SODIUM CHLORIDE-BEKTHOLLET'S LAWS 425 



water is pumped, run along the top. On flowing from these troughs, 

 through the openings, a, the water spreads over the brushwood and 

 distributes itself in a thin layer over it, so that it presents a very large 

 surface for evaporation, in consequence of which it rapidly becomes 

 concentrated in warm or windy weather. After trickling over the 

 brushwood, the solution collects in a reservoir under the graduator, 

 whence it is usually pumped up by the pumps P P', and again run a 

 second and third time through the graduator, until the solution reaches 

 a degree of concentration at which it becomes profitable to extract 

 the salt by direct heating. Generally the evaporation in the graduator 

 is not carried beyond a concentration of 12 to 15 parts of salt in 

 100 parts of solution. Strong natural solutions of salt, and also the 

 graduated solutions, are evaporated in large shallow metallic vessels, 

 which are either heated by the direct action of the flame from below 

 or from above. These vessels are made of boiler plate, and are called 

 salt-pans. Various means are employed for accelerating the evapora- 

 tion and for economising fuel, which are mainly based on an artificial 

 draught to carry off* the steam as it is formed, and on subjecting the 

 saline solution to a preliminary heating by the waste heat of the steam 

 and furnace gases. Furthermore, the first portions of the salt which 

 crystallise out in the salt-pans are invariably contaminated with gypsum, 

 since the waters of saline springs always contain this substance. It is 

 only the portions of the salt which separate later that are distinguished 

 by their great purity. The salt is ladled out as it is deposited, left to 

 drain on inclined tables and then dried, and in this manner the so- 

 called bay salt is obtained. Since it has become possible to discover 

 the saline deposits themselves, the extraction of table salt from the 

 water of saline springs by evaporation, which previously was in general 

 use, has begun to be disused, and is only able to hold its ground in 

 places where fuel is cheap. 



In order to understand the full importance of the extraction of 

 salt, it need only be mentioned that on the average 20 Ibs. of table salt 

 are consumed yearly per head of population, directly in food or for cattle, 

 In those countries where common salt is employed in technical pro- 

 cesses, and especially in England, almost an equal quantity is consumed 

 in the production ot substances containing chlorine and sodium, and 

 especially in the manufacture of washing soda, &c., and of chlorine 

 compounds (bleaching powder and hyrdochloric acid). The yearly pro- 

 duction of salt in Europe amounts to as much as 7J million tons. 



Although certain lumps of rock salt and crystals of bay salt some- 

 times consist of almost pure sodium chloride, still the ordinary com- 

 mercial salt contains various impurities, the most common of which are 



