356 SCIENCE OF COMMON THINGS. 



Simmering. "Why a kettle sometimes boils over. 



Because the particles of water near the bottom of the 

 kettle (being formed into steam sooner than the rest) 

 shoot upwards, but are condensed again (as they rise) 

 ~by the colder water, and produce what is called " sim-. 

 mering." 



1008 What is meant by simmering } 



A gentle tremor or undulation on the surface of the 

 water. When water simmers, tha bubbles collapse 

 beneath the surface, and the steam is condensed to water 

 again; but when water boils, the bubbles rise to the 

 surface, and the steam is thrown off. 



1009 Why does boiling water swell? 



Because it is expanded by the heat / that is, the heat 

 of the fire drives the particles of water farther apart 

 from each other, and (as they are not packed so closely 

 together) they take up more room / in other words, the 

 water swells. 



1010 Why does boiling water bubble ? 



Because the vapor (rising through the water) is 

 diffused, and forces up bubbles in its effort to escape. 



All the air of water is expelled at the commencement of its boiling. 



1011 Why does a kettle sometimes boil over f 



Because the water is expanded by heat / if, therefore, 

 a kettle is filled with cold water, some of it must run 

 over as soon as it is expanded by heat. 



1O1J3 But I have seen a kettle boil over, although it has not been fitted 

 full of water : how do you account for that? 



If a fire be very fierce, the air and vapor are expelled 

 so rapidly, that the bubbles are very numerous, and 

 (towering one above another) reach the top of the kettle, 

 and fall over. 



1013 Why is a pot (which was full to over/lowing while the water was 

 boiling hot) not full after it has been taken off the fire for a short time f 



Because (while the water is boiling) it is expanded 

 by the heat, and fills the pot even to overflowing ; but 

 when it becomes cool, it contracts again, and occupies a 

 much less space. 



1014 When steam pours out from the ''spout of a kettle, the stream 

 "begins apparently half an inch off Hie spout : why does it not begin close to 

 the spout f 



