INTERFERENCE OF LIGHT 325 



with a film of solid paraffin, then wiped nearly off again, 

 coloured rings will be exhibited more readily. 



186. Soap Films. These offer the most impressive de- 

 monstrations. Plateau's solution is made by dissolving 1 

 ounce of soda oleate in 40 ounces of distilled water, and 

 mixing this with 30 ounces of Price's best glycerine. This is 

 violently shaken up at intervals for several days, and then 

 filtered clear at as cool a temperature as practicable. When- 

 ever the solution becomes turbid, it must be filtered again, 

 and must always be brilliantly clear for use, and all vessels 

 (such as saucers) made scrupulously clean. In very cold 

 weather, however, the turbidity thus caused may be removed 

 by gently warming the solution and all the vessels used. I 

 think a little Marseilles soap ( ounce to the above quantity) 

 somewhat toughens this for the variable English climate; 

 and for immediate use a little gelatine added to a small 

 quantity of Plateau mixture will add toughness, but it de- 

 composes, and therefore will not keep. A Eoyal Institution 

 recipe, not nearly so tough, but which is free from streakiness 

 and gives the black effect rapidly, consists of five volumes 

 glycerine and six volumes of a 3 per cent, solution of potash 

 oleate, a ' soft ' soap. I am however indebted to Herr Dahne, 

 of Dresden, for a still handier method, which he has worked 

 out with care, and which allows the solution to be mixed as 

 required, even in a test-tube, from separate ingredients kept 

 in stock. Keep in bottles (1) a saturated solution of soda 

 oleate in distilled water, carefully neutralised and filtered clear 

 in the cold ; (2) distilled water ; (3) Price's best glycerine, 

 tested free from acid. Then the liquid is mixed variously as 

 follows, according to the purpose in view : 



(a) For greatest toughness or lasting properties (not 

 always desirable, as extreme toughness is apt to produce a 

 ropy or streaky appearance) : take one volume oleate solu- 

 tion, one volume glycerine, and two volumes distilled 

 water. 



