322 



NATURE 



\_Feb. 2, I 



With regard to the second point, the statement in the text of 

 the book which I questioned is as follows : — "The main principle 

 is to employ thin layers of well-packed and pressed moss in trays 

 with perforated bottoms, the eggs being separated from the moss 

 by muslin, mosquito-netting, swans' down, calico, or butter-cloth, 

 and that each tray contains two or three layers." In all the 

 methods of packing salmonoid eggs in which moss is employed, 

 the descriptions I have read state that the eggs are placed in 

 direct contact with the moss, and Mr. Day does not justify the 

 statement above quoted by referring to another statement in his 

 notes, that for shorter journeys eggs are thrown off the frames on 

 to swans' down. I doubted, and still doubt, if there is any 

 method practised in which layers of moss are used, and are 

 separated from the eggs by muslin or similar material. 



With regard to the third point, it is true that on p. 249 of his 

 book, in the chapter on 5. fontinalis, Mr. Day refers to Brown 

 Goode's " Game Fishes of the United States," and to the 

 statement in that work that S. namaycush has, as its nearest 

 relative, S. fontinalis. But I think a more direct reference to a 

 speciegraphical description of 6". namaycush might have been 

 expected in a footnote referring to errors in the descriptions of 

 this fish by certain writers. My remark about the omission of 

 such reference was not made under the impression that S. 

 namaycush was not a char, for I am aware that it is described as 

 such in recent American reports on pisciculture, and have no 

 doubt that such description is correct. But reference to specie- 

 graphical determinations are rare in such reports, and I think 

 readers of Mr. Day's book would have been glad of the references 

 which he now supplies in his letter. Your Reviewer. 



MODERN VIEWS OF ELECTRICITY} 



Part III. {continued). 



VI. 



T ET us now pass in review the various facts and 

 -*— ' experiences which have led us to a dual view of 

 electricity ; a kind of two-fluid theory, but in a very 

 modified form. 



First, there are the old experiments which vaguely 

 suggest the separate existence of negative electricity, 

 such as : — 



(i) The wind from a point whether positive or negative ; 

 so that a candle gets blown always away from it, whether 

 the point be on the prime conductor and the candle held 

 in hand, or whether the point be held in the hand and 

 presented to the candle or prime conductor ; so, also, 

 that a point whirligig turns the same way, whether 

 supported on the prime conductor, or whether attached 

 to the earth and placed near it. 



(2) Phenomena connected with the spark discharge, 

 such as Wheatstone's old experiment on what he called 

 the velocity of electricity, with the three pair of knobs ; 

 arid the double burr produced in cardboard when pierced 

 with a spark, suggesting that something has pierced it 

 both ways at once. 



Then there are the more recently observed facts ; as, 

 for instance : — 



(3) The fact that an electrostatic strain scarcely affects 

 the volume of a dielectric ; thereby at once suggesting 

 sornething of the nature of a shearing or distorting stress, 

 which alters shape but not size ; a displacement of positive 

 outwards and simultaneous negative inwards. 



(4) The facts of electrolysis, and the double procession 

 of atoms past each other in opposite directions. 



(5) The phenomena of self-induction, and the behaviour 

 of a thick wire to an alternating current. The delay also 

 in magnetizing iron, and especially the possibility of 

 permanent magnetism ; combined with 



(6) The absence of momentum in an electric current, 

 or moment of momentum in an electro-magnet, as tested 

 by all mechanical means yet tried. 



I admit at once that many of these are mere superficial 

 suggestions which may hardly bear examination and 



' Continued from p. no. 



criticism. Only (3), (4), (5), and (6) can be at all seriously 

 appealed to ; but (5) and (6), in conjunction, seem to me to 

 afford a sort of provisional and hypothetical proof, which 

 (3) greatly strengthens. 



At this point we must for the present again leave the 

 question. 



Representation of a Magnetic Field. 



The disturbance called magnetism, which we have 

 shown to be something of the nature of a spin— a rotation 

 about an axis — is conspicuously not limited to the steel 

 or iron of the magnet : it spreads out through all adjacent 

 space, and constitutes what is called the magnetic field. 

 A map of the field is afforded by the use of iron filings, 

 which cling end to end and point out the direction of the 

 force at every point. 



These lines of force so mapped are to be regarded as 

 the axes of molecular whirls. They are continuous with 

 similar lines in the substance of the steel, and every line 

 really forms a closed curve, of which a portion is in the steel 

 and a portion in the air. In a wire helix, such as Figs. 

 16 or 29, the lines are wholly in the air, but in one part 

 of their course they thread the helix, and in another part 

 they spread out more or less between its faces. 



But according to Ampere's theory of molecular currents 

 there is no essential difference between such a helix and 

 a steel magnet ; directly the currents in the molecules of 

 the magnet are considered, everything resolves itself into 

 chains of molecular currents, threading themselves along 

 a common closed curve or axis. Each atom, whether in 

 the steel or in the air, is the seat of a whirl of electricity, 

 more or less faced round so as on the average to have 

 its plane at right angles to the lines of force. The 

 simplest plan of avoiding having to consider those only 

 partially faced round, is to imagine the whole number 

 divided into a set which face accurately in the right 

 direction, and a set which look any way at perfect 

 random, and to neglect this latter set. 



Fig. 30. — A, an element of a magnetic line of force with the electric whirl 

 round it ; b, a bit of an electric circuit with one of its magnetic lines of 

 force shown round it, and the electric whirl round this ; each magnetic 

 line of force round a current being an electric vortex ring. 



Well now try and picture a chain of whirls like beads 

 spinning on a wire threading them all, and think of the 

 effect of a material fluid thus rotating. Obviously it 

 would tend to whirl itself fatter, and to shorten its length. 

 An assemblage of such parallel straight whirls would thus 

 squeeze each other laterally, or cause a lateral pressure,, 

 and would tend to drag their free ends together, causing 

 a longitudinal tension. 



Such whirls cannot in truth have free ends except at 

 the boundary of a medium — as at the free surface of a 

 liquid. Magnetic whirls are in reality all closed curves ; 



