STEAM NAVIGATION. 



steam, and, mixed with the condensed steam, it is thence con- 

 ducted as feeding water into the boiler. 



Sea water holds, as is well known, certain saline and alkaline sub- 

 stances in solution, the principal of which is muriate of soda, or 

 common salt. Ten thousand grains of pure sea water contain two 

 hundred and twenty grains of common salt, the remaining ingre- 

 dients being thirty-three grains of sulphate of soda, forty-two grains 

 of muriate of magnesia, and eight grains of muriate of lime. The 

 heat which converts pure* water into steam does not at the same 

 time evaporate those salts which the water holds in solution. As 

 a consequence it follows, that, as the evaporation in the boiler is 

 continued, the salt, which was held in solution by the water 

 which has been evaporated, remains in the boiler, and enters into 

 solution with the water remaining in it. The quantity of salt 

 contained in sea water being considerably less than that which 

 water is capable of holding in solution, the process of evaporation 

 for some time is attended with no other effect, than to render the 

 water in the boiler a stronger solution of salt. If, however, this 

 process be continued, the quantity of salt retained in the boiler 

 having constantly an increasing proportion to the quantity of 

 water, it must at length render the water in the boiler a saturated 

 solution ; that is, a solution containing as much salt as, at the 

 actual temperature, it is capable of holding in solution. If, 

 therefore, the evaporation be continued beyond this point, the salt 

 disengaged from the water evaporated, instead of entering into 

 solution with the water remaining in the boiler, will be precipitated 

 in the form of sediment ; and if the process be continued in the 

 same manner, the boiler would at length become a mere salt-pan. 



But besides the deposition of salt sediment in a loose form, some 

 of the constituents of sea water having an attraction for the iron 

 of the boiler, collect upon it in a scale or crust, in the same manner 

 as earthy matters, held in solution by spring water, are observed to- 

 form and become incrusted on the inner surface of land-boilers 

 and of common culinary vessels. 



The coating of the inner surface of a boiler by incrustation, and 

 the collection of salt sediment in its lower parts, are attended 

 with effects highly injurious to the materials of the boiler. The 

 crust and sediment thus formed within the boiler are almost non- 

 conductors of heat, and placed, as they are, between the water 

 contained in the boiler and the metallic plates which form it, they 

 obstruct the passage of heat from the outer surface of the plates 

 in contact with the fire, to the water. The heat, therefore, accu- 

 mulating in the boiler-plates so as to give them a much higher 

 temperature than the water within the boiler, has the effect of 

 softening them, and by the unequal temperature which will thus 

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