December 23, 1897] 



NA TURE 



LETTERS TO THE EDITOR 



\The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither (an he undertake 

 to return, or to correspond with the writers of rejected 

 tnamiscripis intended for this or any other part of NATURE. 

 No notice is taken of anonymous co/nmnti>canofis.'\ 



Transpiration into a Space Saturated with Water. 



For some time past^ I have been endeavouring to decide 

 whether the energy used in raising the water of the transpiration 

 current is derived solely and directly from the inflow of heat at 

 the evaporating surfaces of the leaf-cells, or whether stored 

 energy (i.e. vital force) is in any way responsible for work done. 



It has been observed that plants transpire into a space 

 saturated with moisture. But I am not aware that it has been 

 pointed out that this transpiration will continue even when no 

 light falls on the leaves. Without this precaution we cannot 

 assume that the space is really saturated at the surface of the 

 leaves ; for they will convert some of the light into heat, and so 

 lower the state of saturation at their surfaces. 



In my experiments small leafy branches were cut and set in a 

 watery solution of eosin under a glass receiver. Beside the 

 vessel containing the eosin, and under the receiver, a beaker 

 filled with boiling water was placed. The receiver became imme- 

 diately filled with water vapour, and, as the space was continually 

 falling in temperature, owing to the cooling of the beaker, it 

 remained always in a state of saturation. These arrangements 

 were made in a feeble light, and then the receiver branch and 

 all were set in total darkness. It is to be mentioned that a wet 

 board cut off" the direct radiation of the beaker from the branch. 

 After an hour it was found that the eosin had risen into the 

 leaves of the branch. In order to ensure that this rising was not 

 due to reduced air pressure, previously obtaining in the water 

 conduits of the branch, experiments were made in which any 

 reduced pressure was equalised by setting the branch for one 

 hour standing in water under the receiver, before setting it in 

 eosin. The same result was obtained in these cases. 



The raising of the eosin in this experiment seems probably 

 due to a pumping action in the cells of the leaf, depending upon 

 vital processes taking place there. This surmise is confirmed 

 by the fact that the elevation of the eosin does not take place in 

 a saturated atmosphere if the leaves have been killed. This 

 may be proved either by leaves killed by immersion in water at 

 about 90^ C, or by exposure to chloroform vapour. 



If the pumping action be a " vital " process we would expect 

 it to be dependent upon a supply of oxygen, like growth and 

 grotropic curvatures, &c. We, in fact, find this to be the case. 

 Thus a branch wholly surrounded by water will draw up eosin 

 from a vessel below, if exposed to light. The raising of the 

 eosin will be but little if the light be cut off" from the submerged 

 branch. The action of light supplies the leaves with oxygen set 

 free by assimilation ; in the dark, however, the leaves can only 

 obtain the small amount of oxygen dissolved in the water, and 

 perhaps a little, too, derived by intramolecular respiration. 

 With this limited supply the elevation of the eosin is incon- 

 spicuous. 



The oxidising processes taking place in the leaf-cells must 

 bring about some minute rise in temperature. This will, of 

 course, favour evaporation. But I think this eff"ect would be far 

 too small to account for the whole phenomenon of transpiration 

 into saturated spaces, as I have here described it. 



That a very considerable amount of the pumping action is 

 located in the leaves, may be shown by employing large leaves 

 set upright in the eosin. It will be found that in a dark 

 saturated space the veins of such large leaves severed from the 

 stem will quickly become injected with the colouring fluid. 



Henry H. Dixox. 



Trinity College, Dublin, December 14. 



The Zeeman Effect Photographed. 



In the number of Nature issued on September 2, 1897, a 

 short account is given (p. 420) of the recent work which has 

 been done in the study of " the radiation of light in the magnetic 

 field," and it is there remarked that it would be very desirable if 

 the eff'ects described by Prof. Zeeman were reproduced by pho- 

 tography. This, indeed, appeared to be all the more necessary 



1 Cf. Report of a Discussion on the Ascent of Water in Trees, Ann. of 

 Bot., December 1896. 



in consequence of the doubts expressed and entertained as to 

 whether the eff'ect was a simple broadening of the spectral lines, 

 or the production of doublets and triplets, or a combination of 

 both eff'ects. I accordingly availed myself of the opportunity 

 aff"orded me, through the courtesy of the Royal University, of 

 using for this purpose the splendid Rowland's concave grating 

 mounted in the Physical Laboratory at Earlsfort Terrace, 

 Dublin. 



After the usual amount of preliminary difficulties and failures, 

 I have finally succeeded in actually photographing all the ap- 

 pearances described by Prof. Zeeman, and I herewith enclose 

 three small negatives which show the general character of 

 the phenomena, and verify Zeeman's observations. 



I do not now propose to enter into any particulars as to 

 measurements, &c. , so I shall merely describe the photographs. 



The line represented is the violet line of cadmium which lies 

 nearest to the blue, its wave-length being 4678. Plate A is 

 taken with the slit viewing the spark across the lines of force of 

 the magnetic field. The electro-magnet being excited, we have 

 the triplet marked i. The current was then turned off, and 2 

 was taken, which shows the line in its normal condition. A 

 nicol was then inserted between the lens and the slit, and 3, 4, 

 5 were taken. Of these 4 shows the line when the magnet is 

 not excited, while 3 and 5 were taken with the magnet excited. 

 The position of the nicol in 5 was at right angles to that in 3, 

 and as in 5 the side lines of the triplet have disappeared, it is 

 proved that they are plane polarised. A faint middle line is 

 shown in 3, but in my other photographs this line of the triplet 

 is very weak, showing that it is mainly plane polarised in a 

 plane at right angles to that of the side lines of the triplet. This 

 is shown more distinctly in plate B, where 6 and 8 were taken 

 with the excited field and the nicol interposed, the position of 

 the nicol in 8 being at right angles to that in 6. The line 

 marked 7 was taken with the magnet unexcited. 



The third plate, c, was taken with the slit viewing the spark 

 along the lines of force in the usual way through an axial 

 aperture in one of the pole-pieces. In this plate the line marked 

 9 was taken with the magnet unexcited, whereas 10 was taken 

 with the field excited. The latter is a distinct doublet, and a 

 photograph which I took to-day with a quarter wave-plate and 

 a nicol interposed, shows that the lines of the doublet are 

 circularly polarised in opposite senses. 



I wish to thank Dr. W. E. Adeney, the curator of the 

 University laboratories, by whom the grating was mounted, for 

 all the trouble he took to facilitate my work, and also Prof. 

 Barrett, of the Royal College of Science, who kindly lent me 

 his electro-magnet. Thomas Preston. 



P.S. — Vou will observe that all the eff"ects described above 

 are clearly visible on the plates (which I have forwarded) by aid 

 of any ordinary magnifying glass. They lend themselves 

 admirably to lantern projection, and when thrown on a screen 

 the eff'ects may be shown to a large audience. It is to be clearly 

 understood, however, that the description above applies to this 

 particular line (it is also true for other particular lines) ; but it 

 is not implied that the same eff"ect precisely is produced in 

 every other line, either of the same or of diff"erent substances. 



I am making further observations on this latter point, and 

 hope to publish my results shortly. T. Preston. 



November 19. 



[The negative.s referred to by Mr. Preston show clearly the 

 eff"ects described, but they do not lend themselves to satisfactory 

 reproduction, even when enlarged. — Ed. Nature.] 



The Small Tortoiseshell Butterfly in December. 



I SEE in more than one daily paper of this morning's date a 

 paragraph announcing the appearance of a small tortoiseshell 

 butterfly in Highgate Police Court yesterday as something un- 

 usual. 



This butterfly is more or less common wherever nettles 

 grow freely, and there is a succession of broods throughout the 

 fine season, the last of which hibernates and reappears early 

 in spring (in mild winters in the southern counties as early as 

 February, or perhaps occasionally even in January). It is 

 easily disturbed in its winter quarters, so there is really nothing 

 surprising about its appearance now. The small tortoiseshell is 

 usually one of the latest butterflies to retire from notice in 

 autumn, and one of the first to reappear in spring. 



Chiswick, December 21. W. F. Kirby. 



NO. 1469. VOL. 57] 



