PREPARATION OF FRESH WATER FROM BRINE. 45 



the water is left behind in the retort or still, and pure water 

 obtained after condensation. 



Expt. 37. To prove that Fresh Water can be obtained from 

 Salt Water by Distillation. Place in the retort as above a tea- 

 spoonful of common salt and half fill the retort with water, and 

 warm until the salt is all dissolved. Pour out a few drops into a 

 wine-glass and add a little solution of nitrate of silver ; the two 

 colourless clear fluids will at once become turbid (just as in Expt. 

 12, No. 1 glass), a flocculent white solid precipitate being formed 

 which by and by falls to the bottom of the glass. This solid 

 matter is due to the chemical action of the salt on the nitrate of silver 

 by double decomposition; if pure water be used instead of salt 

 solution in another wine-glass, the addition of the nitrate of silver 

 will of course produce no result and the liquid will remain clear ; 

 so that the nitrate of silver thus serves as a chemical test or reagent 

 for salt, giving rise to the formation of a precipitate if salt be 

 there, but not if it be absent. 



Now distil off some of the water, taking care to avoid frothing 

 over of the brine in the retort, which would contaminate the dis- 

 tilled portion ; on testing the distilled fluid with nitrate of silver 

 it will remain clear, showing that the salt has not passed over with 

 the steam but is left behind. This chemical test is far more 

 delicate than the taste, and will easily detect small quantities of 

 salt quite imperceptible to the most sensitive palate ; it is, in fact, 

 so delicate that if the hands be dipped in some distilled water and 

 a few drops of nitrate of silver be then added, the liquid will 

 become more or less cloudy, owing to the formation of a small 

 amount of precipitate, due to the circumstance that salt (sodium 

 chloride) is contained in the perspiration, so that a minute quantity 

 becomes dissolved in the water from contact with the skin. 



This method of preparing fresh water from sea-water is largely 

 used aboard ships, especially steamers. Boilers fed with sea-water 

 will produce steam which on condensation is free from the salt and 

 other impurities of the sea-water, and can be used for cooking 

 directly. For drinking purposes this distilled sea-water is un- 

 pleasantly flat, like boiled water, and for the same reason, viz., 

 that the air and other gases present which give the fresh taste to 

 ordinary spring water, have been removed by the heating. Con- 

 sequently it is usual to aerate and freshen up the distilled water 

 before using it for drinking in order to improve its taste ; but for 

 boiling vegetables, making tea and coffee, &c., this treatment is not 

 necessary. In certain tropical cities, where a supply of good 

 natural water is difficult to obtain all the year round owing to the 

 comparatively short period when rain falls and the difficulty of 



