EFFECTS OF HEAT 73 



rises up the tube (Fig. 58). Take the flask out of the warm water, and see 

 that the coloured water gets smaller as it cools, and that it sinks in the tube. 

 (6) Arrange two other flasks as in the last experiment, but filled respec- 

 tively with alcohol and turpentine. Push in the corks till the liquid stands 

 in each tube at the same height. Put the flasks to the same depth into a 

 vessel of warm water. Notice that the expansion of the glass causes a 

 momentary sinking of the liquids ; and that ultimately the expansions of 

 the liquids are very different. 



iii. Expansion of gases. (a) Procure a well-made paper bag and tightly 

 tie a piece of tape round the open end. Hold the bag in front of the fire 

 and notice that the air inside gets larger and inflates the bag. 



(6) Or, obtain a flask with a cork and tube as in Fig. 59, i. Remove 

 the cork and tube, and, by suction, draw a little red ink into the end of the 

 tube near the cork. Re-insert the cork, and 

 gently warm the flask by clasping it in the 

 hands. Notice that the air in the flask gets 

 larger and pushes the red ink along the 

 tube. 



(c) Turn over and place the open end of the 

 tube beneath the surface of some coloured 

 water in a beaker. Warm the flask with the 

 hand or a flame so as to expel some of the air, 

 and let the liquid rise in the stem (Fig. 59, ii.). 

 This arrangement constitutes an air thermo- 

 meter. 



(d) Fasten in an air-tight manner two bulbs 



or flasks together by a tube bent six times at FlQ 60 _ A gim le form 

 right angles, and containing some coloured differential thermometer, 

 liquid in the middle bends (Fig. 60). Show 



that the liquid moves if one flask is warmed more than the other. This 

 instrument is known as a differential thermometer. 



Change of size. Expansion. As a rule all bodies, whether solid, 

 liquid, or gaseous, get larger when heated, and smaller when cooled. 



The change of size which a body undergoes is spoken of as the amount 

 it expands or contracts ; or, heat is said to cause expansion in the 

 body. This expansion is regarded in three ways. When dealing with 

 solids, expansion may take place in length (linear expansion), in area 

 (superficial expansion), and in volume (cubical expansion). In the case 

 of liquids and gases we have only cubical expansion. Similar terms 

 can be used with reference to contraction. 



The expansion which substances undergo when heated must be 

 allowed for in many engineering projects. Railway lines, for instance, 

 are usually not placed close together, but a little space is allowed 

 between the separate rails, so that they can expand in summer without 

 buckling. Steam pipes used for heating rooms are also not fixed firmly 

 to the walls at both ends, but are left slightly loose or are loose- jointed, 

 so that they can expand or contract without doing any damage. For 



