370 THE FORMS OF CELLS [ch. 



be experimentally illustrated by means of soap-bubbles, or better 

 still, after another method of Plateau's, by means of a large globule 

 of oil, supported when necessary by wire rings, and lying in a fluid 

 of specific gravity equal to its own. 



To prepare a mixture of alcohol and water of a density precisely 

 equal to that of the oil-globule is a troublesome matter, and a 

 method devised by Mr C. R. Darhng is a great improvement on 

 Plateau's*. Mr Darhng used the oily hquid orthotoluidene, which 

 does not mix with water, has a beautiful and conspicuous red 

 colour, and has precisely the same density as water when both 

 are kept at' a temperature of 24° C. We have therefore only to 

 run the liquid into water at this temperature in order to produce 

 beautifully spherical drops of any required size : and by adding a 

 little salt to the lower layers of water, the drop may be made to 

 rest or float upon the denser liquid. 



Fig. 105. 



We have seen that, the soap-bubble, spherical to begin with, is 

 transformed into a plane when we release its internal pressure and 

 let the film shrink back upon the orifice of the pipe. If we blow 

 a bubble and then catch it up on a second pipe, so that it stretches 

 between, we may draw the two pipes apart, with the result that 

 the spheroidal surface will be gradually flattened in a longitudinal 

 direction, and the hubble will be transformed into a cyhnder. But 

 if we draw the pipes yet farther apart, the cyhnder narrows in the 

 middle into a sort of hour-glass form, the increasing curvature of 

 its transverse section being balanced by a gradually increasing 

 negative curvature in the longitudinal section; the cyhnder has, in 

 turn, been converted into an unduloid. When we hold a soft glass 

 tube in the flame and "draw it out," we are in the same identical 

 fashion converting a cylinder into an unduloid (Fig. 105, A)\ when 

 on the other hand we stop the end and blow, we again convert the 

 cylinder into an unduloid (B), but into one which is now positively, 

 while the former was negatively, curved. The two figures are 



* See Liquid Drops and Globules, 1914, p. 11. Robert Boyle used turpentine 

 in much the same way; for other methods see Plateau, op. cit. p. 154. 



