58 CHEMISTRY OF FARM PRACTICE 



The normal calcium carbonate (CaCOs) is soluble only 

 to the extent of 2-^ grains per gallon. Magnesium bicar- 

 bonate, when heated, decomposes in the same way as cal- 

 cium bicarbonate, although the normal magnesium car- 

 bonate is soluble to the extent of 14 grains per gallon. 



As a second method, temporarily hard water may be 

 softened by chemical means as already shown. Four 

 pounds of quicklime will soften as much water as 80 

 pounds of soap, hence the use of the lime will be far more 

 economical. Needless to say, this treatment must take 

 place in a different receptacle from the boiler. The reac- 

 tion is the same as that previously given for milk of lime. 



Pure calcium carbonate does not produce a very hard 

 scale at first, but it hardens with heating and drying. 

 When it is heated rapidly, it deposits as mud; but when 

 heated slowly it forms calcite, which will become a hard 

 scale when baked. Magnesium carbonate behaves simi- 

 larly to calcium carbonate. 



Permanently hard water is less desirable as a boiler 

 supply. Calcium sulphate is more troublesome than cal- 

 cium carbonate, because it forms a hard and adhesive 

 boiler incrustant, beneath which the iron is often corroded 

 and overheated. 



Boiler water may, also, be helped by filtration, al- 

 though the suspended matter strained out by the filter 

 as a rule does not cause a deposit of scale. One very 

 simple precaution may save much trouble, never empty a 

 boiler while it is hot, because the incrustation in that case 

 will be baked on. Never blow out the boiler under steam 

 pressure, because the incrustation, becoming dry, absorbs 

 carbon dioxide from the air, which helps to fix the deposit 

 more firmly. 



