Energy and Heat 131 



as an invisible vapour diffused through the air of the 

 room in which the experiment is carried out. This 

 vapour we know as steam, and, by holding a clean 

 cold object over the dish while the water is boiling, 

 some of the steam may be recovered in the form of 

 water, which forms small drops on the outside of the 

 cold object. 



By similar treatment of other solids, it may readily 

 be shown that, in many cases, the application of heat 

 causes them to assume the liquid form, which may be 

 again transformed into a solid by allowing the liquid 

 to cool. Again, most liquids, when heated, may be 

 caused to assume the gaseous state, and, on cooling, 

 become liquid once more. 



As the general result of the above considerations, 

 we may say that 



Another effect of heat upon bodies is to bring about 

 a change in the state of matter in which they exist. 



86. EXPERIMENT 44. Into a small test-tube put 

 about 1 gram of the red powder called red oxide of 

 mercury. Heat the end of the tube containing the oxide 

 in a small Bunsen flame. The first change noted will 

 be that the powder assumes a darker hue, becoming 

 brown and finally almost black. Stop the heating at 

 this stage and allow the tube and its contents to cool. 

 The oxide will slowly change in colour once more, the 

 changes Baking place in the reverse order to that first 

 noted, until its appearance is not distinguishable from 

 that of some of the same powder that has not been 

 heated. Heat up the oxide again, and continue heating 

 after the colour has reached its deepest shade. Ignite 

 a thin splinter of wood in the flame, blow out the flame 

 of the wood, and insert the still glowing end of the 

 splinter into the upper end of the tube. It will be 



92 



