DEMONSTRATIONS IN PHYSICS 229 



which is a film of vaseline or other viscid substance free from 

 bubbles. This being put into any sort of a slide-frame, the 

 glasses being kept together by a spring, on introducing a pen- 

 knife between the bevelled edges and separating them, curious 

 arborescent figures are projected on the screen. 



121. Vortex Rings. These may be partially projected in 

 a tank, vertically or otherwise, dropping in milk or aniline 

 dye. They are best shown, however, by using a vortex-box 

 in the ordinary way, and sending the rings along a track 

 illuminated by the full beam from the nozzle of the lantern. 

 Rings thus illuminated are very impressive. 



122. Molecular Strains and Forces, With the usual 

 precautions, the breakage of a Rupert's drop or Bologna 

 flask in a vessel of water is very easily projected ; and the 

 apparently sudden annihilation of the drop is very curious. 



Crystallisation is easily projected upon plates of glass. 

 About 3 inches square is a convenient size, clipped at one 

 corner in a * holder ' and focussed in the condenser field. A 

 saturated solution of ammonium chloride, or urea, in water, 

 or of magnesium sulphate in stale beer, crystallise perhaps 

 most readily. 1 



123. Gases. A great many of the phenomena of gases 

 are readily projected, using the shadow method for larger 

 apparatus. Apparatus is easily arranged on a small scale for 

 exhibiting absorption, or the expulsion by heat of occluded 

 gas. The whole of a manometer can be projected by the 

 shadow method, or the scale portion only may be projected 

 with the objective. A barometer, Sprengel pump, apparatus 

 exhibiting Marriotte's law and its applications (including all 

 sorts of fountains), are best shown large by the shadow 

 method. Siphons can be projected either way. The action 

 of an aspirator is projected in the condenser field with exceed- 

 ingly good effect. After what has been said, the methods of 

 arranging such projections will suggest themselves. 



1 See also under ' The Projection Microscope,' and ' Polarised Light. 



