June 3, 1909] 



NA TURE 



39: 



is quite obvious tliat llie air rL-quired fur ventilation 

 would not only have to be supplied in huge quantities 

 bv special fans, but also that it would have to be 

 carried away in closed ducts to the outside of the 

 engine-room. 



When dealing with gas engines for alternating- 

 current generators the author falls into a strange 

 error. He mentions as one of the drawbacks that 

 power is lost through the damping coils which the 

 irregular motion of a gas engine renders necessary. 

 Now it is well known that damping coils must not be 

 used in such cases. His remarks on slot insulation, 

 on which subject he is an authority, are highly in- 

 teresting; he believes that eventually it will be possible 

 to reduce this to something liks 2 mm. for a 10,000- 

 v'olt machine, but unfortunately he does not say in 

 what manner this improvement is to be achieved. He 

 is evidently an advocate of severe testing, and the 

 subject of insulation tests gives him the opportunity 

 of a homily on the ethics of the inspecting engineer. 

 His suggestion that the sufficiency of the mechanical 

 support of the winding should be tested by short- 

 circuiting at full excitation the terminals of, say, a 

 5000-kw. alternator sounds rather heroic, and his 

 anticipation that not more than six times normal 

 current would flow at the instant of closing the switch 

 may be doubted, although he is quite right in saving 

 that, a moment after, the current would only be about 

 three times the normal value. 



The chapter on the design of the central station as 

 a whole is particularly interesting and useful. Here 

 we find an enormous mass of information collected 

 from a variety of stations and tabulated in a con- 

 venient form. The same may be said of the chapter 

 on transmission plant. The author gives us, not onlv 

 technical details, but also the cost from actual 

 e.xperience, and one cannot but admire the industry 

 with which he has collected so much really valuable 

 information. As regards electric traction, his sym- 

 pathies are all for the direct-current system, for which 

 he predicts a rise of working pressure up to some- 

 thing like 1200 volts. The single-phase system he 

 condemns entirely, but as regards the three-phase he 

 admits a slight superiority in the matter of weight 

 over the direct-current .system. As the limits of the 

 power of motors at the ordinary one-hour rating he 

 takes 150 h.p. for the single-phase, 300 h.p. for the 

 continuous, and 400 h.p. for the three-phase system. 

 The three-phase system is a little lighter, and the 

 single-phase system more than twice as heavy as the 

 continuous-current system. .A new and very simple 

 formula for the tractive resistance in kg. per ton of 

 train is given on p. 231. It is as follows: — 



R = 270 +-0-09 



W 



for railways in the open, and 

 R = 3 + o-3 



for tube railways. V is the speed in km. per hour, 

 and W is the weight of the train in tons. 



GiSBERT Kapp. 

 NO. 2066, VOL. 80] 



\VH\ LEAVES ARE GREEN. 

 Zur Biolngie des Chlorophylls, Laiibjarbe iind Him- 

 niclslicht, Vergilbiing und Etiolement. By Ernst 

 Stahl. Pp. V+153. (Jena: Gustav Fischer, 1909.) 

 Price 4 marks. 



IN this interesting and suggestive book. Prof. Stahl 

 presents us with the results of his observations 

 and speculations upon the ever-interesting problems 

 of the biology of chlorophyll and its related colouring 

 matters. One of the most interesting of these is the 

 cause of ihe prevailing green colour of our vegeta- 

 tion. How does it arise that the various photo- 

 synthetic organs of plants are green, and not some 

 other colour? 



Engelmann has already shown that the colours of 

 the algal vegetation of the sea are complementary to 

 the light which falls upon them, and Gaidukov has 

 made experiments to show that the Cyanophyceae, or 

 blue-green algse, undergo a change in colour com- 

 plementary to the light which falls upon them, when 

 grown under different coloured lights. Prof. Stahl 

 thinks that these observations may lead to an 

 explanation of the green colour of land plants. The 

 chlorophyll spectrum may be regarded as a combina- 

 tion of two absorption spectra. The absorption at the 

 blue end of the spectrum agrees very nearly with that 

 of etiolin and the colouring matter of yellow leaves, 

 whilst the absorption in the red corresponds to that of 

 the green colouring matter which is formed when 

 etiolated plants are exposed to light, and dis- 

 appears in the autumn, when the leaves again turn 

 vellow. The yellow-green colour of the leaf may, 

 therefore, be an adaptation to the prevailing colour of 

 the diffuse light which falls upon it, the yellow being 

 complementary to the blue of the heavens, and the 

 green to the orange and red which mostly prevail when 

 the sun is low. 



The region of least absorption in the chlorophyll 

 corresponds with that of maximum energy in the 

 spectrum. The plant does not, therefore, depend 

 for its assimilative work upon the rays of greatest 

 energy. On the other hand, the possibility of using 

 these rays is shown by the red alga, which absorb the 

 green as well as the blue, the maximum of their 

 assimilative activity lying exactly in the green. 



The author tries to show that the non-absorption of 

 the green rays is not only due to the fact that the 

 chlorophyll makes no use of those rays which usually 

 reach it in a weakened form, but also to the fact that 

 the absorption of these rays in direct sunlight would 

 be. dangerous to the plant, because of their great 

 heating power. Under normal conditions an in- 

 tense illumination is unnecessary. The amount 

 •of energy used up by the chlorophyll grain in 

 carbohydrate assimilation is only a small part 

 of the total energy it absorbs. In light of 

 lower intensities, however, it is clear that the 

 amount of energy absorbed by the plant becomes more 

 nearly proportional to the amount used for assimila- 

 tion, and thus a complementary colour adaptation to 

 the light is understandable. In the red and brown 

 seaweeds, the blue-green algae, &c., the absorption of 



