June 23, 1921] 



NATURE 



523 



could be regulated by a Gaede pump and measured 

 by a McLeod gauge) a continuous discharge was 

 maintained for about eight hours on several consecu- 

 tive days, and I have observed a gradual hardening in 

 spite of the maintenance of a comparatively high pres- 

 sure. Further experiments carried out in this direction 

 have revealed a remarkable effect which takes place 

 in an X-rav bulb or, more generally, in any vacuum 

 tube after a suflficiently long and continuous run — an 

 effect similar to the polarisation of an electrolytic cell. 

 This is, that after the discharge has been kept running 

 for a sufficiently long time through a tube (inside 

 which the pressure is kept nearly constant) a time 

 arrives when the current flowing through the tube 

 begins to decrease, and finally ceases altogether. To 

 continue the discharge it is then necessary to 

 increase either the potential difference applied to the 

 electrodes or the pressure inside the tube. B\' 

 repeating this operation several times I could ulti- 

 mately reach a stage where a potential difference of 

 more than 50,000 volts was not sufficient to produce a 

 discharge in the bulb, although the pressure was as 

 high as 0060 mm., whereas under ordinary conditions 

 in the same bulb a much smaller potential difference 

 was sufficient to produce a discharge under a pressure 

 of the order of 0001 mm. After the discharge has 

 been stopped the bulb gradually returns to its normal 

 condition, but afterwards a comparatively short run 

 is sufficient to bring the bulb back to the state of 

 polarisation. 



It could be further shown that the effect is not due 

 to changes in the nature of the gas in the bulb brought 

 about bv the discharge. .A large side-tube containing two 

 electrodes, the shape and distance apart of which were 

 essentiallv the same as in the X-ray bulb, was fused 

 into it. ' When after a long run the main bulb 

 became polarised, so that the highest available voltage 

 could not break down its resistance in spite of a high 

 pressure of about 1/20 mm., a potential difference of 

 1200 volts, supplied by a battery of small cells, when 

 put across the side-tube was found to produce a normal 

 discharge. 



Experiments, which will be described elsewhere, give 

 some evidence in support of the view that this effect 

 is due to the destruction by the discharge of the 

 gaseous layer on the surface of the electrodes. 



It seems probable that the hardening of an X-ray 

 bulb with usage is due not onlv to the disappearance 

 of the gas in the bulb, but also to the phenomenon 

 described In this letter. S. Ratnkr. 



The Physical T.aboratory, Victoria University, 

 Manchester, June i. 



Observations of Plant-growth with the Recording 

 Ultramicrometer. 



At the meeting of the Royal Dublin Society on 

 January 25 last, as reported in Nature for February 

 24, p. 850, I described a form of ultramicrometer in 

 which the minute movement of one plate of a parallel 

 plate condenser, forming part of a thermionic-valve 

 oscillating circuit, is recorded by a galvanometer. 

 We are now applying this apparatus to the study of 

 plant-growth, and as some of the preliminary ob- 

 servations show verv clearlv the pulsations of growth 

 described by Sir J. C. Bose, it may be of interest to 

 give a short account of these results. 



In applying the method to this problem the upper 

 olate of the condenser is a thin aluminium disc, about 

 6 cm. in diameter, supported by a long flat strip of 

 spring steel, so as to be situated about i/io mm. 

 above the lower fixed (horizontal) plate. The latter 

 plate may be given small vertical movements by 

 means of a micrometer screw for adjustment or cali- 



NO. 2695, VOL. 107] 



bration of the apparatus. In many of our observa- 

 tions the apparatus is adjusted to give 150 divisions 

 on the galvanometer scale for a displacement of the 

 upper plate through i/iooo cm., but it can be made 

 many times more or less sensitive simply by altering 

 the galvanometer shunt. 



To the recording (upper) plate is rigidly attached a 

 short wooden arm against which the plant member 

 presses lightly. It is found that a weight of i/io 

 gram placed on this causes a galvanometer deflec- 

 tion of 100 divisions. This indicates the order of 

 magnitude of the stress on the plant under observa- 

 tion. During experiments on roots the root-tip presses 



Fig. I. — Broad bean root-shoot (four days old). 



the plate downwards ; in other cases the movement is 

 upwards. 



As an example of one type of record obtained the 

 accompanying curve is appended. It represents the 

 (downward) growth of the root-shoot of a broad- 

 bean which had been planted some four days before 

 and had just been removed from the ground. A con- 

 siderable time had been allowed to elapse after plac- 

 ing the plant in position before observations were 

 commenced. 



I have to thank two botanical studentSj Miss 

 Cannon and Mr. Saunders, for the part they are 

 taking in the work. Johx J. Dowling. 



Department of Phvsics, University College, 

 Dublin. 



Cup and Ring Markings. 



Referring to the note anent the above which 

 appeared in Natlre of June 9, p. 468, may I men- 

 tion that these peculiar surface-features can fre- 

 quently be seen upon old mortar, stucco, and 

 calcareous sandstones, and that they are due to mole- 

 cular re-arrangement of the calcium carbonate, and 

 not to any artistic efforts on the part of prehistoric 

 man, as is frequently supposed? 



At the Royal Society in 1896 I exhibited photo- 

 graphs of some remarkable examples of " cup and 

 ring markings " which had developed on the stucco 

 of one of the houses in Warrior Square, St. Leonards- 

 on-Sea. Similar " patterns " may sometimes be seen 

 on old "American cloth " which has been subjected to 

 tension, and also on old oil paintings. In these cases 

 the gradual shrinkage of the canvas backing has pro- 

 duced the effect by causing lines of fracture in the 

 more homogeneous layers of paint. 



June II. C. Carus-Wilson. 



