IX, A, 4 Cox et al. : Water Supplies in the Philippines 377 



through a comparatively thin layer comes out thoroughly softened. The 

 substance may be regenerated by treatment with warm 10 per cent salt 

 solution. The cost of softening water by this method is about 1.25 to 1.5 

 centavos per cubic meter. 



Crude oil has been suggested to assist in the prevention of scale in steam 

 boilers, but it is injurious in that its residual tar combines with the sedi- 

 ment to form an undesirable scale. Kerosene has been used in the preven- 

 tion of scale without deleterious results," because this is the fraction of 

 crude petroleum from which the naphtha or volatile products have been re- 

 moved in the process of refining and it has no residual product. The action 

 in this case is mostly mechanical in that it forms a protective greasy film 

 over the interior boiler parts and greases the precipitated matter so that 

 it does not coagulate but remains in suspension, in which condition it can 

 be blown off easily. Kerosene is very satisfactory as a scale preventive, but 

 it should be handled with care, and it must be remembered that it will 

 attack the rust and open up leaky parts and is apt to be carried in the steam 

 in sufficient quantity to injure the rubber packing of the engine and of all 

 valves. Tannin and other wood extracts are excellent scale preventives in 

 that they cause the precipitate to form as soft scale or sediment that can be 

 easily removed from a boiler by blowing off. On the other hand, some wood 

 extracts contain acid in sufficient quantity to attack the metal of the boiler. 



Of the chemical softeners, soda ash (crude sodium carbonate) and 

 lime are the ones most widely used. From the chemical analysis of 

 the water, the necessary amounts of each of these can readily be calculated. 



According to Stabler " the number of pounds '" of 90 per cent lime 

 (CaO) required=:0.00931Fe + 0.0288A1 + 0.0214Mg + 0.258H -|- 0.00426 

 HCOa -|- O.OII8CO2; and the number of pounds of soda ash (95 per cent 

 Na.COs) =:0.0167Fe + 0.0515A1 -1- 0.0232Ca + 0.0382Mg + 0.462H - 

 0.0155COr, ~ 0.00763HCO3 for every 1,000 gallons *' of water. 



CORROSION 



There are many causes of corrosion, and a substance which may resist 

 one cause may readily yield to another. Boiler corrosion does not neces- 

 sarily depend on the quality of the boiler plate; in fact, steel often 

 corrodes more quickly than wrought iron." Continued or interrupted use 

 of a properly cared for boiler has little to do with corrosion, but when not 

 properly cared for a boiler in continued use will not corrode as rapidly 

 as one in interrupted use. Stabler " has calculated the "coefficient of corro- 

 sion," c=H -1- 0.1116A1 -f 0.0361Fe + 0.0828Mg - 0.0336CO3 - O.OI65HCO3. 

 If c is positive, the water will certainly corrode a boiler. If c+ 0.0503 Ca 

 is negative, the mineral constituents in the water will not cause corrosion. 

 If c is negative but c-f 0.0503 Ca is positive, corrosion may or may not 

 occur, the probability of corrosive action varying directly with the value of 

 the expression c -1-0.0503 Ca. 



All natural waters contain oxygen in solution, and this is continually 

 being introduced into the boiler by means of the feed pump and often 



"Lyne L. F., Trans. Am. Soc. Mech. Eng. (1888), 10. 



" U. S. Geol. Surv., Water-Supply Paper (1911), 274, 170. 



'"1 pound=0.45359 kilogram.. 



""1 U. S. gallon =3.78543 liters. 



"Eng. News (1903), 50, 286, 296, and 502. 



"Loc. cit., 175. 



