June 15, 191 1] 



NATURE 



517 



same way as I imagine the dust of the flame to be de- 

 posited on the plates, namely, by difference of tempera- 

 ture, our rooms being warm and windows cold at night. 

 The only difference is in the rate of deposit, due to there 

 being less dust in the room air and to the smaller differ- 

 ence in temperature in tlie case of the window. Cleaning 

 a small part of a window which had been clean ten days 

 previously, a piece of ice was rubbed over the outer sur- 

 face until dew began to deposit on the inside. The deposit 

 on the ten-day-old surface was different from the newly 

 cleaned one, but not greatly so. While the newly cleaned 

 part was covered with the usual little lens-like discs, the 

 older surface was covered with much larger and irregularly 

 shaped blotches of film. As the nights had not been cold 

 since the window was cleaned, we can hardly expect much 

 dust to have collected in ten days ; so another window was 

 tried which had not been cleaned for some months. 

 Repeating the cleaning and cooling on this window, it was 

 found that while the newly cleaned part carried a ground- 

 glass-like deposit, the uncleaned part was sufficiently clear 

 for the landscape to be seen through it. These tests show 

 that dust on glass does tend to cause water condensed on 

 its surface to spread and take the film form in the same 

 way as glass that has been exposed to flame or to hot gases. 



Coming, now, to Quincke's experiment, referred to by 

 Lord Rayleigh, in which sulphuric acid is shown to pro- 

 duce the same effect as the blow-pipe flame, this and the 

 experiment with hydrofluoric acid seem to have inclined 

 Lord Rayleigh to think that cleanliness was the cause of 

 the breath figures. But does either sulphuric acid or 

 hydrofluoric acid prove cleanliness? I have doubts. I 

 know I am on dangerous ground in differing from Lord 

 Rayleigh on anything connected with surface action, yet I 

 have recently had my lesson on how some substances 

 cling to glass in spite of efforts to get rid of them, and I 

 think it is not improbable that some residual of both 

 sulphuric and hydrofluoric acid may cling to the glass in 

 spite of washing. Recently I was making an investigation 

 in which was required a little iodine vapour, and for this 

 purpose put a small crystal of iodine in a flask from 

 which the vapour was drawn as required. Afterwards the 

 investigation took another turn, and the flask was used 

 for other purposes, but many days' work were lost owing 

 to that flask. Results were obtained with it which were 

 contrary to previous experience. As suspicion centred on 

 the flask it was discarded, and not until a new flask had 

 replaced it could satisfactory work be done. Yet all this 

 loss of time was occasioned by a residual quantity of 

 Iodine, which the washings with alcohol, acids, soap water 

 and a sponge, had not succeeded in removing. After that 

 e.vperience I confess to being sceptical of absolute cleanli- 

 ness of glass after being touched with sulphuric or hydro- 

 fluoric acid. Any residual of these substances, as they 

 have an affinity for water, would tend to form films and 

 not little lens-like patches. Though breath figures may 

 be formed by dust, yet there are evidently other ways of 

 altering the surface of the glass and causing it to repel or 

 attract water, and so making the surface capable of giving 

 breath figures. John Aitken. 



.Ardenlea, Falkirk, June 5. 



The "Vernal Phytoplankton Maximum." 



In Xatlrf, for .April 27 it was stated, in connection 

 with the plankton statistics taken periodically in the Irish 

 Sea from the Port Erin Biological Station, that (p. 280) 

 " the outstanding fact in this season's work, so far, is 

 that the diatoms are unusuallv scarce and late. The 

 vrrnal phytoplankton maximum has not yet arrived." 



That statement referred to the collections up to the 

 middle of April. During the rest of April the catches 

 r-mainf>d small— for the most part i, 2, or 3 cubic centi- 

 metres in a standard haul of the fine silk net. In May 

 the approximate auantities (thcv have not vet been 



"■ ■ thr. 



The large catches on May 13-22 were mainly composed 

 of Chaetoceras (C debile, and a few other species of 

 diatoms), while the sudden increase in the beginning of 

 June is due almost wholly to Rhizosolenia (mainly R. 

 semispina). 



Last year we found that the vernal phytoplankton 

 appeared as two well-marked maxima, one in April, caused 

 by species of Chaetoceras and Lauderia, and a second in 

 June, composed of Rhizosolenia and Guinardia. 



It is evident that the " vernal maximum " is really a 

 complex made up of the maxima of several different species 

 or groups of species which seem to occur in a definite 

 sequence, but may be earlier or later, more spread out in 

 one year or more telescoped together to form a single 

 diatom maximum in another. Moreover, the individual 

 species or groups of species may be more abundant one 

 year than another. 



In the present year, if we look at the three genera 

 that usually bulk largest in our spring and early summer 

 collections, we find that Biddulphia reached its climax in 

 March (but continued throughout most of April in fair 

 abundance), Cha;toceras not until the latter part of May, 

 and Rhizosolenia in early June. Rhizosolenia is usually as 

 late as or later than this ; Biddulphia is always one of 

 the first forms to appear, sometimes causing a slight 

 increase in the plankton as early as February, so it is 

 really Ch.-ctoceras (some of the species of which are usually 

 the most abundant of our April diatoms) that is this year 

 unusually late. 



I have just returned from Port Erin, where, during the 

 first few days of this month, Rhizosolenia seemed to be 

 unusuallv abundant. The water of the bay is visiblv dis- 

 coloured by it ; when a tow-net at the end of a fifteen 

 minutes' haul is raised from the sea it contains a column 

 of thick soupy fluid, which runs out very slowly, as the 

 meshes of the silk are largely clogged up with the 

 diatoms ; on a calm surface, with the sun shining, the 

 peculiar iridescent glistening appearance characteristic of 

 Rhizosolenia can be seen from the boat, and anything put 

 into the water is at once covered by a slimy layer of the 

 slender needles. It may be that Rhizosolenia has not even 

 yet attained its climax, but a week ago the June increase 

 in diatoms had certainly set in with unusual force. Ihe 

 " vernal maximum " has, then, this year been spread out 

 and divided into three parts— a slighter increase in March 

 (Biddulphia), a much greater one, later than usual, in 

 May (Ch£Etoceras), and another great increase (Rhizo- 

 solenia) early in June. W. A. Herdm.w. 



Liverpool, June 10. 



On the Action of the Latex of Euphorbia peplus on a 

 Photographic Plate. 



In August, 1909, our attention was directed to certain 

 properties of surgical importance possessed by the milky 

 juice of Euphorbia peplus, a spurge naturalised in this 

 colony. During the examination of the milky juice or 

 latex,' we exposed during two days in the dark a highly 

 sensitive photographic plate some millimetres above a sheet 

 of glass on which we had dried a few drops of the juice. 

 On development, a sharp image of the dried juice 

 appeared on the plate. We propo.se to describe briefly a 

 few results from the many experiments since made to 

 determine the conditions and nature of the action on the 

 photographic plate. 



Thirtv drops of the juice were spread on a sheet ot 

 glass as the letters EUPHORBIA, and the glass dried in an 

 oven at 100° C. for two hours. A sharply defined image 

 of the letters was produced on development of a photo- 

 graphic plate (speed 325) placed 5 mm. above the dried 

 juice after an exposure of forty-eight hours or longer. 

 The density of the images increased with greater length 

 of exposure, with thicker films of dried juice, and with 

 less distance between the plate and the film. The ^dges 

 of the images were well defined, as if focussed on the 

 photographic plate. No imag.- appeared with short 

 exposures of twentv-four to forty-eight hours unless the 

 films were very thick. With long exposures, up to thirty 

 davs, the images were more dense, but showed .n littl<' 

 diffusion at the edges of the letters. When a distance of 

 IS mm. separated the photographic plate from the film. 

 fho plate was unaffected. Action on the plato diminished 



NO. 2172, VOL. 86] 



