28 



NATURE 



{May r r, 1876 



mind will tend to prevent the student from forming any clear and 

 distinct conception of the phenomena. 



Let us now examine how far M. Volpicelli's experimental skill 

 and extensive reading have enabled him to give an accurate 

 account of the phenomena, and how far he may have fallen into 

 error from not availing himself of the idea of electric poten- 

 tial, but continuing to employ that of latent electricity. 



Melloni, in his exposition, has represented the homonymous 

 electrification (j8) as greater on the side of the induced body 

 further from the inductor. The fact, however, is that the elec- 

 trification is distributed in the same way as it would be if the 

 inductor were in its actual position and insulated, but without 

 charge. It will therefore be densest on the projecting parts of 

 the induced body ; but if the two extremities ol this body are 

 geometrically similar, and if the inductor is made of a conducting 

 substance, it will be somewhat denser on the extremity (ib) next 

 the inductor, because the surface of the inductor itself (c) will 

 become electrified, and the electricity on the side next to b will 

 be negative. 



But the inequality of the distribution of the negative electrifi- 

 cation (a) is so much greater that it completely masks that of 

 (;3), so that from an experimental point of view we must regard 

 this error of Melloni as a very tiifling one. 



The next point we must notice is the mode in which objection 

 (3) is expressed. It is as follows: — 



"(3) Because of the two kinds of electricity which coexist 

 upon the induced insulated body, only the homonym of the in- 

 ductor is dissipated by contact with the air," (The italics are our 

 own.) 



We have no evidence whatever that electricity is ever dissi- 

 pated by contact with air, whether dry or moist, unless the 

 electric density is so great that a disruptive discharge takes place 

 in the forms of " glow," " brush," or " spark," from sharp points 

 connected with the electrified body. 



If the electrified body and the surrounding conductors have 

 rounded surfaces, and if the potential is moderate, it appears 

 from the experiments of Boltzmann^ that no measureable quantity 

 of electricity passes through air or other gases, even when 

 greatly rarefied, and when the experiment is continued for 

 fourteen hours. 



I have myself been unable to detect any conduction through a 

 stratum of still air of two millimetres thickness, even when the 

 temperature was raised to a red heat, and when steam, or the 

 vapour of mercury or of sodium was introduced between the 

 oppositely elecirihtd surfaces. If, however, smoky air was intro- 

 duced, there was a considerable effect arising from convection by 

 the solid particles. 



The cause of the powerful electrical effects of the stream of 

 heated matter rising from a Bunstn's burner or Irom a red-hot ball, 

 as in Guthrie's experiments, requires a special investigation. 



The dissipation of the charge of insulated bodies which we 

 actually observe seems to depend principally on the insulating 

 supports on which they are placed, and if these are of good 

 glass the conduction is almost entirely due to moisture on the 

 surface of the glass. If the air which is in contact with the glass 

 insulator is perfectly dry the dissipation of electricity will be ex- 

 tremely small, even when the air in contact with the electrified 

 body itself is loaded with moisture. 



It is not, therelore, by contact with the air that the electricity 

 escapes, but by conduction to the earth along the so-called insu- 

 lating supports, and the effect of this conduction is of course to 

 reduce the potential to zero by discharging electricity of the same 

 kind with that of the inductor. 



We come next to the fourth of the five facts mentioned under 

 the head of the First Experiment. It is stated as follows : — 



" 4. Points applied to the extremity of the cylinder nearest to 

 the inductor allow only the homonym of the inductor to escape, 

 and not at all the opposite electricity." 



This will be the case it the point is electrically connected with 

 the earth, and made to apj, roach any part of the surface of the 

 cylinder ; but if, as the words seem rather to imply, the point 

 is attached to the cylinder and projects into the air, then the 

 statement is exactly opposite to that given by Riess in Art. 247 

 of his book, who corrtciiy tells us that if the cylinder has a sharp 

 point at one end, then if the point is turned towards the in- 

 ductor, the cylinder becomes charged simdarly to the inductor, 

 whereas if the point is turned away from the inductor, the cylinder 

 becomes charged oppositely to the inductor, the discharge from 



;i Sitzb. der k. Akad. (Wien), April 23, 1874. .- ' 



the point being always of that kind of electricity which exists 

 on the part of the cylinder where the point is placed. 



The fifth fact stated to be established by the experiment is — 



"5. Induced electricity of the first kind (opposite to that of 

 the inductor) is not transferred from the induced body to the in- 

 ductor, but the electricity of the inductor may certainly be trans- 

 ferred to the induced body," 



For the sake of distinctness, let us say that the inductor is 

 positive, then it is here asserted that negative electricity does not 

 pass from the cylinder to the inductor, but that positive electri- 

 city passes from the inductor to the cylinder. 



If M. Volpicelli can give us an experimental method of dis- 

 tinguishing between the passage of negative electricity from B 

 to A, and the passage of positive electricity from A to B, we 

 may expect to learn more of the nature of electricity than any 

 of our physicists have hitherto even hoped for. 



J. Clerk Maxwell 



Cherry Blossoms 



In the last number of Nature (vol. xiv., p. 10), Mr. 

 Pryor states that the flowers of the wild cherry aie bitten off in 

 large numbers in much the same manner as I formerly described 

 in the case of the primrose. Some days ago I observed many 

 cherry blossoms in this state, and to-day I saw some actually 

 falling. I approached stealthily so as to discover what bird was 

 at work, and behold it was a squirrel. There could be no doubt 

 about it for the squirrel was low in the tree and actually had 

 a blossom between its teeth. It is none the less true that birds 

 likewise bite the flowers of the cherry tree. 



Down, Beckenham, May 6 Charles Darwin 



The Pollen of the Cherry 



The practice of the indefinite reproduction of woodcuts by means 

 of clichh has frequently given rise to the repetition of erroneous 

 drawings in one scientific text-book after another. Botanical 

 text-books seem to have suffered especially in this way, in con- 

 sequence of the great dearth of new and original illustrations 

 by which they are characterised. Many botanical students must 

 have been puzzled by the peculiar appearance presented by the 

 pollen of the cherry in a very familiar drawing. It is hardly 

 sufficiently explained that "the escape of the fovilla in an 

 irregular jet," as there represented, has nothing to do with the 

 process of fertilisation, but is an altogether abnormal pheno- 

 menon depending on the bursting of the pollen-grain from arti- 

 ficial moistening. The shape of the pollen-grain, as drawn, for 

 example, in Balfour's "Class-book of Botany," Le Maout and 

 Decaisne's "General System of Botany," and Dr. Hooker's 

 Science Primer " Botany " ^ is also incorrectly indicated. The 

 perfectly spherical form represented in these drawings is almost, 

 if not altogether, confined to anemophilous plants, fertilised by 

 the wind. The cherry is, on the contrary, entomophilous, and 

 its pollen partakecs of the general character of this class of plants. 



Though somewhat variable in size and form, the grains are, I 

 believe, never spherical, but ellipsoidal, with three longitudinal 

 furrows, as represented in the longitudinal and apical aspects, 

 a, b, in the accompanying figure. The pollen has, however, 

 well-marked characters of its own, which distinguish it from that 

 of allied plants, the ends often appearing truncated, as repre- 

 sented in c, and some or all of the grains more gibbous on one 

 face than another (d). Most poilen-grains assuma a more 

 spherical form on being moistened with water, 



Alfred W, Bennett 



I In Hooker's Primtr there is the further complication of the accidental 

 transposition of the figures of the cherry and evening-prinitose, the well- 

 known triangular form of the latter being attributed to the lormer. 



