8 VETERINARY HYGIENE 



oldest rocks are the softest, while those from chalk and limestone 

 are the hardest. Intermediate between these two extremes are the 

 moderately hard waters collected from the sandstones and shales. 



The Rivers Pollution Commissioners tabulate the relative hard- 

 ness of waters according to their source as follows, descending from 

 softest to hardest : 



(1) Rain water; (2) upland surface water; (3) water from 

 cultivated land; (4) river water; (5) spring water; (6) deep- 

 well water; (7) shallow-well water. 



Shallow wells may, however, contain soft water. 



The hardness of a water may be indicated conveniently by 

 (1) expressing the number of grains of calcium carbonate per gallon 

 of water as so many degrees of hardness ; or (2) in parts of calcium 

 carbonate per 100,000, 1 per 100,000 being 1 degree. The first 

 method is expressed as so many " degrees Clark." The latter 

 method is being generally adopted. Since one gallon of water 

 weighs 70,000 grains, degrees Clark can be readily converted into 

 parts per 100,000 by multiplying by 10 and dividing by 7. To 

 convert parts per 100,000 into degrees Clark, multiply by 7. 



The following table is a classification of water according to 

 its hardness : 



Parts of 



Description. CaCOs per Degrees Clark. 



100,000. 



Very soft 5 3 5 



Fairly soft 5-10 3-5-7 



Medium 10-15 7-10-5 



Fairly hard 15-20 10-5-14 



Hard 20 - 30 14 - 21 



Very hard 30 and over 21 and over 



The Significance of Hard and Soft Waters. Hard waters 

 possess many economic drawbacks. When water is heated suffici- 

 ently to drive off the CO 2 the calcium and magnesium carbonates 

 are precipitated, forming the " fur " in kettles and boilers. As this 

 deposit increases there is a corresponding decrease in the available 

 heat, with consequent financial loss. " Boiler scale " is due to the 

 deposition of the sulphates, which become thrown out of solution 

 when the water is heated under pressure. The wastage of soap 

 when hard water is the only water available for washing purposes 

 is very great compared with what it would be if the water were 

 moderately soft. With hard waters the waste pipes from kitchens 

 and lavatories become coated with the insoluble stearate, palmitate 

 and oleate salts, which on decomposing make the pipes foul. 



