Of the Excellent Qualities of Coffee. 625 



sarily puts the whole mass of the liquid in motion. 

 The warmer and lighter particles are continually 

 rising towards the surface of the liquid, while the 

 colder and heavier particles are descending ; and these 

 motions never can cease, till the whole of the liquid has 

 acquired the precise temperature of the surrounding 

 atmosphere. 



When the liquid is heated, similar motions take 

 place, but in an opposite direction. The particles 

 first heated, being rendered specifically lighter by this 

 augmentation of temperature, rise upwards and give 

 place to the colder and heavier particles which de- 

 scend. 



These motions may be rendered visible by a very 

 simple contrivance. 



If one ounce of common salt be dissolved in eight 

 ounces of water, a brine will be formed, which will 

 have the same specific gravity as yellow amber ; con- 

 sequently, if a small quantity of that solid substance 

 be pounded in a mortar, so as to be reduced to a coarse 

 powder (of about the size of mustard-seeds), this pow- 

 der on being put into the brine will remain suspended 

 in that liquid, and in all parts of it, without either sink- 

 ing or rising to its surface, and the particles of the 

 amber being visible in the brine will, by their motions, 

 indicate the motions and directions of the currents in 

 the liquid, which take place when the temperature of 

 the liquid is changed.* 



If now two like glass tumblers be filled, the one 

 with the pure brine moderately heated, the other with 

 an equal quantity of the same brine at the same tem- 



* In order that the brine may be rendered perfectly transparent, it should be 

 filtered or made to pass through filtering paper. 

 VOL. iv. 40 



