758 



EXPANSION. 



few millimetres below the surface of the water in a 



small saucer, as in fig. 380. 

 If the hand be placed upon 

 the wider portion of either 

 vessel, the heat of the 

 hand is sufficient to cause 

 :a perceptible expansion of 

 the air within the vessel. 

 If a spirit or gas flame be 

 used for heating the vessel, 

 the expansion of the air 

 will be so considerable that 

 a large quantity of air will 

 escape through the water in 



PIG. 380 ( real size}. ^ Q form Qf bubbleg . when 



the flame is withdrawn and the vessel allowed to cool 

 the air within it contracts again, and, in consequence 

 of the atmospheric pressure outside, the water rises to 

 some height in the neck of the vessel. The expansion 

 and contraction of air, which is heated or cooled 

 respectively, may be very strikingly shown by clamping 

 the contrivance represented in fig. 168 (page 244) 

 horizontally in the retort-stand, and applying the. heat 

 of a flame to the portion of the tube which contains air ; 

 the mercury will be pushed towards the opening of the 

 tube as the air expands, and will recede again when 

 the air is allowed to cool. 



The brass piece, fig. 379, is made of wire, 6 mm thick, which is 

 softened over a charcoal fire and hammered into the required shape 

 with the mallet, the wire being either clamped in the jaws of the 

 vice or placed upon one of the beaks. The short ends a and d must 

 be bent first, and the two sides, which are horizontal in the figure, 

 afterwards. The extremities of a and d, and also those of the bar 



