THE BOOK OF THE LANTERN. 191 



of these are of a chemical nature, but there are others 

 which exhibit physical phenomena in a manner which is 

 perhaps, impossible by any other means, or rather, we may 

 say, that experiments which can only under normal con- 

 ditions be viewed by one or two pairs of eyes on the lecture 

 table, can by means of this tank be made visible to a large 

 audience. 



A fine experiment showing the formation of vortex 

 rings may be shown in the following way. Having filled 

 the tank with clean water, take a penholder or a piece of 

 stick pointed for the purpose and dip it into some milk, so 

 that a drop forms at the end of it. Bring this carefully over 

 the tank and allow the milk to just graze the surface of 

 the water, when it will form a white ring in the fluid 

 which will fall gradually downward, but on the screen, 

 of course, it will appear to rise upward. This ring as it 

 travels to the bottom of the tank will give rise to other 

 similar rings, so that presently there will be quite a number 

 of circles slowly moving upwards on the screen. This 

 experiment is one which will illustrate well the formation 

 of smoke rings, and of the more important phenomena of 

 whirlpools and whirlwinds. 



Another experiment of a similar nature, and giving a fine 

 effect on the screen, may be performed by filling the tank 

 to within half an inch of the top with methylated spirit. 

 Take now instead of a wooden rod one of glass, or a camel 

 hair brush will do as; well. Dip it into an alcoholic solution 

 of any of the aniline dyes, and just allow the drop which 

 hangs from it to touch the inner side of one of the glasses 

 of the tank. Directly this drop reaches the alcohol, it will 



