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duct heat much more slowly than the metals; when, therefore, 

 the surface, either of a plate or tube of metal, which should 

 be in immediate contact with the water, is so encrusted, the 

 caloric passes slowly through, and accumulates proportionably 

 in the metal, which oxidates with corresponding rapidity on 

 the side exposed to the fire. When this state of things 

 exists to a great degree, the fire which would raise, with a 

 clean boiler, the needful quantity of steam, becomes insuffi- 

 cient, and its increased heat augments the injury to the 

 boiler. 



Besides the injury to the different parts of the boiler, an 

 increased consumption of fuel, and even difficulty in obtaining, 

 by any means, a sufficiently rapid generation of steam, are 

 additional reasons for attention to the quality and selection 

 of water. The substances present in common spring or 

 river waters are soda, lime, magnesia, as bases ; and sul- 

 phuric acid, carbonic acid, and chlorine, which represents 

 hydrochloric or muriatic acid. Iron and organic matter, 

 either animal or vegetable, occur occasionally. Several 

 other substances, as potash, iodine, nitric acid, and others 

 are met with, either in proportions so minute, or with such 

 comparative rarity, that they need not be taken into account 

 on the present occasion. 



We may confine our attention, then, to the bases and 

 acids above-mentioned. Respecting the manner in which 

 these are combined or grouped by nature in the water in its 

 original state, chemists are by no means agreed. Carbonate 

 of lime, for instance, as such, is insoluble in water alone ; 

 yet it is separated from it frequently and abundantly, carbonic 

 acid being in most cases disengaged at the same time. Now, 

 one chemist believes that in these cases the lime is in the 

 state of a true bi-carbonate, and reports it to contain that 

 substance. Another chemist denies the existence of such a 

 salt as bi-carbonate of lime ; for he considers the excess of 



