380 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



Coffey's, 1 represented in Fig. 261. We may refer to Wurtz's Dic- 

 tionary of Chemistry for a description of these, as it is sufficient here 

 just to indicate their physical principle. 



IV. EVAPORATION OF SALT WATERS. WATER-COOLEKS. 

 MANUFACTURE OF TOE IN BENGAL. 



A great part of the salt (sodium chloride) we require comes from 

 sea-water, of which it forms about T a T or ^\ part. By evaporation in 

 the open air in large shallow basins, the sea water is concentrated by 

 degrees, and the salt is deposited on the bottom of the basins in the 

 form of crystals, and on the surface of the water in a thin solid crust. 

 The evaporation is hastened by the rise of temperature due to the 

 sun's rays, and by the wind. It is therefore in the hot season that the 

 salt is collected in the brine pits, and the series of very simple mani- 

 pulations carried out that constitute this industry. The salt is piled 

 up in heaps which are left exposed to the air for a certain length of 

 time to allow of the diliquescent substances which may be mixed 

 with it to dissolve ; the salt thus drained is afterwards sent into the 

 market. 



Salt is also obtained by the evaporation of the water of salt 

 springs, but as these contain ordinarily only a small proportion of 

 salt, they have first to be concentrated by being submitted to a prior 

 evaporation in the open air, after which the process is completed by 

 submitting the concentrated waters to the action of heat. The salt is 

 deposited in the boilers by which this second operation is carried on. 



The evaporation of salt waters in the open air is accomplished in 

 the following manner. Heaps of faggots are piled up and supported 

 by a frame-work fixed over the basin in which the water is received 

 (Fig. 263), this water escapes by a series of flow pipes a,a. . . , from the 

 troughs AB, CD, situated on the upper part of the frame-work, to 

 \\hich is given the name of graduation pile (bdtiment de graduation] , 



1 "To give an idea of the dimensions of Coffey's apparatus, we need only 

 mention that Messrs. Currie, of Bow, obtain annually more than 4,675,000 litres of 

 alcohol, 65 degrees above proof, from the distillation of the fermented must of barley 

 and oats, with the addition of malt. This single house pays a duty of ,400,000 

 per annum." Dictionary of Chemistry, Art. "Alcohol." 



