38 SCIENTIFIC AMUSEMENTS. 



from below exhibit the phenomenon of boiling or ebullition ; 

 bubbles of steam or vapour form at the bottom of the kettle and 

 pass upwards, breaking on the surface and keeping up a con- 

 tinuous agitation. This is more easily seen when water is boiled 

 over a spirit-lamp in a thin glass flask. 



When the kettle boils vigorously, in ordinary language steam is 

 said to issue from the spout ; on looking at the spout it will be seen 

 that half an inch or more of the space beyond the spout is perfectly 

 clear and transparent, after which a little mist is visible, and 

 further off still a thick cloud of mist. What is properly called 

 steam is not the visible mist, to which, however, the term is often 

 applied ; the true steam is the clear transparent vapour forming 

 the column first issuing from the spout, as imperceptible to the eye 

 as the air itself. Only when this becomes partly chilled by 

 mixing with the cooler air does it become visible, and then only 

 because it alters its state and condenses, i.e., becomes water again, 

 but in droplets so small that they float about in the air forming a 

 fog. In short, the visible mist is condensed steam, or liquid water 

 in fine spray, resulting from the condensation on cooling of invisible 

 steam. 



What is true of water is equally true of all liquids that boil ; 

 the vapours given off are transparent, and whilst hot are usually 

 wholly invisible (some few substances form coloured vapours), and 

 condense again to visible droplets of the original liquids on cool- 

 ing. 



Some liquids boil far more readily than others ; thus whilst 

 water boils at a temperature of 100 centigrade, strong alcohol boils 

 20 or more lower ; ether will boil with the heat of a hot summer's 

 day, whilst many liquids are known to the chemist which will 

 boil below zero centigrade. On the other hand, mercury does not 

 boil till a temperature of 360 is attained; whilst melted zinc 

 requires a pretty bright red heat to make it boil, and other sub- 

 stances only boil at the very highest temperatures obtainable in 

 the most powerful furnaces or by means of intense electric 

 currents. 



Whether a substance boil at a high or a low temperature, how- 

 ever, the vapour or steam given off on boiling possesses the same 

 kind of physical texture as atmospheric air; indeed this latter, 

 like all other kinds of "gases" obtained by chemical means, is 

 simply the steam or vapour of highly volatile liquids so constituted 

 that they boil at extremely low temperatures. 



Expt. 32. Circumstances modifying the Boiling-Point. If 

 the atmospheric pressure be lessened, water boils more readily, so 

 that a thermometer placed in boiling water indicates a lower 



