PHYSICS. NINETEENTH CENT. HEAT. 



533 



reaching the boiling temperature. It rolls about in the dish and 

 evaporates quietly. It is then, according to Boutigny, in the "sphe- 

 roidal state" The true explanation of this experiment was given by 

 Sir W. Armstrong, who showed that the liquid received heat from the 

 metal by radiation and not by contact, for it rests upon a stratum of 

 its own vapour, continually renewed, as upon an elastic cushion. It 



FIG. 247. 



is the escape of the vapour from beneath the liquid which occasions 

 the movements, and the metal must be at a sufficient temperature to 

 maintain the layer of vapour, otherwise the drop of water comes into 

 contact with the vessel and is instantly converted into steam. 



Scarcely any discoveries in the science of Acoustics have yet been 

 noticed in these pages. Those of Pythagoras (page 14) and Galileo's 

 researches (page 164) are indeed almost the only steps in the progress 

 of this science that have yet been referred to. As the present century 

 has witnessed an advance in acoustical science beyond all comparison 

 greater than that of former times, the few brief notes which must 



