7^ 



NATCH. 



J\cv. 25, iSi.o 



of momentum we have loss of visible kinetic energy, except when 

 the coefficient of restitution — i. This l^inetic energy is trans- 

 formed into the vibrational kinetic energy of sound and heat in 

 general. 



But cannot we have it partly transformei into potential energy 

 by "soaring" against gravity? On thi, suppoition we have 

 the two laws, conservation of momentum where no forces act, 

 and conservation of energy, holding. But we have visible kinetic 

 energy lost and partly traiti^onued into fottntiat energy -i'ith 

 respect to the caith, partly (as usual) into vibrational kinetic 

 energy of sound and heat. [The j iiiid is evident in the ' ' singing ' ' 

 of the wings.] 



It seems to me that the swooping referred to by your corre- 

 spondent is only a matter of convenience to the bird, and does 

 not really affect the mechanical question ; and that the compari- 

 son to a kite (which is held by a string) is not very satisfactory. 

 But from my own observation of sea-gulls I do not think one can 

 say that all the manosuvres and turns of the bird in the air are 

 .performed without real muscular effort, though cerfainly without 

 flaps of the wing ; and if tliere be muscular effort there can be 

 work done — against gravity in this case. 



The above is only a suggestion. I wish to induce some more 

 mathematical re.tder to write a clear answer on this interesting 

 question. W. Larde.n' 



Cheltenham, November S 



The Photophone 



On reading the description published in IvATURE, vol. xxiii. 

 p. 15, of Prof. Graham Hell's wonderful discovery, the trans- 

 mission of speech by light, I notice that in "the photophone" 

 the varying of the intensity of the beam of light thrown on the 

 receiving instrument is accompli-hed by the simple and ingenious 

 means of allowing the soundwaves to beat on the back of a 

 thin plane mirror. It seems to me, however, that this arrange- 

 ment is not complete, and is open to some objection. As the 

 plane mirror will, if provision be not made against it, become 

 convex and concave alternately, it must, unless the vibrations be 

 confined within very narrow limit", give in one vibration /tci? 

 periods of maximum and minimum illumination at the receiver, 

 and therefore the received sound-, apparently, should be (as- 

 suming the periods between each maximum and minimum 

 illumination to be of tlie same duration, which cjuld never 

 exactly occur) an octave higher than those transmitted. This 1 

 think follows from the fact that the rays from the mirror would 

 be dispersed not only when convex, but nl,o when concave, aflci- 

 they had passed the focus. If, therefore, the vibrations of the 

 mirror are sufficiently great to bring its focus between the mirror 

 and the receiving instrument, there would be a second point of 

 minimum illumination. If however the mirror were mide 

 slightly convex, or were constrained by a spring or otherwise, 

 this delect woukl be cured. 



Curiously enough, tlicoretically "the photophone" is the 

 more effective the greater the distance between the transmitter 

 and the receiver, as the degree of variation of the intensity of 

 light falling on the selenium will be, when perfectly adjusted, 

 greater as the distance increa-es, and it is on this element that 

 the intensity of tlie sound depends. A. R. MOLISON" 



Ffynone Club, Swansea, November 15 



[Cur correspondent is obviously right in supposing that with a 

 beam of light focussed accurately upon the selenium receiver a 

 single complete vibration of the transmitting disk would produce 

 t7vo periods of maximum and minimum illumination. This would 

 not however be the case if the lenses were not set originally to 

 exact focus, for then a displacement of the disk in one direction 

 would scatter the rays more, while a displacement in the other 

 would concentrate them more. In practice, we believe, exact 

 focussing Is never obtained or even attempted. — Ed.] 



Salts of Zinc 



In Roscoe and Schorlemmer, vol. 11. p. 264, it states: "The 

 salts of zinc do not Impart to the non-luminous flame any tint ;" 

 and on p. 258, "the metal burns with a bright white flame." 



What then is the green colour imparted to the Bunsen flame 

 by zinc sulphate due to? Also the green flame obtained by 

 heating metallic zinc on charcoal before the blowpipe ? S. 



The green tint referred to by " S." (supra) as imparted by zinc 

 sulphate to the Bunsen flame is only observed whilst the water of 



cr5stalli-ation contained in the salt is being given off; the dry 

 salt which remains imparts no colour to the flame. It therefore 

 appears probable that the green colouration of the flame is 

 caused by very finely divided particles of the salt being carried 

 ofi into the upper part of the flame by the escaphig water of 

 crystallisation. These particles then become so intensely heated 

 as to emit the peculiar greenish light and very likely suffer pre- 

 vious reduction by the carbon of the flame. Other zinc salts, 

 especially the acetate, impart to the flame, when first heated, a 

 greenish-blue tint resembling that observed when met.allic zinc 

 is burnt in the air, this being.doubtlesslydue to a partial reduction 

 of the acetate. The characteristic zinc lines (6362 and 6099 in 

 the red, and 492S, 4924, and 4911 in the blue) are not seen in 

 the case of the salts or when the metal is burnt. A more correct 

 description of the c:imbustion of zinc than that referred to would 

 be ; "the metal burns with a bluish-white flame." 



Chemical Laboratory, Owens College W. BoTT 



THE IVORKS OF CARL VON NAGELI 



'X'HE beginning of the forties in the present century 

 -■■ marks an important epoih in the history of botany. 

 The " Naturphilosophie" which had for many years 

 so banefully influenced the development of the science, 

 was being routed by the energetic attacks of -Schleiden. 

 Botanists were becoming alive to the fact that if their 

 study was to have a place as a science by the side of 

 physics and of chemistry, it must be pursued by the in- 

 ductive method ; that speculation must give way to re- 

 search, and, above all, that development must be studied 

 before any conclusions could be drawn from the investi- 

 gation of mature forms. The early discoveries of von 

 Mohl, and the demonstration of the cellular structure of 

 the tissues by Schleiden, were among the first fruits of 

 this awakening. To this period belongs also Nageli's 

 first contribution to science — a paper on the Development 

 of the Pollen (1842). The first sentence in the introduc- 

 tion shows how thoroughly Nageli was imbued with the 

 same spirit which possessed Schleiden. He says : — " The 

 right knowledge of an object includes an acquaintance 

 with its mature form and a study of its development : the 

 one is dependent upon the other, and the one without the 

 other is insufficient to afford a complete conception of the 

 object." The .actual observations detailed in the paper 

 appear from the drawings to have been accurate, and they 

 were an important addition to the knowledge of the sub- 

 ject ; but their interpretation was so far influenced by 

 .Schleiden's theory of cell-formation, which was then pre- 

 valent, that the process of the development of the pollen 

 grains is described as being one of free cell-formation. 



In the year 1844 appeared the first number of the 

 Zeitschriftfiir wissctischaftlichc Botanik, edited — probably 

 on account of the sympathy existing between them — by 

 Schleiden and Niigeli. This short-lived periodical (1S44 

 to 1846) was practically an organ for the publication 

 of Nageli's researches and for the expression of his 

 views, for it does not contain a single contribution from 

 Schleiden's pen. The first number opens with an article 

 — a sort of confession of scientific faith — " On the Present 

 Aims of Natural History, and especially of Botany," in 

 which he gives an account of the actual state of botanical 

 knowledge, and strongly urges the necessity of empirical 

 study in order that the generalisations of the science 

 might be in the future, not baseless speculations, but in- 

 ductions resting upon a firm foundation of ascertained 

 fact. The Ztitsc/irifl further contains an important paper 

 " On the Nuclei of Cells and the Formation and Growth 

 of Cells," in which the process of free cell-formation, 

 which Schleiden had asserted to be universal, is shown to 

 be only one of the processes by wdiich a multiplication of 

 cells is effected ; these processes are clearly defined and 

 classified. This is followed by a number of researches on 

 the morphology of the lower cryptogams, which are of 

 interest inasmuch as they open up new lines of approach 

 to the study of the complicated morphology of more highly 



