Tension of Liquid Films. 195 



briimi of tension on the skew helicoidare the rectilinear genera* 

 ting lines, the axis, and the helices of the same pitch as the di- 

 recting helix*. 



In order to submit these theoretical deductions to the test of 

 experiment, I produced the surface in question in the shape of a 

 film. M. Plateau, applying a general principle which he gave 

 in one of his memoirs f, has produced tlfis figure and pointed it 

 out in a research which is as yet unpublished; but he has ail* 

 thorized me to use it and describe it 

 here. A helix (fig. 1) is made of iron Fig- !• 



wire, the distance between the turns of 

 which is 30 millims. for example, and 

 which belongs to a cylinder of 45 mil- 

 lims. radius. Any number of turns of 

 this helix may in general be taken ; but 

 for the present purpose it is best to work 

 with only one turn. The ends of this 

 latter are joined by straight iron wires to 

 another straight wire, serving as axis of 

 the helix and furnished with a prolonga- 

 tion, by which the skeleton can be easily immersed in the gly- 

 cerine liquid. It is clear that we have obtained a closed contour 

 belonging to a skew helicoid ; and according to Plateau's prin- 

 ciple we should obtain a helicoidal film when the skeleton is 

 withdrawn from the liquid : this is confirmed in every respect 

 by. observation. 



Since the verification of the theory requires in the present case 

 very accurate experiments, I thought it necessary to give up using 

 silk or cotton threads ; for their weight, together with that of the 

 liquid in which they are immersed, is too great to be disregarded ; 

 if, therefore, there were a change in the shape of a laminar sur- 

 face, it might be either wholly or partially attributed to the dis- 

 turbing cause arising from the weight of the moistened thread. 

 Hence I have had recourse to threads out of a cocoon, the weight 

 of which may be entirely neglected ; these threads, to be sure, are 

 not inextensible, and therefore short lengths only must be used. 

 I pass now to the description of the experiments. And, first, 

 I have never succeeded in reproducing the rectilinear generating 

 lines of the helicoid : when, after having stretched a cocoon- 

 thread in the direction of one of these generating lines, I pro- 

 duced a laminar surface, and then burst the portion of the film 

 on cither side of the thread, this always formed a concave line on 



* This result is announced in M. Lamarle's report. 



f ' Recherches experhnentales et theoriques sur les figures d'equilibre 

 d'une mass liquide sans pesanteur,' 7th series, last paragraph (Memoires de 

 VAcademie, vol. xxxvL). 



