o.\ WATKi; AND ITS COMPOUNDS 



1!) 



receiver. By standing exposed to the atmosphere, however, the water 

 in time absorbs air, and dust carried in the air, and ceases to be en- 

 tirely pure. However, the amount of impurities in distilled water is 

 so small that they have hardly any effect on the properties of the 

 water, and it is fit for many purposes. Nevertheless, in distillation, 

 water retains, besides air, a certain quantity of volatile impurities 

 (especially organic) and the walls of the distillation apparatus are 

 partly corroded by the water, and a portion, although small, of their 

 substance renders the water not entirely pure, thus a sediment is ob- 

 tained on evaporation'. 8 



Still, for certain physical and chemical researches it is necessary to 

 have completely pure water. To obtain it a solution of potassium 

 permanganate is added to distilled water until it all becomes tinted 

 light rose colour. By this means the organic matter in the water is 

 destroyed (converted into gases or non-volatile substances). An excess 



metal still with worm condenser (fig. 4), or on a small scale in the laboratory in a glass 

 retort (fig. 5) heated by a lamp (see footnote 19, Introduction). Fig. 5 illustrates 

 the main parts of the usual glass laboratory apparatus used for distillation. The steam 



FIG. 5. Distillation from a glass retort. The neak of the retort fits into the inner tube of the 

 Ltebig's condenser. The space between the inner and outer tube of the condenser is filled with 

 col< I water, which enters by tliu tube g and Hows out at/. 



issuing from the retort (on the right-hand side) passes through a glass tube surrounded 

 by a larger tube, through which a stream of cold water passes, by which the steam is 

 cuii.lcnsed and trickles into a receiver (on the left-hand side). 



8 One of Lavoisier's first memoirs (1770) referred to this question. He investigated 

 the formation of the earthy residues in the distillation of water in order to prove whether 

 it was possible, as was affirmed, to convert water into earth, and he found that the 

 residue was produced by the action of water on the walls of the vessel holding it, a:id 

 not from the water itself. He proved this to be the case by direct weighing. 



VOL. I. E 



