256 SCIENTIFIC AMUSEMENTS. 



centre. In exactly the same way a flat circular soap film can be 

 bent as it were downwards if a bit of paper moistened with soap 

 water be carefully dropped on its centre, and a gentle pressure be 

 then exerted on the paper by touching it with a pencil or thick 

 knitting needle ; or it may be pulled outwards if the paper have 

 a knotted string attached to it to serve as a sort of handle 

 (fig. 107). 



Expt. 301. Solutions suitable for Soap Films. When the 

 soap water serving to form a bubble or film is of proper strength 



the permanence of the film is very 

 considerable, and its tenacity or 

 power of cohesion very remarkable. 

 The strongest films of this kind are 

 made from a clarified soap solution 

 containing glycerine, such as that 

 obtained in the following way. A 

 piece of good soap (oil soap, castile 

 soap, or even ordinary yellow soap, is 

 usually preferable to transparent 

 kinds for this purpose) is shaved 

 with a knife, and the shavings well 

 Fig. 107. Flat Soap Film sup- stirred up with about 30 or 40 times 

 ported by Wire Ring, and their weight of warm water till all is 

 dissolved or nearly so; half an ounce 

 of soap to a pint of water is about 

 the proportion, distilled water being preferable; or failing this, 

 the softest rain water you can obtain. The solution is then 

 filtered through blotting paper, and some glycerine added, about 

 half its bulk being usually sufficient. 



The following receipt for making exceedingly permanent soap 

 films is due to M. Plateau, a celebrated French physicist. Marseilles 

 soap (or better, pure oleate of soda) is dissolved in about 40 parts of 

 warm distilled water and the solution filtered and mixed with f 

 of its bulk of pure glycerine and allowed to stand for a week ; the 

 vessel containing the fluid is then immersed in fragments of ice 

 for six hours and again filtered to remove the deposited matter, 

 care being taken to prevent the liquid in the filter from becoming 

 warmed again (which would redissolve the deposit) by placing 

 inside the filter a little corked bottle filled with fragments of ice. 

 Films and bubbles made with this solution will often last 12 

 hours and more if kept under a glass shade in slightly moist air. 

 Instead of a tobacco pipe a piece of glass tube about J inch bore 

 may be conveniently used as blowing tube. 



Collodion films may be prepared by means of the following 



