28 



SC/£.VCE-G0SS/J>. 



COMU-rilin HV .IA\IK> ■jril.k. 



The Electric Arc. — The investigations con- 

 ducted by Mrs. Avrton, and her paper read before the 

 Institution of Electrical Engineers on March 23rd 

 last, have considerably extended our knowledge of the 

 conditions governing the electric arc. Everyone is 

 fainiliar with the hissing that takes place with an arc 

 lamp, when the current or other conditions are altered. 

 Detailed work, however, upon the manner in which 

 this hissing is brought about has not hitherto been 

 done. Among other things, Mrs. Ayrton has found 

 that when the length of the arc is constant and the 

 arc is silent, it may be made to hiss by increasing the 

 current : also when the current is constant and the 

 arc is silent, shortening the arc will make it hiss. If 

 the arc is a silent one, it is found that the difi'erence 

 of potential varies as the current, and that this varia- 

 tion is different with solid and with cored carbons. 

 On the other h.ind, with a hissing arc the difference 

 of potential is the same for a given length of arc and 

 a given pair of carbons whatever current is flowing. 

 This law also is true whether the carbons be cored or 

 solid. There thus seems to be a sudden breakdown 

 when hissing occurs. Furthermore, it is found that 

 the longer the arc, the less does the difi'erence of 

 potential beHseen the carbons diminish, when the arc 

 changes from silence to hissing. Next, considering 

 the appearance of the crater under various conditions, 

 quite a distinct difference takes place when a current 

 is reached of a certain magnitude, depending only on 

 the length of the arc with a given pair of car- 

 bons. The crater becomes partly covered with 

 what are apparently bright and dark bands in con- 

 centric circles. The directions of rotations change 

 continually, and the motion grows faster as the 

 current increases. When due to the latter cause, the 

 motion becomes too fast to be followed by the e_\e, 

 the arc begins to hum. Mr. A. 1'. Trotter in 1S94 

 made measurements upon the velocity of rotation in 

 these circumstances, and found it to varj- from 50 to 

 450 revolutions per second. At about this highest 

 speed the arc coinmences to hiss, and then the whole 

 appearance of the crater again changes. Many more 

 interesting results were brought forward by Mrs. 

 -Vyrton. The paper elicited a lengthy discussion. 



I'HOTOGRAIIIIC .VCIION WITHOUT Li<;nT.— Some 

 striking phenomena have recently been worked out 

 by Dr. W. J. Russell upon the action exerted by 

 certain metal and other bodies on a photographic plate 

 in the dark. These "active" bodies have been 

 divided into two groups— the metals magnesium, 

 cadmium, zinc, and some five or six others forming one 

 group, and the class of organic bodies known as the 

 terpencs forming the other. When any of these 

 Uidies are placed cither in contact or in close 

 proximity to the film of a plate, they exert, under 

 certain conditions, a more or less strong photographic 

 action upon it, and an image is produced by the 

 ordinaiy melhotls of developing. These results 

 should certainly Ix: welcome, especially to iihoto- 

 graphcrs, as proljably they will explain many a hitherto 

 inexplicable fogging on an unexp<.sed plate. Dr. 

 kussell's experiments go at present tf) prove that hydro- 

 gen peroxide is the active agent in the^e phenomena. 



CO.NTRIBCTEI) liY F1.0R,\ UINSTO.NE. 



La Xature (I'aris), April 29th, contains an article 

 by M. J. Poisson on the Echinocactus of Lower 

 California. The directors of the "' lardin des Plantes'" 

 of I'aris have placed for a stated time a certain 

 amount of room at the disposal of travellers, that they 

 may have opportunity for showing any objects of 

 interest obtained from foreign countries. This novel 

 exhibition was opened in February last, and contains 

 some interesting specimens in zoolog}-, ethnography, 

 chiefly of the Indian races, and botany. The 

 notes by M. Poisson relate more particularly to the 

 flora of Lower California, especially the Family Echin- 

 ocactus. Two fine illustrations are given, one in which 

 are a number of young plants, and another, giving a fine 

 specimenof £. digucti Webb. Its height is more than 

 twice that of a medium-sized man. Its trunk, how- 

 ever, never hardens into wood, and it can always be 

 pierced with ease. The flesh-like mass makes a ver)' 

 nice sweetmeat, which is sold under the name of 

 " Sweet of Visnaga," that being the colloquial term 

 for cactus. In the same number. M. Aclogue writes 

 on the metamorphoses of insects, selecting as his 

 example one of the Ephemeridac. A figure is given 

 of the insect after emergence from chr)'salis. ."Another 

 new generator for acetylene gas is noticed by M. J. F. 

 Gall. The apparatus is illustrated by three figures, 

 but though new in some of the details, it does not 

 appear to difier in general principles from those 

 already in the market. (May 6th) M. Albert Vilcoq 

 contributes an illustrated article on the gadfly. 

 This family of the Diptera are especially interesting 

 because, although they are Inveterate enemies of farm 

 animals, they are little understood by the fanners, 

 and the means employed to destroy them are often 

 inefiectual. The various divisions of the Family are 

 described, as are also the organs which they respec- 

 tively attack, and the symptoms that attend their 

 presence. A few methods of treatment are suggested, 

 but none of them appear to be of much use. There 

 is a short note by Jacques Boyers on the Telegraph and 

 Telephone in Persia. 



CoMiTES Reni)1.:s (Paris), .M.ay ist. This number 

 contains a note by M. Armand Gautier on Iodine in 

 sea-water. It is usually supposed that the sea con- 

 tains a large proportion of iodines principally as 

 ioduretted alkalis. The writer, however, in the 

 course of various experiments demonstrates that the 

 open sea does not contain, either on the surface or 

 several metres below, any trace of iodide or 

 ioduretted alkalis. M. Gautier s.ays he cannot at 

 present explain the presence of organic iodides in 

 algae, sponges, and seaweed, but he proposes to 

 carr)' on a series of experiments likely to elucidate 

 the cause. M. Stanislas Meunier notifies the 

 observation of the fall of a meteorite in Finland at 

 the end of .March. The circumstances were 

 peculiarly favourable for observation. It was seen to 

 travel along a considerable portion of the sky above 

 the littoral of the liallic Sea, falling at last not far 

 from the town of Korgo. It will be presented to the 

 collection of meteorites in the .Museum of the 

 Academy of Sciences, 1'' ranee. 



