July i6, 1891] 



NATURE 



251 



means of instantaneous photography the individual drops 

 can be made evident. I will first project the jet itself on 

 the screen, in order that you may appreciate the subject 

 which we shall see presently represented by photography. 

 [Experiment.] Along the first part of its length the jet 

 of water is continuous. After a certain point it breaks 

 into drops, but you cannot see them because of their 

 rapidity. If we act on the jet with a vibrating body, 

 such as a tuning-fork, the breaking into drops occurs 

 still earlier, the drops are more regular, and assume a 

 curious periodic appearance, investigated by Savart. I 

 have some photographs of jets of that nature. Taken as 

 described, they do not differ much in appearance from 

 those obtained by Chichester Bell, and by Mr. Boys. We 

 get what we may regard as simply shadows of the jet 

 obtained by instantaneous illumination ; so that these 

 photographs show little more than the outlines of the 

 subject. They show a little more, on account of the lens- 

 like action of the cylinder and of the drops. Here we 

 have an instantaneous view of a jet similar to the one we 

 vyere looking at just now (Fig. 2). This is the con- 

 tinuous part ; it gradually ripples itself as it comes 

 along ; the ripples increase ; then the contraction be- 

 comes a kind of ligament connecting consecutive drops ; 



the ligament next gives way, and we have the individual 

 drops completely formed. The small points of light are 

 the result of the lens-like action of the drops. [Other 

 instantaneous views also shown.] 



The pictures can usually be improved by diffusing 

 somewhat the light of the spark with which they are 

 taken. In front of the ordinary condensing lens of the 

 magic lantern we slide in a piece of ground glass, 

 slightly oiled, and we then get better pictures showing 

 more shading. [Photograph shown.] Here is one done 

 in that way ; you would hardly believe it to be water 

 resolved into drops under the action of a tremor. It 

 looks more like mercury. You will notice the long liga- 

 ment trying to break up into drops on its own account, 

 but not succeeding (Fig. 3). 



There is another, with the ligament extremely pro- 

 longed. In this case it sometimes gathers itself into two 

 drops (Fig. 4), 



[A number of photographs showing slight variations 

 were exhibited.] 



The mechanical cause of this breaking into drops is, I 

 need hardly remind you, the surface tension or capillary 

 force of the liquid surface. The elongated cylinder is an 

 unstable form, and tends to become alternately swollen 

 and contracted. In speaking on this subject I have often 

 been embarrassed for want of an appropriate word to 

 describe the condition in question. But a few days ago, 

 during a biological discussion, I found that there is a 

 recognized, if not a very pleasant, word. The cylindrical 

 jet may be said to become varicose, and the varicosity 

 goes on increasing with time, until eventually it leads to 

 absolute disruption. 



There is another class of unstable jets presenting many 

 points of analogy with the capillary ones, and yet in 

 many respects quite distinct from them. I refer to the 

 phenomena of sensitive flames. The flame, however, is 

 not the essential part of the matter, but rather an in- 

 dicator of what has happened. Any jet of fluid playing 

 NO. 1 133, VOL. 44] 



into a stationary environment is sensitive, and the most 

 convenient form for our present purpose is a jet of 

 coloured in uncoloured water. In this case we shall 

 use a solution of permanganate of potash playing into 

 an atmosphere of other water containing acid and sulphate 

 of iron, which exercises a decolourising effect on the 

 permanganate, and so retards the general clouding up of 

 the whole mass by accumulation of colour. [Experi- 

 ment.] Mr. Gordon will release the clip, and we 

 shall get a jet of permanganate playing into the 

 liquid. If everything were pefectly steady, we 

 might see a hne of purple liquid extending to the 

 bottom of the trough ; but in this theatre it is almost im- 

 possible to get anything steady. The instability to which 

 the jet is subject now manifests itself, and we get a break- 

 ing away into clouds something like smoke from chimneys. 

 A heavy tuning-fork vibrating at ten to the second acts 

 upon it with great advantage, and regularizes the disrup- 

 tion. A little more pressure will increase the instability, 

 and the jet goes suddenly into confusion, although at first, 

 near the nozzle, it is pretty regular. 



It may now be asked " What is the jet doing? " That 

 is just the question which the instantaneous method 



e 



Fig. 4. 



enables us to answer. For this purpose the permanganate 

 which we have used to make the jet visible is not of much 

 service. It is too transparent to the photographic rays, 

 and so it was replaced by bichromate of potash. Here 

 the opposite difficulty arises ; for the bichromate is invi- 

 sible by the yellow light in which the adjustments have to 

 be made. I was eventually reduced to mixing the two 

 materials together, the one serving to render the jet 

 visible to the eye and the other to the photographic plate. 

 Here is an instantaneous picture of such a jet as was 

 before you a moment ago, only under the action of a 

 regular vibrator. It is sinuous, turning first in one direc- 

 tion and then in the other. The original cylinder, which 

 is the natural form of the jet as it issues from the nozzle, 

 curves itself gently as it passes along through the water. 

 It thus becomes sinuous, and the amount of the sinuosity 

 increases, until in some cases the consecutive folds come 

 into collision with one another. [Several photographs of 

 sinuous jets were shown, two of which are reproduced in 

 Figs. 5 and 6.] 



The comparison of the two classes of jets is of great 

 interest. There is an analogy as regards the instability, 

 the vibrations caused by disturbance gradually increasing 

 as the distance from the nozzle increases ; but there is a 



