192 



KNOWLEDGE ♦ 



[July 1, 1889. 



In plate No. i, forming part of the frontispiece, was 

 shown the well-known but puzzling appearance to which the 

 rather paradoxical title of " The Dark Flash " has been given. 

 This, it will be remembered, is a black line, similar in 

 character to ordinary litjhtning-flashes, and in this case 

 especially like those on the same plate. It ha.s been sug- 

 gested that this dark line on the plate may result, through 

 some photographic complication, from an actual and imme- 

 diate bright flash — that is, that the black line of the plate 

 may really be a picture of a hri'jht line. Professor Stokes, 

 however, as explained in the text, is of the opinion that the 

 dead flash is really the dark line which it appears on the 

 photograph, and that it follows in the track of an ante- 

 cedent bright flash. Steps have been proposed by which 

 direct evidence upon this point may perhaps be obtained. 



In the meantime it has been suggested (as was also set 

 forth in Kxowledce last month) that the dark line is due 

 to absorption of light by a column of dark-coloured — to wit, 

 reddish-yellow — gases, ])roduced by the direct union, under 

 the influence of electricity, of the oxygen and nitrogen in 

 the air. It is well known to chemists that if the sparks of 

 an induction coil be passed continuously through a vessel 

 containing air, yellowish fumes will presently form, con- 

 sisting of nitrogen trioxide (NiOj) and nitrogen peroxide 

 (NOj) ; and there is no doubt that the same thing occurs 

 when a thunderstorm takes place, these gases being found 

 dissolved in rain-water collected after thunder. Whether 

 they are formed indirectly or directly there is not, so far as 

 I am aware, any evidence to show. It may well be that 

 nitrogen dioxide (NjO™) is first formed, and that this 

 colourless gas subsequently absorbs additional oxygen, thus 

 forming the familiar red fumes of the higher oxides. How- 

 ever this may be, it does not at present afiect the ques- 

 tion in hand, and the point need not have been raised, 

 except that I can imagine it coming up later on, when 

 further steps have been taken experimentally to ascertain 

 the cause of the dark flash. 



Now this experiment, though not often performed on the 

 lecture-table, is fully set forth in all the test-books. But 

 there is nothing, so far as I can ascertain, to show that, 

 even in the undisturbed air of a laboratory gas-holder, the 

 fumes exhibit any tendency to follow the line of the electric 

 sparks. I have never been able to see that they do so, and 

 indeed it would be a remarkable deviation from the usual 

 behaviour of free gases if they did. Gases invariably tend to 

 difluse themselves, and this property would surely prevent 

 the nitrous and nitric fumes from forming, even for a 

 very short space of time, a line or column as narrow and as 

 sharply defined as the dark flash is clearly shown to be. It 

 is noteworthy that on this hypothesis the photograph of the 

 dark flash must be impressed upon the plate some seconds 

 at least after the flash which it belongs to. 



As an alternative explanation it has been suggested that 

 the dark track shows the smoke of burnt dust floating in the 

 air and preserving the shape of the flash which burned it up. 

 This is ingenious ; and the dark flash in photographs cer- 

 tainly looks more like a truly black line than a reddish- 

 yellow one, which would hardly be expected to produce so 

 decided an effect. On the other hand, however, it must be 

 remembered that the dust particles are very minute indeed ; 

 being, in fact, often invisible even with high microscope 

 powers. Even if any smoke at all be produced, it must 

 therefore be very small in amount, and fine in texture, 

 which renders doubtful the possibility of photographing it. 

 Moreover, it is probable that the extreme heat of the flash 

 would be capable of entirely volatilising such infinitesimal 

 matter, which would disappear instantaneously, and, like 

 Prospero's insubstantial pageant, 



" Leave not a wrack behind." 



I believe, nevertheless, that the true explanation of this 

 phenomenon is to be found in connection with the floating 

 matter of the air. Before coming direct!}' to the point, 

 however, it will be necessary to work back a little. 



We have all observed the stream of intangible dust 

 which shows itself in the track of any sharply-defined 

 sunbeam, giving the latter an almost solid appearance. It 

 is not generally known that the path of the ray could not 

 be traced at all if this floating matter were absent. It is 

 the fact, however, that if the floating matter of the air be by 

 special means removed from a room, or, more conveniently, 

 from a glass tube, rays of light, though still passing unin- 

 terruptedly through the vacant space, can no longer be 

 followed by the eye in their course.* Now, if from a 

 certain portion of a beam of light the suspended solid 

 matter of the air be artificially abstracted, that portion of 

 the beam will appear to be traversed by a distinct black 

 line. This circumstance was first fully explained by 

 Professor Tyndall in his essays on " The Floating Matter 

 of the Air." f 



He writes : — " In a cylindrical beam which strongly 

 illuminated the dust of the laboratory I placed an ignited 

 spirit-lamp. Mingling with the flame and round its rim 

 were seen curious wreaths of darkness, resembling an 

 intensely black smoke. On placing the flame at some 

 distance below the beam the same dark masses stormed 

 upwards. They were blacker than the blackest smoke. . . ." 

 At first he was inclined to think that the blue flame of 

 alcohol did not, after all, represent complete combustion. 

 " But is the blackness smoke 1 " he goes on to ask. " This 

 question presented itself in a moment, and was thus 

 answered : — A red-hot poker was placed under the beam; 

 from it the black wreaths also ascended. . . . Smoke was 

 therefore out of the question. What, then, was the black- 

 ness ? It was simply that of stellar space — that is to say, 

 the blackness resulting from the absence from the track of 

 the beam of all matter competent to scatter its light. When 

 the flame was placed below the bciim, the floating matter was 

 destroyed in situ, and the air, free from this matter, rose into 

 the beam " — being forced up by the heat of the flame. A 

 subsequent experiment in the same fleld bears directly on the 

 subject of this paper. Professor Tyndall tried the eSect of 

 stretching a platinum wire, afterwards heated to redness 

 by a current of electricity, just under the path of the beam 

 from a powerful light. He writes — " A stream of air rose 

 from it, which, when looked at edgeways, appeared darker 

 and sharper than the blackest lines of Fraunhofer in the 

 purifled spectrum. Eight and left of this dark black band 

 the floating matter rose upwards, bounding definitely the 

 non-luminous stream of air." J Now is it not evident from 

 this that the dark flash — so called — may well be due to the 

 destruction of the floating dust by the ordinary flash, and 

 the consequent inability of its track to reflect the light of a 

 subsequent flash 1 The extreme sharpness of the line agrees 

 well with Tyndall's observations, and the intensely black 

 appearance of the smoke-effects which he mentions will 

 account for the ease with whicli the dead flash is photo- 

 graphed, in a way which neither the nitric fume explanation 

 nor the dust-smoke explanation will do. It is fair to 

 remark, however, that Tyndall does not speak of the appear- 

 ance of the beam after the platinum wire was removed ; 



* The electric beam is well known to bacteriologists as a test of 

 floating matter far surpasfiug in delicacy the most powerful masni- 

 fying instruments. The air of a chamber is known to be completely 

 sterilised only when it fails to reveal the track of a ray of light. 

 To vessels in this condition Professor Tyndall applies the term 

 " optically empty." 



t Longmans (18S1), p. .5. 



J Jbid., p. 6. The Italics are mine.— T. B. E. 



