March 27, 1879] 



NATURE 



487 



These experiments may be repeated with extreme ease 

 and with hardly any apparatus. The diameter of the jet 

 may be about ^}q inch, and may be obtained either from 

 a hole in a thin plate or from a drawn-out glass tube. I 

 have generally employed a piece of glass tube fitted at 

 the end with a perforated tin plate, and connected with a 

 tap by india-rubber tubing. The pressure may be such as 

 to cause the jet to rise eighteen or twenty-four inches, or 

 €ven more. A single passage of a rod of gutta-percha, 

 or of sealing-wax, along the sleeve of the coat is sufficient 

 to produce the effect. The seat of sensitiveness may be 

 investigated by exciting the extreme tip only of a glass 

 rod, which is then held in succession to the root of the 

 jet and to the place of resolution into drops. An effect is 

 •observed in the latter but not in the former position. 

 Care must be taken to use an electrification so feeble as 

 to require close proximity for its operation, otherwise the 

 ■discrimination of the positions will not be distinct. 



The behaviour of the colliding drops becomes apparent 

 under instantaneous illumination. I have employed 

 sparks from an inductorium, whose secondary terminals 

 ■were connected with the coatings of a Leyden jar. The 

 jet should be situated between the sparks and the eye, 

 and the observation is facilitated by a piece of ground 

 glass held a little beyond the jet, so as to diffuse the 

 light ; or the shadow of the jet may be received on the 

 ground glass, which is then held as close as possible on 

 the side towards the observer. 



If the jet be supplied from an insulated vessel, the 

 coalescence of colliding drops continues for a time after 

 the removal of the influencing body. This is a con- 

 sequence of the electrification of the vessel.* If the 

 electrified body be held for a time pretty close to the jet, 

 and be then gradually withdrawn, a point may be found 

 where the rebound of colliding drops is re-established. 

 A small motion to or from the jet, or a discharge of the 

 vessel by contact of the finger, again induces coalescence. 

 Although in these experiments the charges on the 

 oUiding drops are undoubtedly of the same name, it 

 [appeared to me very improbable that the result of contact 

 ■of two equal drops, situated in the open, could be affected 

 by any strictly equal electrifications. At the same time 

 opposite opinion makes the phenomena turn upon the 

 very small differences of electrification due either to 

 irregularities in the drops or to differences of situation, 

 and is at first difficult of acceptance in view of the 

 efficiency of such very feeble electric forces. Fortunately 

 I am able to bring forward additional evidence bearing 

 upon this point. 



When two horizontal jets issue from neighbouring holes 

 in a thin plate, they come into collision, for a reason that 

 I I need not now stop to explain, and after contact they 

 frequently rebound from one another without amalgama- 

 tion. This observation, which I suppose must have been 

 made before, allowed me to investigate the effect of a 

 passage of electricity across two contiguous water sur- 

 faces. The jets that I employed were of about yV inch 

 in diameter, and issued under a moderate pressure (5 or 

 6 inches) from a large stoneware vessel. Below the place 

 of rebound, but above that of resolution into drops, was 

 placed a piece of insulated tin plate in connection with a 

 length of gutta-percha-covered wire. The source of elec- 

 tricity was a very feebly excited electrophorus, whose 

 cover was brought into contact with the free end of the 

 insulated wire. When both jets played upon the tin plate 

 the contact of the electrified cover had no effect in deter- 

 mining the union, but when only one jet washed the plate 

 union instantly followed the communication of electricity 

 I r and this notwithstanding that the jets were already in 

 llB communication through the vessel. The quantity of elec- 

 !~ tricity required is so small that the cover would act three 

 or even four times without being re-charged, although no 



k precautions were taken to insulate the reservoir. 

 In subsequent experiments the colliding jets, about 

 ( 



Y§jy inch in diameter, issued horizontally from similar 

 glass nozzles, formed by drawing out a piece of glass 

 tubing and dividing it with a file at the narrowest part. 

 One jet was supplied from the tap, and the other from the 

 stoneware bottle placed upon an insulating stool. The 

 sensitiveness to electricity was extraordinary. A piece of 

 rubbed gutta-percha brought near the insulated bottle at 

 once determined the coalescence of the jets. The influ- 

 encing body being held still, it was possible to cause the 

 jets again to rebound from one another, and then a small 

 motion of the influencing body to ox from the bottle again 

 induced coalescence, but a lateral motion without effect. 

 If an insulated wire be in connection with the contents of 

 the bottle, similar effects are produced when the electritied 

 body is moved in the neighbourhood of the free end of 

 the wire. With care it is possible to bring the electrified 

 body into the neighbourhood of the free end of the 

 wire so slcwly that no effect is produced ; a sudden move- 

 ment of withdrawal will then usually determine the 

 coalescence. 



Hitherto statical electricity has been spoken of ; but the 

 electromotive force of even a single Grove cell is sufficient 

 to produce these phenomena, though not with the same 

 certainty. For this purpose one pole is connected 

 through a contact key with the interior of the stoneware 

 bottle, the other pole being to earth. If the fingers be 

 slightly moistened, the body may be thrown into the 

 circuit, apparently without diminution of effect. This 

 perhaps ought not to surprise us, as in any case the elec- 

 tricity has to traverse several inches of a fine column of 

 water. On the other hand, it appeared that most of the 

 electromotive force of the Grove cell was necessary. 



Further experiment showed that even the discharge of 

 a condenser charged by a single Grove cell was sufficient 

 to determine coalescence. Two condensers were used 

 successively ; one belonging to an inductorium by Ladd, 

 the other made by Elliott Brothers, and marked " Capa- 

 city i Farad." Sometimes even the "residual charge" 

 sufficed. 



It must be understood that coalescence of the jets 

 would sometimes occur in a capricious manner, without 

 the action of electricity or other apparent cause. I hare 

 reason to believe that some, at any rate, of these irregu- 

 larities depended upon a want of cleanness in the w^ater. 

 The addition to the water of a very small quantity of 

 soap makes the rebound of the jets impossible. 



The last observation led me to examine the behaviour 

 of a fine vertical jet of slightly soapy water : and I found, 

 as I had expected, that no scattering took place. Under 

 these circumstances the approach of a moderately electri- 

 fied body is v/ithout effect, but a more powerful influence 

 scatters the drop as usual. The apparent coherence of 

 a jet of water when the orifice is oiled was observed by 

 Fuchs, and appears to have been always attributed to a 

 diminution of adhesion between the jet and the walls of 

 the orifice. 



Some further details on this subject, and other inves- 

 tigations respecting the phenomena of jets, are reserved 

 for another communication, which I hope soon to be able 

 to present to the Royal Society ; but I cannot close with- 

 out indicating the probable application to meteorology of 

 the facts already mentioned. It is obvious that the 

 formation of rain must depend very materially upon the 

 consequences of encounters between cloud particles. If 

 encounters do not lead to contacts, or if contacts result in 

 rebounds, the particles remain of the same size as before ; 

 but, if the issue be coalescence, the bigger drops must 

 rapidly increase in size and be precipitated as rain. Now, 

 from what has appeared above, we have every reason 

 to suppose that the results of an encounter will be dif- 

 ferent according to the electrical condition of the particles, 

 and we may thus anticipate an explanation of the remark- 

 able but hitherto mysterious connection between rain and 

 electrical manifestations. 



