150 



NA TURE 



[June i8, 1896 



one with the finger from various parts of the tube (but not from 

 all), best from the circular section of the tube in a plane with 

 the edge of the kathode mirror, or in the very immediate neigh- 

 bourhood of this line, the discharge producing the X, rays could 

 be induced at will in a tube which was not otherwise giving 

 them, or at any rate only giving them very feebly. Thinking, 

 therefore, that the production of the X, rays must be in some 

 way connected with intermittent leakage of the charge which 

 resides on the outside of the tube (a continuous drain stops 

 fluorescence and the emission of any rays capable of exciting 

 my fluorescent screen altogether), I tried various ways of 

 drawing off this charge intermittently, with several curious 

 results ; but the plan I find to work best is to place a ring of 

 plain copper wire round the tube in the plane of the kathode 

 mirror's edge, not touching the glass, but very near it, and then 

 to cause a very rapid but intermittent discharge by bringing a 

 wire connected to earth within a very small distance of some 

 part of this ring, so far I cannot discover any particularly 

 favourable position. The sparks between this ring and the 

 earth wire are very small, but the effect on the fluorescent screen 

 exceedingly striking. 



What is still more interesting is that not only is the discharge 

 of X-rays made much more regular (when the adjustment of the 

 ring and wire is carefully made), but the X-rays can thus be 

 induced in a tube with a far weaker current ; the weakest 

 current capable of inducing sparks between the ring and 

 conductor seems capable of giving the X-rays, though they are 

 more copiously emitted with a stronger current.' And not only 

 so, but the tubes I have experimented with seem to show as yet no 

 symptoms of growing fatigued. I have caused a brilliant 

 emission of the X-rays from a tube which was before 

 " fatigued" — at least, my coil seemed too weak to excite it, and 

 the emission ofX, rays has been sustained for over two hours with 

 but a few short intervals, withoutshowingany signs of diminution, 

 judging by the screen effects ; but on (he withdrawal of the ring 

 and wire, it at once failed to give any. This seems an important 

 result, for it must greatly shorten the exposure and fatigue 

 necessary for the photography of thick objects, and also greatly 

 save the expense of the operations. Even breathing very 

 gently, or blowing gently for a moment on the tube, specially on 

 the parts mentioned, produces a marked bright flash which on 

 examination will prove very rich in X-rays (a fact first noticed by 

 Mr. r. H. Walter, my assistant). It would seem to follow from 

 this that the "fatigue" is not altogether due to the diminution 

 of the number of free particles in the tube, but to a kind of 

 polarisation in which the glass acts as the dielectric separating a 

 negative charge inside from a positive outside, the X-rays and fluor- 

 escence being dependent in some way on the oscillations conse- 

 quent on the intermittent discharge of this condenser. I find that 

 the tube also gives X-rays plentifully (though not so plentifully as 

 when /loth terminals of the .secondary circuit are used), when the 

 positive terminal of the coil is connected in the usual way, and a 

 wire from earth leads to what is usually the kathode ; but only, 

 so far as my experiments go, when the ring and other earthed 

 wire are used. In this experiment the negative terminal was 

 not connected with the earth, but insulated. The tube did not 

 give X-rays, scarcely indeed any sign of the passage of electricity 

 when the kathode wire only was retained in its usual position. 

 The positive anode therefore seems in some respects to govern 

 the emission of these rays. 



In one experiment I placed a ring round the glass in the 

 plane of the concave mirror of a Crookes' tube showing the 

 "independence" of the positive pole from which I could not 

 get any X-rays, and found on extracting a series of small sparks 

 from it, with a wire leading to earth, a very decided increase in the 

 still general fluorescence of the tube, but it gave no rays. Incident- 

 ally it was noticed that when a wire brush connected to earth 

 was pressed lightly against the glass over the dark spot opposite 

 the kathode, in every place touched by the wire, a most brilliant 

 green fluorescence was excited, which faded away very quickly 

 when the brush was withdrawn. It is therefore certain that 

 much may yet be done to increase the efinciency of the tubes 

 used for the production of X-rays by a further study of the 

 action of neighbouring conductors upon them, and it seems that 

 such a study cannot fail to throw light upon the cause of these 

 hitherto unexplained phenomena. 



Eton College, June 8. T. C. Porter. 



' 1 hnve succeeded in eliciting feeble X-rays from a Newton's focus lube 

 with a small coil giving only i/srd of an inch spark — using the ring and 

 earthed wire or linger. 



Addendum, June 13. — After trying various forms of conductor, 

 coatmg different parts of the X-ray tube with Dutch gold, and alu- 

 minium leaves, I find the following a most effective plan, and feel 

 no hesitation in recommending it. First coat the external part 

 of the tube between the kathode wire loop and the afore-men- 

 tioned plane of the edge of the kathode mirror, with any con- 

 ducting metallic leaf; being careful that none of it projects 

 beyond the trace of this plane on the glass of the tube. Next 

 coil a stout piece of copper wire intoa circular loop with a stem, 

 and place it so that whilst the loop is in the plane of the edge of 

 the kathode mirror, it does not touch either the glass of the 

 tube or the metallic coating — I find an interval of about the 

 ^'jth of an inch answers excellently ; and lastly, instead of using 

 a wire to earth, bring the stem of the looped wire, or better still, 

 a more pliable piece of wire connected with the loop stem, 

 within a very short distance of the part of kathode wire from 

 the coil close to the tube. The adjustment is easily made it> 

 practice, and the emission of X, rays which follows will, I think, 

 be found satisfactory, to say the least, especially when it is 

 remembered that without the looped wire the tube may be 

 giving no X-rays at all. — T. C. P. 



Koch's Gelatine Process for the Examination of 

 Drinking- Water. 



I DO not find that Dr. Percy Frankland advances any sub- 

 stantial evidence in his letter of May 12, in support of his broad 

 and unqualified claim that he was the first person in this country 

 who adopted the Koch method and applied it to the London 

 water supply. 



Dr. Percy Frankland states that he has " failed to find in Dr. 

 Angus Smith's publications any mention whatsoever of cultiva- 

 tion on plates or their equivalents in any shape or form," and 

 which he ' ' holds to be the essence of the process which bears 

 the name of Koch, and to which modern bacteriology is so pro- 

 foundly indebted. ' 



Now if Dr. Percy Frankland will turn to page 2S of Dr. Angus 

 Smith's report to the Local Government Board, he will find Dr. 

 Angus Smith writing as follows : — " It may be better to give up 

 test-tubes entirely — equally good results have been obtained by 

 using other vessels." As a matter of fact, photographic glass- 

 plates were used in some of Dr. Angus Smith's experiments, also 

 flat-bottomed flasks, desiccators, &c. ; consequently if Dr. Percy 

 Frankland's contention is correct, that the essence of the process 

 which bears the name of Koch consists in the use of plates or 

 their equivalents, Dr. Angus Smith at all events can be credited 

 with having adopted the essence of the process. The main 

 value, however, of Dr. Koch's invention was in the use of 

 gelatine for preserving, as Dr. Angus Smith said, "the indica- 

 tions of organic vitality, and of aflbrding an opportunity for the 

 expansion of living germs in water, keeping a record for a time 

 both of the quality and intensity of life in the liquid, and 

 enabling the smallest points to exert their energies, and, as it 

 were, to build their structures, the size and numbers of which 

 can to some extent be measured and counted " 



Dr. Koch was the first to use gelatine, and it was from him 

 Dr. Angus Smith learned its use ; for he says, " I seized on the 

 use of gelatine with great earnestness, and soon satisfied myself 

 that there was much to be gained by its use. . . . Dr. Koch 

 has himself informed me that he is glad I have taken up the 

 subject ... a subject being more fully developed under Dr. 

 Koch by Dr. Rozahegyi, and chemists must prepare for a new 

 condition of things." 



With regard to Dr. Percy Frankland's statement that Dr. 

 Angus Smith distinctly deprecated rendering the medium more 

 nutritive, I do not agree with it, inasmuch as Dr. Angus Smith 

 clearly stated that ' ' experience must discover whether this is an 

 advantage . . . the use of sugar in addition to the gelatine 

 renders the examination of water by this method less dependent 

 on the opinion of the operator, ami a photograph may he taken 

 of each specimen, and the result preserved as evidence." 



Whilst wishing in no way to detract from the value of the 

 work which has been done by the use of the modified Koch's 

 process, and from developments of bacteriological methods since 

 the investigation of Dr. Angus Smith. I believe that I have 

 quoted sufficient evidence to show' that Dr. Angus Smith was 

 the first chemist in this country who " seized on " and applied 

 to practical purposes Dr. Koch's gelatine process of 1 881 ; and 

 his name w ill ever be associated with the historical development 

 and application of Koch's gelatine process in this country ; and 



NO. 1390, VOL. 54] 



