March 3, 1881] 



NA rURE 



423 



Viscosity of Carbonic Oxide. — The results with this gas are 

 remarkable as showing an aljnost complete identity with those of 

 nitrogen both in po^ition and shape. The viscosity at 760 

 milliins. is in each cise o'log2. 



Like tliat of nitrogen the curve of carbonic oxide is seen to be 

 vertical — i.e., assuming the curve to represent the viscosity, the 

 gas obeys Maxwell's law, at pressures between 90 millims. and 

 3 millims. The straight portion in nitrogen is at a little higher 

 pre-sure — between too millims. and 6 millims. 



The curve of repulsion resultin;^ from radiation is lower in 

 carbonic oxide than in any other gas examined, and, unlike the 

 other gaic^, there is no sudden rise to a maximum at about 

 40 M . At lower exhaustions the curve is, however, higher than 

 it is in nilritgen. 



During exhaustion obierv.itims were continued on the varia- 

 tions in the spectrnm. The ordinary band spectrum is first seen 

 with a few sharp lines terminating the bands. 



At 12 millims. pressure a sharp green line is fir.-.t seen, \ 515 

 ms of mm. This line rapidly grows brighter as exhau-.tion 

 continues, and then fades out ; it is last seen at a pressure of 

 about O'g millim. This line is probably the bright oxygen-line, 

 the wave-length of which is given by Plucker at S'4'4' 



At a pie.-sure of 2-8 milhm^. the spectrum agrees in appear- 

 ance with the " Carbon No. 2" in VVatts's "Index of Spectra. " 



At 553 M the bands between the sharp lines appear to be 

 breaking up into masses of fine lines. 



At 211 M thes? fine lines are distinctly visible. The bright- 

 ness of this spectrum is now near its maximum. 



At 100 M the general spectrum is growing faint, but a sharp 

 green line at A 534 makes its appearance by fits and starts. 

 This is coincident with Pliicker's bright oxygeji line \ 534. 



After this degree of exhau4ion the spectrum rapidly gets 

 fainter. The line A 534 soon disap[)ears, and the carbon lines 

 also go one after the other, until at an exhaustion of 4 M only 

 two lines are visible, A 560 and A 519. 



Viscosity of Hydrogen. — It his been found that hydrogen has 

 much less viscosity than any other gas ; the fact of the lig dec. 

 not decreasing by additional attempts at purification is the 

 test of its being free from admixture. This method of ascer- 

 taining the purity of the gas, by the uniformity of its viscosity 

 coefficient at 760 millims., is more accurate than collecting 

 samples and analysing them eudiometrically. 



Several series of observations in hydrogen have been taken. 

 For a long time it was con-idercd that hydrogen, like other 

 gases, showed the same slight departure from Maxwell's law of 

 viscosity being independent of density that appeared to be indi- 

 cated with other gases ; for the log dec. persistently diminished 

 as the exhaustion increased, even at such moderate pressures as 

 could be measured by the barometer gauge. Had it not been 

 that the rate of decrease was not uniform in the different series of 

 observations, it might have been considered that this variation 

 from Maxwell's law was due to some inherent property of all 

 gases. After working at the subject for more than a year it was 

 discovered that the discrepancy arose from a trace of water 

 obstinately held by the hydrogen. .Since discovering this 

 property extra precautions (already described at the commence- 

 ment of the paper) have been taken to dry all gases before 

 entering the apparatus. 



The remarkable character of hydrogen is the uniformity of 

 resistance which it pre-ents. It obeys Maxwell's law almost 

 absolutely up to an exhaustion of about 700 m., and then it com- 

 mences to break. down. Up to this point the line of viscosity is 

 almost perfectly vertical. It then commences to curve over, 

 and when the mean free path assumes proportions comparable 

 with the dimensions of the bulb and approaches infinity, the 

 vis:;osity curve in like manner draws near the zero line. 



The repulsive force of radiation is higher in hydrogen than in 

 any other gas. It commences at as low an exhaustion as 14 

 millims., but does not increase to any great extent till .an exhaus- 

 tion of 200 M i^ attained ; it then ri>es rapidly to a maximum at 

 between 40 and 60 M, after which it falls away to zero. The 

 maximum repuKion exerted by radiation in hydrogen is to that 

 in air as 70 to 42'6. This fact is now utilised in the construction 

 of radiometers and similar instruments when great sensitiveness is 

 required. 



Taking the viscosity of air at 760 millims. as o"li24, and 

 hydrogen as o"0499, the proportion between them iso'4439. 



The ifeclniiii of Jlydiogen. — The red line (A = 656), the 

 green line (A = 486), and the blue line (A — 434) are seen at 

 their brightest at a pressure of about 3 millims., and after that 



exhaustion they begin to diminish in intensity. As exhaustion 

 proceeds a variation in visibility of the three lines is observed. 

 Thus at 36 millims. the red line is .seen brightly, the green faintly, 

 whilst the blue line cannot be delected. At 15 millims. the blue 

 line is seen, and the three keep visible till an exhaustion of 418 

 M is reached, when the blue line becomes difficult to see. At 

 38 M only the red and green lines are visible, the red being 

 very f.iint. It is seen with increasing difficulty up to an exhaus- 

 tion of 2 M, when it can l>e seen no longer. The green line now 

 remains visible up to an exhaustion of o"37 M, beyond which it 

 has not been seen. 



It is worthy of remark that although when working with pure 

 hydrogen the green line is always the last to go, it is not the 

 first to a|ipear when hydrogen is prcNcnt as an impurity in other 

 gases. Thus, when working with carbonic anhydride insuffi- 

 ciently purified, the red hydrogen line is often seen, but never 

 the green or the blue line. 



(To be continued.) 



SEEING BY ELECTRICITY' 

 /■ \N being called upon by the chairman to .show bis experi- 

 ^-' ments, I'rof. Ayrton stated that he and Mr. Perry thought 

 that the occasion of the reading of Mr. Bidwell's paper was a 

 suitable one for their .showing to the Society that they were 

 constructing the apparatus described by them in a letter in 

 Nature, vol. xxii. p. 31. The feasibility of their plan had been 

 combated, and at the last meeting of the liritish Association at 

 Swan ea it was confidently asserted that the action of selenium 

 was n )t quick enough to register rapid change^ of light intensity — 

 an idea, however, which they stated in the discussion at the time 

 there was experimental evidence to disprove. After that came 

 the publication and exhibition of the photophone, proving that 

 selenium changed its electrical properties .synchronously with 

 rapid changes in light intensity. For a light telegraph however 

 not only was this property necessary, but in addition that the 

 electric changes in the selenium should be considerable for a 

 comparatively small change in the light. They had, therefore, 

 tried to make sensitive selenium cells of low resistance. The 

 method they had employed consisted in winding two wires 

 parallel on strips of box-wood, ivory, and other non-conductors 

 in section, somewhat like that of a paper-knife in the manner 

 subsequently described by Mr. Bidwell in Nature, but they 

 had not found it necessary to cut a screw on the wood or mica 

 in a lathe. Of the twenty-five cells that they had constructed 

 they had invariably found, like Mr. Bidwell, that only those 

 were sensitive that had a high resistance. They were aware 

 that Prof. Adams had made sensitive cells of low resistance, 

 and had he been present they would have liked to ask 

 whether it was not only for very small electromotive forces 

 that the cells were sensitive. They had also found that 

 when sensitive cells of lOOfOOO ohms resistance diminished 

 in resistance to only a few hundred ohms by natural annealing 

 extending over some months, the cells lost entirely their sensi- 

 bility. Further that certain sensitive cells of high resistance 

 were sensitive as long as an electromotive force of not more than 

 about seven volts was employed to send a current through them, 

 but for electromotive forces much above this the cells v\ ere com- 

 paratively unsensitive to light, but the sensibility was not de^troyed 

 for electromotive forces smaller than seven volts used subsequently. 

 These jjhenomena, which they believed hid not been previou4y 

 noticed, pointed, they suggested, to the sensibility of selenium 

 being due almost entirely to a polarisation and not merely to 

 a change of resistance, as was commonly supposed and stated. 

 Might it not be possible, they asked, that there was an electro- 

 motive force developed in selenium by light, which, for different 

 cells, increased more rapidly than the resistance of the cell, and 

 which was the greater, the greater the electromotive force of the 

 auxiliary battery employed ; that in fact selenium became 

 rapidly polarised by the auxiliary current flowing through 

 it, and that this polarisation, the amount of which depended 

 on this current, was removed in proportion to the inten- 

 sity of the light. That a small electromotive force was de- 

 veloped in selenium by light when no auxiliary current was 

 sent through it, had been conclusively shown by Prof. Adams 

 and Mr. Day in 1876, a result that they had also experienced; 

 and they Mould mention that a careful examination which they 

 had recently made of the paper published by Prof. Adams and 

 ' P.iper communicated to the Physical Society, February 26.] Ji 



