224 THE FORMS OF CELLS [ch. 



It represents, in fact, that portion of the nodoid, which 



in Fig. 66 hes between such points as o, p. While it is easy to 



draw the outhne, or meridional 



section, of the nodoid (as in 



Fig. 66), it is obvious that the 



sohd of revolution to be derived 



from it, can never be reahsed in 



its entirety : for one part of the 



solid figure would cut, or en- 

 Fie. 66. . 



tangle with, another. All that 



we can ever do, accordingly, is to realise isolated portions of the 

 nodoid. 



If, in a sequel to the preceding experiment of Plateau's, we 

 use solid discs instead of annuli, so as to enable us to exert direct 

 mechanical pressure upon our globule of oil, we again begin by 

 adjusting the pressure of these discs so that the oil assumes the 

 form of a cyhnder: our discs, that is to say, are adjusted to 

 exercise a mechanical pressure equal to what in the former case 

 was supphed by the surface-tension of the spherical caps or ends 

 of the bubble. If we now increase the pressure shghtly, the 

 peripheral walls will become convexly curved, exercising a pre- 

 cisely corresponding pressure. Under these circumstances the 

 form assumed by the sides of our figure will be that of a portion 

 of an unduloid. If we increase the pressure between the discs, 

 the peripheral surface of oil will bulge out more and more, and 

 will presently constitute a portion of a sphere. But we may 

 continue the process yet further, and within certain limits we shall 

 find that the system remains perfectly stable. What is this new 

 curved surface which has arisen out of the sphere, as the latter 

 was produced from the unduloid ? It is no other than a portion 

 of a nodoid, that part which in Fig. 66 lies between such limits as 

 M and N. But this surface, which is concave in both directions 

 towards the surface of the oil within, is exerting a pressure upon 

 the latter, just as did the sphere out of which a moment ago it 

 was transformed ; and we had just stated, in considering the 

 previous experiment, that the pressure inwards exerted by the 

 nodoid was a negative one. The explanation of this seeming 

 discrepancy lies in the simple fact that, if we follow the outline 



