388 



OPTICAL PROJECTION 



dropped into the vessel, from which coloured streams will 

 proceed. The little apparatus shown in fig. 213 will both show 

 convection currents, and illustrate their every-day applica- 

 tion to hot-water systems of heating. A few small bubbles 

 or particles of sawdust show the movement well in this 

 apparatus. 



226. Evaporation. Boiling in a small flask is easily pro- 

 jected, and by the well-known apparatus in fig. 214, boiling up 



a flask of water, which is 

 closed with a cork and then 

 inverted in the field of the 

 lantern, the familiar experi- 

 ment of producing ebullition 

 by the application of cold 

 water, is also shown. The 

 loss of heat in evaporation 

 may be projected either by 

 wrapping the bulb of a 

 thermometer in a piece of- 

 rag moistened with the liquid, 

 or by applying a moistened 

 plate to the face of a thermo- 

 pile, the galvanometer in 

 connection with which is 

 projected direct, or by a re- 

 flected pencil. (See 240.) 

 227. Conductivity. To 

 show the different conducting power of various metals and 

 other substances, nothing more is necessary than to modify 

 the well-known apparatus of Ingenhouz (in which rods of 

 the substances project from a trough filled with boiling water, 

 which heats the inner ends of the rods simultaneously), so 

 that the rods project from the 'bottom of the trough, and their 

 lower ends at least stand in the field of the condensers. Glass 

 balls of equal weight being attached by wax softened with a 



FIG. 214 



