COMMON THINGS. WATER. 



Passing next through the coils of the worm, it is exposed to the 

 contact of its cold surface, and is entirely condensed and reduced 

 to the liquid state before it arrives at the lower extremity, a, from 

 which it trickles in drops into the jar, c. 



The heat disengaged from the vapour in the process of conden- 

 sation being constantly imparted to the water in the cistern, 

 P QJ r > that water would be gradually warmed, and if it were not 

 discharged and replaced by cold water, it would no longer keep 

 the worm cold enough to condense the vapour. A supply of cold 

 water is therefore introduced through a pipe, T T, while the 

 heated water flows away through the pipe of discharge, o. 



Heated water being lighter, bulk for bulk, than cold water, will 

 float upon the latter without mixing with it, unless the liquid be 

 agitated. The cold water, therefore, being introduced at the 

 lowest part, T, of the cistern, will form the inferior strata, while 

 the heated water will collect at the superior strata, and being 

 pressed upwards by the cold water will flow out at o. The supply 

 pipe, p, which feeds the pipe, TT, and the discharge pipe,- o, 

 may be, and generally are, so regulated that the water discharged 

 from o is very little below the temperature of the vapour coming 

 from the boiler, while the water of the lowest strata is as cold as 

 the external atmosphere. The vapour, therefore, which enters at 

 (?, is at first only partially condensed, the condensation being 

 rapidly increased, as winding through the worm it passes in 

 contact with a surface colder and colder, until, at length, arriving 

 at the lowest coil, it is wholly condensed. 



The heated water which flows from the discharge pipe, o, may 

 be used to feed the boiler, E ; and being already at a high tempe- 

 rature, an economy of fuel is thus effected. 



When extreme purity is required in the distilled water, it is 

 evaporated at a temperature lower than 212, because at that 

 temperature a certain small portion of the foreign matters which 

 it holds in solution sometimes go over in the vaporous state 

 through the worm, and are ultimately deposited in the jar, c. 

 The lower the temperature at which the water in the boiler is 

 evaporated, the less of this impurity will pass through the worm. 



By these expedients, with proper precautions, water absolutely 

 pure, and entirely free from all foreign matter, may be obtained. 



35. It remains now to show how the compound nature of this 

 liquid can be demonstrated, and the characters and proportions of 

 its constituents ascertained. 



This may be accomplished by either of two methods ; by 

 COMPOSITION or DECOMPOSITION, or, if the Greek derivatives be 

 preferred, by SYNTHESIS or ANALYSIS. 



The method by synthesis presumes the previous knowledge of 

 106 



