364 



NATURE 



\_Aitgii.st 19, 1880 



In this new earthquake springs are said to have burst out on 

 the side of Mount Sipylus. Hyde Clarke 



32, St. George's Square, S.W., August 9 



New Biological Term 



In writing certain parts of a book on water-beetles, I find my- 

 self frequently desirous of indicating briefly but emphatically 

 that some particular genus I may be mentioning consists of only 

 a single species. If we talvc a rational or theoretical view of 

 classification rather than an empirical one, it must be admitted 

 that a genus consisting of only one species is almost as great an 

 anomaly as a species that should consist of a single individual ; 

 and a special term to indicate the fact would be desirable. Mr. 

 Pascoe has suggested to me that tlie expression " monotypical 

 genus " meets the want ; but I am not satisfied with this, for in 

 the first place it is a phrase, not a word ; and in the second place 

 the use of the " typical " interferes with concentration of thought 

 by the introduction of an alien suggestion. I therefore propose 

 to use either the word "autogenus" or the word "mouogenus" 

 for the purpose, and on the whole prefer the former. Perhaps 

 some one else may be able to suggest a better term, and I shall 

 be very glad of an expression of opinion on the point. 



Thornhill, Dumfriesshire D. Sharp 



Depraved Taste in Animals 



Your correspondent, Mr. Nicols, draws attention this week 

 to what he terms the "depraved taste" for tobacco exhibited 

 by several individuals of that species of Phalangistidfe known 

 as the koala. 



Whilst in Australia some years ago I myself remarked the 

 same propensity amongst numerous luild specimens of the 

 J-harcolarctos cincrcics, in an abandoned tobacco-clearing not far 

 from my residence, and, like Mr. Nicols, I also observed that 

 no ill effects seemed to follow the consumption of the tobacco 

 by tlie Koaloe. Now since the Phalangistidx I had the oppor- 

 tunity of observing were perfectly wild, I cannot agree with Mr. 

 Nicols that their taste for tobacco is a depraved one, although 

 the desire for spirits which he mentions is of course decidedly 

 unnatural. 



These observations induced me to make several analyses of 

 the Victorian tobacco, with the result of isolating an hitherto 

 undiscovered vegetable allcaloid. A detailed account of my 

 various experiments is contained in a paper read by me before 

 the Melbourne Medical and Chemical Society, and printed in 

 the fourteenth volume of the Society's Transactions. 



F. R. Greenwood 



St. Bartholomew's Hospital, E.C., August 14 



Firing a Tallow Candle through a Deal Board 

 Will the wiiter of " Pliysics without Apparatus " be good 

 enough to specify the conditions of success for the above 

 experiment ? C. J. WOODWARD 



Birmingham and Midland Institute, August 9 



[Set up a J-inch or |-iiich plank of deal in the ground. It 

 should be 6-8 inches wide. Ram small charge of gunpowder 

 into gun with wad. Select a dip candle just fitting bore ; cut 

 down to about 5 inches long, with flat end. Be very particular 

 to ram it down well ; for if there is air space between it and the 

 wad there is risk of bursting gun. Take care that the rest of barrel 

 IS cleared of bits of tallow. Fire at say 3 yards from plank. 

 If you don't miss aim, there Hill be a hole lorn, about 2 inches in 

 diameter.— The Writer of "Physics without Apparatus."] 



V - I must send his name and address. 



THUNDERSTORMS ^ 

 II. 

 gEFORE I can go farther with this subject it is neces- 

 sary that I should give some simple facts and 

 Illustrations connected with ordinary machine electricity. 

 Ihese will enable you to follow easily the slightly more 



difficult steps in this part of our subject which remain to 

 be taken. 



Since we are dealing mainly with motion of electricity, it 

 is necessary to consider to what that motion is due. You 

 all know that winds, i.e. motions of the air, are due to dif- 

 ferences of pressure. If the pressure were everywhere the 

 same at the same level we should have no winds. Similarly 

 the cause of the motion of heat in a body is difference of 

 temperature. When all parts of a body are at the same 

 temperature there is no change of distribution of heat. 

 Now electricity presents a precisely analogous case. It 

 moves in consequence of difference of potential. Poten- 

 tial, in fact, plays, with regard to electricity, a part pre- 

 cisely analogous to the role of pressure, or of temperature, 

 in the case of motions of fluids and of conducted heat. 

 Now the power of an electrical machine may be measured 

 by the utmost potential it can give to a conductor. The 

 greater the capacity of the conductor the longer time will 

 be required for the machine to charge it : but no elec- 

 tricity passes between two conductors charged to the 

 same potential. Hence the power of a machine is to be 

 measured by using the simplest form of conductor, a 

 spliere, and finding the utmost potential the machine can 

 give it. It is easily shown that the potential of a solitary 

 spliere is directly as the quantity of electricity, and 

 inversely as the radius. Hence electricity is in equi- 

 librium on two spheres connected by a long thin wire 

 when the quantities of electricity on them are proportional 

 — not to their surfaces, nor to their volumes, as you might 

 imagine — to their radii. In other words, the capacity is 

 proportional to the radius. This, however, is only true 

 when there are no other conductors within a finite 

 distance. When a sphere is surrounded by another con- 

 centric sphere, which is kept in metallic connection with 

 the ground, its capacity is notably increased, and when 

 the radii of the spheres are nearly equal the capacity of 

 the inner one is directly as its surface, and inversely as 

 the distance between the two spheres. Thus the capacity 

 is increased in the ratio of the radius of one sphere to the 

 difference of the radii of the two, and tliis ratio may 

 easily be made very large. This is the principle upon 

 which the Leyden jar depends. 



It is found that the work required to put in a charge is 

 proportional to the square of the charge. Conversely, 

 the damage which can be done by the discharge, being 

 equal to the work required to produce the charge, is pro- 

 portional to the square of the charge, and inversely to the 

 capacity of the receiver. Or, what comes to the same 

 thing, it is proportional to the square of the potential and 

 to the capacity of the conductor directly. Thus a given 

 quantity of electricity gives a greater shock the smaller 

 the capacity of the conductor which contains it. And 

 two conductors, charged to the same potential, give 

 shocks proportional to their capacities. But in every 

 case, a doubling of the charge, or a doubling of the 

 potential, in any conductor, produces a fourfold shock. 



The only other point I need notice is the nature of the 

 distribution of electricity on a conductor. I say on a 

 conductor, because it is entirely confined to the surface. 

 Its attractions or repulsions in various directions exactly 

 balance one another at every point in the substance of the 

 conductor. It is a most remarkable fact that this is 

 always possible, and in every case in one way only. 

 When the conductor is a single sphere the distribution is 

 uniform. When it is elongated the quantity of electricity 

 per square inch of its surface is greater at the ends than 

 in the middle ; and this disproprotion is greater the greater 

 is the ratio of the length to the transverse diameter. 

 Hence on a very elongated body, terminating in a point, 

 for instance, the electric density — that is, the quantity 

 per square inch of surface — may be exceedingly great at the 

 point while small everywhere e!se. Now in proportion to 

 the square of the electric density is the outward pressure 

 of the e'ectricity tending to escape by forcing a passage 



