved 

 that before the arrival of the Europeans, pumpkins were used as 

 far north as northern New York. 



ELECTRICAL NEWS. 

 Electrical Lines of Force. 



This subject was brought before the members of the Royal In- 

 stitution, London, some years ago by Mr. Gordon ; and recently a 

 lecture was delivered on the same subject at the institution by 

 Professor A. W. Riicker, an abstract of which appears in Nature 

 of March 7. In the interval a considerable amount of work has 

 been done upon it, both in England and Germany, and many ex- 

 periments have been devised to illustrate it. Some of the more 

 striking of these, though of great interest to the student, are rarely 

 or never shown in courses of experimental lectures. The lecturer 

 and Mr. C. V. Boys, F.R.S., last year devised a set of apparatus 

 which has made the optical demonstration of electrical stress com- 

 paratively easy, and most of the results obtained by Kerr and 

 Quincke can now be demonstrated to audiences of a considerable 

 size. Before discussing this portion of his subject, the lecturer in- 

 troduced it by an explanation of principles on which the experi- 

 ments are founded. 



Magnetic lines of force can easily be mapped out by iron filings, 

 but the exhibition of electrical lines of force in a liquid is a more 

 complex matter. In the first place, if two oppositely electrified 

 bodies are introduced into a liquid which is a fairly good non-con- 

 ductor, convective conduction is set up. Streams of electrified 

 liquid pass from the one to the other. The highly refracting liquid 

 phenyl thiocarbamide appears to be specially suitable for experi- 

 ments on this subject. If an electrified point is brought over the 

 surface, a dimple is formed, which becomes deeper as the point 

 approaches it. At the instant at which the needle touches the 

 Uquid the dimple disappears, but a bubble of air from the lower 

 end frequently remains imprisoned in the vortex caused by the 

 downward rush of the electrified liquid from the point. It oscil- 

 lates a short distance below the point, and indicates clearly the 

 rapid motions which are produced in the fluid in its neighborhood. 

 When the needle is withdrawn, a small column of liquid adheres 

 to it. This effect is, however, seen to greater advantage if a small 

 sphere about five millimetres in diameter is used instead of the 

 needle-point. When this is withdrawn, a column of liquid about 

 five millimetres high and two millimetres in diameter is formed be- 

 tween the sphere and the surface. A similar experiment was made 

 by Faraday on a much larger scale with oil of turpentine ; and he 

 detected the existence of currents, which are in accord with the 

 view that the unelectrified liquid flows up the exterior of the cylin- 

 der, becomes electrified by contact, and is repelled down its axis. 

 In view of this explanation, and the movements assumed can be 

 ■clearly seen in the phenyl thiocarbamide, the performance of the 

 ■experiment on a small scale is not without interest. The possi- 

 bility of the formation of such violent up-and-down currents in so 

 small a space must depend upon a very nice adjustment between 

 the properties of the hquid and the forces in play. It is obvious 

 that such movements of the liquid must be a disturbing element in 

 any attempt to make the lines of electric force visible. 



Again : if a solid powder be suspended in a liquid into which 

 electrified solids are introduced, it tends to accumulate round one 

 of the poles. This subject has been investigated by W. Holtz. 

 Sometimes the powder appears to move in a direction opposed to 

 that in which the liquid is streaming. Sometimes two powders 

 will travel towards different poles. 



If powdered antimony sulphide be placed in ether, it settles at 

 the bottom of the liquid ; and if either two wires insulated with 

 glass up to their points, or two vertical plates, be used as elec- 

 trodes, on exciting them slightly the solid particles arrange them- 

 selves along the lines of force. If the electrification be increased, 

 they cluster round the positive pole. On suddenly reversing the 

 electrification by means of a commutator, they stream along lines 

 of force to the pole from which they were previously repelled. 

 Other methods of obtaining the lines of force have been devised. 

 They can, for instance, be shown by crystals of sulphate of quinine 

 immersed in turpentine. 



The tendency of the lines of force to separate one from the other 

 was illustrated by Quincke's experiment. A bubble.of air is formed 

 in bisulphide of carbon between two horizontal plates. It is in 

 connection with a small manometer, and when the plates are op- 

 positely excited, the electrical pressure acting at right angles to the 

 lines of force, being greater in the liquid than in air, compels the 

 bubble to contract. 



Kerr's experiments depend upon the fact, that, since the electri- 

 cal stress is a tension along the lines of force, and a pressure at 

 right angles to them, a substance in which such a stress is pro- 

 duced assumes a semi-crystalline condition in the sense that its 

 properties along, and perpendicular to, the lines of force are differ- 

 ent. Light is therefore transmitted with different velocities, ac- 

 cording as the direction of vibrations coincides with, or is perpen- 

 dicular to, these lines ; and the familiar phenomena of the passage 

 of polarized light through crystals may be imitated by an electri- 

 cally stressed liquid. 



The bisulphide of carbon used must be dry, and, to make the 

 phenomena clearly visible, it is necessary that the light should 

 travel through a considerable thickness. Thus, to represent the 

 stress between two spheres, elongated parallel cylinders should be 

 used, the axes of which are parallel to the course of the rays of 

 light. These appear on the screen as two dark circles. Between 

 crossed Nicols, the planes of polarization of which are inclined at 

 forty-five degrees to the horizontal, the field is dark until the 

 cylinders are electrified, when light is restored in the space between 

 them. 



If parallel plates with carefully rounded edges, and about two 

 millimetres apart, are used, the colors of Newton's rings appear in 

 turn, the red of the third order being sometimes reached. If one 

 plate is convex towards the other, the colors of the higher orders 

 appear in the middle, and travel outwards as the stress is increased. 

 The experiments may be varied by using two concentric cylinders, 

 or two sheets of metal bent twice at right angles to represent a 

 section through a Leyden jar. In the first case a black cross is 

 formed ; and in the second, black brushes unite the lower angles 

 of the images of the edges of the plates. By the interposition of a 

 piece of selenite, which shows the blue of the second order, two of 

 the quadrants contained between the arms of the cross become 

 green, and the others red. In like manner the horizontal and 

 vertical spaces between the intier and outer coatings of the " jar" 

 become differently colored. 



There are several phenomena connected with the stress in in- 

 sulators which present considerable difficulties. Thus in a solid it 

 is found impossible to restore the light between crossed Nicols by 

 a uniform electrical field. That the non-uniformity of the field has 

 nothing to do with the phenomenon in liquids, though at first dis- 

 puted, is now generally admitted. It may be readily proved by 

 means of a Franklin's pane, of which half is pierced into windows. 

 The glow is much weakened by thus removing part of the uniform 

 field, though it is thus made much less uniform. 



Again ; though most dielectrics, when placed in an electric field, 

 expand, the fatty oils contract. Professor J. J. Thomson has re- 

 cently pointed out that this indicates that another set of strains are 

 superposed upon those assumed in the ordinary explanations of 

 these phenomena, and by which they may be neutralized or over- 

 come. 



In experiments with carbon bisulphide it is necessary to take 

 ever}' precaution against fire. For this purpose the cell which 

 contains the liquid should be immersed in a larger cell ; so that if, 

 as sometimes happens, the passage of a spark cracks the glass, the 

 liquid may flow into a confined space. This should stand in a tray 



