SNOW AND ICE 49 



other words that in becoming ice water undergoes a consider- 

 able expansion of bulk, and that there is a corresponding 

 diminution when ice becomes water. 



A more striking way of demonstrating the same thing is by 

 allowing water to freeze in a jampot, to show how the expansion 

 splits the pot. If desired this experiment can of course be 

 performed in the schoolroom, when the potful of water should 

 be surrounded with a freezing mixture made of snow and salt. A 

 small flask from the chemical laboratory will show the expansion 

 well, but in some respects the jampot experiment is preferable. 



The connection of the expansion of water with the domestic 

 phenomenon of burst pipes will of course be discussed, and the 

 action of the frost on the ground noticed. The occurrence of 

 frost without snow should be utilised to show the effect on the soil 

 of the expansion of water in becoming ice. 



Something has been already said about hoar frost, but more 

 detailed observations may be usefully made. Notice that hoar 

 frost does not occur when the temperature is very low, hence 

 the common distinction between hoar and black frosts. In 

 the autumn dew formation will have been noticed. By the help 

 of our thermometer we show that, especially when the sky is clear 

 and the air calm, the ground becomes rapidly colder after sun- 

 down. The result of the rapid loss, under the conditions named, 

 is that the air is chilled down below the point where it can hold 

 its load of vapour, and drops of moisture appear on the ground. 

 The appearance of a light mist close to the surface is of course 

 a phenomenon of precisely the same nature. If the chilling is 

 excessive the dew may freeze as it forms, producing the phenom- 

 enon of hoar frost. 



Though any extensive observations either on hoar frost or 

 on dew formation are not likely to be carried out with children, 

 yet an attempt should be made to show that both dew and hoar 

 frost are more noticeable on vegetation than on ground denuded 

 of vegetation, the vegetation parting with its heat more readily 

 than stones or dry ground, and also giving off moisture which 

 assists in the formation of dew. 



More feasible than detailed observations on dew formation 

 are those on methods of protecting from frost. On evenings when 

 VOL. vi. 4 



