HEAT. 145 



front of the condenser of the lantern. A touch of the 

 finger will heat the bulb sufficient to cause the drop to 

 rise in the tube, and it may be made to descend by 

 simply blowing upon the bulb, or by dropping a little 

 water or ether upon it. 



Many of the pieces of apparatus for illustrating the 

 expansion of metals by heat are so small that they may 

 be readily projected. Thus Gravesand's Ring, Pyrom- 

 eters, etc. The latter may have a small bit of mirror 

 fastened to the end of the index, and the light so 

 arranged that as the index rises, the beam will move 

 upward. A rise in temperature of only a few degrees 

 can be then shown, and the alcohol flame may be dis- 

 pensed with ; the warmth of the hand or a little hot 

 water answering the purpose. 



FORMATION OF CLOUDS. 



The condensation of liquid in the form of vapor into 

 minute globules and in the production of a shower of 

 rain may be very well illustrated and projected for 

 class purposes in the following manner : — 



Place about an ounce of Canada balsam in a Flor- 

 ence flask and make it boil. At the top of the flask 

 clouds of globules of turpentine will be seen hovering 

 about, altering in shape very much like sky clouds, and 

 the globules are large enough to be visible by the naked 

 eye. If a cold glass rod be gradually introduced into 

 the flask these clouds may be made to descend in 

 showers. Lawson Tait in Nature. 



Another : Take a flask of one or two litres capacity ; 

 rinse it out with distilled water, and attach to the neck 

 a cork and glass tube of about twenty or thirty centi- 

 metres length. Place the glass tube in the mouth and 

 10 



