228 OPTICAL PROJECTION 



so that the bubble will be in focus when blown, and the sauce* 

 of solution is lifted to the funnel when required, and then with- 

 drawn. If this bubble be pricked, it does not disappear as a soap- 

 bubble would, but ' hangs in rags ' to the funnel, as it were. And 

 secondly, if the air be sucked rapidly out from the bubble, it does 

 not contract smoothly, but hangs in folds like a wet linen bag. 

 That this is owing to extraordinary surface viscosity can be 

 shown in a tank like fig. 117 laid on the vertical attachment, 

 or less perfectly by direct projection in a cubical tank. Tanks 

 like fig. 117 are made by cementing large metal rings or 

 cylinders any required depth, on pieces of plate glass. The 

 tank being supplied with saponine solution, it can first be 

 shown by projection that a magnet 

 suspended above the surface is readily 

 moved about by another magnet. 

 Being then lowered to the surface, 

 hardly any movement can be produced. 



And being finally lowered below the surface, movement is 

 free and easy again. 



120. Cohesion Figures and Motions. In such tanks as 

 fig. 117 all the experiments usually known as ' Tomlinson's 

 Cohesion Figures ' * are readily projected by the vertical 

 method ; also the movements due to varying tension caused 

 by dropping camphor, alcohol, &c., on the surface of water. 

 The water must be perfectly clean for each new experiment ; 

 but cleaning or a fresh tank may often be dispensed with by 

 neatly tossing, as it were, the contents of the tank perpen- 

 dicularly out of it into a convenient receptacle. 



Another class of cohesion figures are produced in an 

 ordinary slide-tank projected direct, by filling with water or 

 alcohol, and dropping on the surface, or against one of the 

 glass sides, with a glass rod, solutions of aniline dyes. Still 

 another class are produced by providing pairs of pieces of 

 plate-glass, bevelled towards the inside surfaces, and between 



For detaila see Phil. Mag. 1861. 



