Energy and Heat 133 



"physical " changes. In the latter change in the mercury 

 oxide, however, the mercury oxide ceases to exist as 

 such, and by no simple means, such as the withdrawal 

 of the source of heat, may it be again produced from 

 the new bodies obtained from it. Changes of this 

 character are called " chemical " changes. Hence a 

 further general statement may be made as follows : 



A third effect of heat upon bodies is to cause the 

 formation from them of entirely distinct bodies. 



87. It may be readily shown that work may be 

 performed during each kind of change mentioned above, 

 or, in other words, that heat is a form of energy. Thus, 

 when a body is caused to expand by the application 

 of heat, very great force may be produced, and similarly, 

 when the body cools, equally great forces may be 

 brought into play. An example of this is seen in the 

 method sometimes used for strengthening old brick- 

 work. On old walls one may see straps of iron of the 

 shapes shown in Fig. 38. In the middle of each strap 

 is a nut which travels upon a screw thread worked on 

 the end of a long iron rod passing through the wall, 

 and provided at the other end with a similar strap and 

 bolt which are affixed to the opposite wall of the house. 

 The object of this arrangement is to prevent the walls 

 of the house from falling outwards, and the manner 

 in which this is effected is as follows : The rod is 

 placed in position with the straps and nuts loosely fitted 

 upon its ends. It is then strongly heated with a conse- 

 quent increase in length. While still very hot, the nuts 

 are screwed home as tightly as possible. The source 

 of heat is then withdrawn and the rod allowed to cool. 

 In cooling it contracts and the force of contraction is 

 sufficient to counteract the bulging of the walls, thus 

 rendering the house a safer habitation. 



