May 31, 1900] 



NATURE 



105 



Krown so appropriately terms the " reduced constituent of 

 chlorophyll." My persistent endeavours resulted in the dis- 

 covery of protophylline, a substance obtainable through the 

 action of nascent hydrogen on chlorophyll solutions.^ Some 

 years later I discovered this substance in the living plant. ^ 



The existence of a reduced consiitU'Ut of chlorophyll may be 

 consequently considered as a perfectly established fact, and will 

 be probably brought to account by the chemical theory of the 

 chlorophyll function. I conclude my French paper with the 

 following words : — " L'etude de ces substances ne manquera 

 pas a jetter une vive lumiere sur le co'^ chimiqtte de la fonction 

 chlorophillienne qui a die ^ludU dans ce dernier temps presqiie 

 excltisivement an point de vne physique" 



To sum up : though it may be clearly seen that for nearly 

 thirty years I have been considering chlorophyll as a chemical 

 sensitiser (or, strictly speaking, an adsorbent of the products of 

 dissociation of COj and W.f)), still even now I must confess 

 that this theory lacks direct experimental proof and may be con- 

 sidered only as a matter for further research, whereas the 

 physical aspect of ihe question {i.e. that COg and H^O are de- 

 composed through the agency of those rays of the spectrum, 

 which are absorbed and somehow transformed by chlorophyll) 

 is but the expression of a fact, put beyond any doubt by my re- 

 searches, both on the decomposition of COo and on the produc- 

 tion of starch in the living plant. ^ But I do not abandon the 

 hope that the discovery of the protophylline may turn out some 

 day to be a step in the direction of a chemical theory of the 

 chlorophyll function, somewhat similar to that of the colouring 

 matter of the blood — an analogy which has been present to my 

 mind ever since I became acquainted with the classical researches 

 of Sir G. G. Stokes in that direction. 



University, Moscow. Ci.kment Timiriazeff. 



I REGRET that M. Timiriazeff should regard the concluding 

 lines of my presidential address as doing him some injustice. 



No one can be more impressed than I have been with the 

 extreme beauty and importance of M. Timiriazeff's work, which 

 cleared away many illusions, and for the first time prominently 

 brought out tiie fact that the rays corresponding to the principal 

 absorption band of the chlorophyll spectrum are those which 

 are mainly active in the assimilatory process. 



I have always regarded M. Timiriazeff's paper of 1885 (Ann. 

 des Sciences Nat. [Bot.], vol. ii. p. 99) as being one of the most 

 convincing and eloquent expositions in scientific literature, and 

 the final proof of the proposition there laid down was given by 

 the author in 1890 [Compt. rend, no, 1346), when he succeeded 

 in showing that the reappearance of starch in a depleted leaf 

 exposed to a pure spectrum only takes place in the region of 

 the red corresponding exactly to the principal absorption l)and 

 of chlorophyll. 



With regard to the first }X)int rai.sed in M. Timiriazeff s letter, 

 I may say that when preparing my address I experienced a diffi- 

 culty in ascertaining who it was that first drew attention to 

 the existing analogy between chlorophyll and a chromatic 

 sensitiser. 



There is no complete list of Sir William Abney's papers, and 

 knowing that he has sent many communications on this and 

 cognate subjects to photographic journals in various parts of 

 the world, I applied to Sir William Abney before writing what I 

 did. There can be no doubt that chromatic .sensitisers were 

 very much "in the air " immediately after Vogel's discoveries of 

 1873, and it is probable that the application of these new ideas 

 to chlorophyll occurred independently to Abney, Timiriazeff 

 and Becquerel. 



M. Timiriazeff's second objection is that I have not sufficiently 

 taken into account his views of the function of chlorophyll as a 

 chemical sensitiser. On this point I may say that I had in 

 view his paper of 1885 : " Etat actuel de nos connaissances sur 

 la fonction chlorophyllienne," which it was fair to imagine fully 

 embodied the author's view up to that date. It is certainly the 

 physical role of chlorophyll which is there insisted upon, as the 

 following quotation indicates : " Le role de la chlorophylle dans 

 le phenomene de la decomposition de I'acide carbonique peut 

 done etre resume ainsi : elleabsorbe les radiations qui possedent 

 J The first description of this curious substance was given in two short 

 notes communicated to these columns: "Colourless Chlorophyll" 

 (Nature, 1885, p. 342) and "Chlorophyll" (Nature, 1886, p. 52). For 

 more ample details, see Coinfiies rendns, 1889. 



o '., 'r* Pi^ot'-phylline dans la plante vivante " (Comftes rendiis, 1889). 



" Enregistiement photographique de la fonction chlorophyllienne 

 par la plante vivante " {Comptes rendus, 1884). 



NO 1596. VOL. 62] 



la plus grande energie et transmet cette energie aux molecules 

 de I'acide carbonique qui, a elles seules, n'eprouveraient pas 

 de decomposition, etant transparentes pour ces radiations 

 energiques." 



That the physical conception was certainly uppermost in M. 

 Timiriazeff's mind at that time is further shown by the diagram 

 and remarks immediately following, in which he regards the 

 molecules of carbon dioxide as suffering "shipwreck" in the 

 luminous undulations corresponding to maximum amplitude. 



It is, however, quite clear from M. Timiriazeff's references to 

 his paper of 1877, and especially to his Russian paper of 1871,. 

 neither of which I have seen, that he has expressed views which 

 are practically identical with those contained in the concluding 

 remarks of my address. It is to be regretted that these ideas 

 were not again clearly brought forward in the 1885 paper, which 

 purported to give the author's latest views on the whole ques- 

 tion, and that the physical idea of the immediate transference of 

 the energy of radiation was there made the dominant one. 



52, Nevern Square, Kensington, Horace T. Brown. 



A Simple Experiment on Thermal Radiation. 



The following experiment, which has been successfully per- 

 formed by our students for several years, may be of interest to 

 teachers of physics. 



Three chemical thermometers are chosen of equal size and 

 shape. The bulb of one is silvered, of the other covered with 

 dead black paint by dipping it into a mixture of lamp-black and 

 alcohol, whilst the third is left unchanged. For silvering, any 

 of the well-known solutions and processes will be applicable. 

 The thermometers indicate the same temperature if there is no 

 source of radiation near them. 



But if a gas flame, for example, an Argand burner, be placed 

 at a distance of 20 centimetres from them, so that the thermo- 

 meters, hanging from a stand, are at equal distances from the 

 flame. the|temperature rises at a different rate, and to a different, 

 though in each thermometer constant, height. The silvered 



thermometer gives the lowest reading, and the blackened the 

 highest, whilst the thread of the uncovered one stops at some 

 point between these readings nearer to that of the blackened 

 than the silvered ; for the different surfaces of the thermometers 

 absorb the radiation of heat generated in the flame in different 

 proportion. The blackened thermometer bulb almost com- 

 pletely absorbs the rays falling on it ; the silvered and polished 

 bulb reflects the radiation reaching it ; the plain glass bulb partly 

 reflects and partly absorbs the rays. Thus, none but the 

 silvered bulb thermometer indicates the temperature of the air 

 communicating heat to it by conduction. As the other thermo- 

 meters rise in temperature, they emit radiation ; and when the 

 amount of heat emitted from them equals the amount received 

 through radiation from the gas flame, they are in the final 

 stationary state, which is, of course, reached by the thermo- 

 meters at different temperatures. 



If the gas flame is put out, the temperatures of the three 



