104 3. HOPKINSON — EELATIVE ADVANTAGES 



soap-destroying power is equal to that exerted by one grain of 

 carbonate of lime (existing as bicarbonate) in one imperial gallon 

 of water (weighing 70,000 grains). 



It is well known that lime is made by driving off from chalk, 

 by means of heat, the carbonic acid which it contains. Chalk, or 

 calcium-carbonate, is thus converted into lime, or calcium- oxide, 

 and carbonic acid gas, or carbon-dioxide, which escapes into the 

 air. The lime has then a great affinity for its former partner, 

 carbonic acid gas, and it is in virtue of this affinity that slaked 

 lime softens water which is hard owing to the presence of 

 bicarbonate of lime, or calcium-bicarbonate, for it combines with 

 half its carbonic acid gas, thus forming chalk, and by this 

 deprivation the rest of the bicarbonate is also left as chalk. The 

 bicarbonate is soluble in water, but the carbonate (chalk) is not, 

 or is only so to a very slight extent ; and therefore the chalk thus 

 formed is deposited as a line powder, or may be removed from the 

 water by filtration, thus rendering it soft. 



The hardening of water by bicarbonate of lime and its softening 

 by lime may be thus expressed quantitatively. A gallon of rain- 

 water charged with 7 grains of carbonic acid gas, which it may 

 take up from the air or from decaying vegetable matter, passing 

 through chalk, will carry with it in solution about VI ^ grains 

 of the chalk, of which 10 grains will be in chemical combination 

 with the carbonic acid gas, forming 23 grains of bicarbonate of 

 lime, and the water will be said to be of I7i degrees of hardness. 

 If now 9 grains of lime be added, they will combine witli the 23 

 grains of bicarbonate of lime and form 32 grains of chalk, for 

 7 grains of carbonic acid gas will have abandoned the bicarbonate 

 of lime, and have formed, with the 9 grains of lime, 16 grains 

 of chalk. The whole of these 32 grains of chalk can now be 

 removed from the water by settlement or filtration, leaving it with 

 only a grain and a half of chalk dissolved in it, and thus reducing 

 it from 17+° of hardness to U°. 



This is the reaction which takes place in the now well-known 

 and extensively-adopted method of softening water called Clark's 

 process, though it is by no means a complete explanation of tlie 

 process. For instance, in practice the lime is added in the form 

 of lime-water in the proportion of about one gallon of lime-water 

 to every ten gallons of hard water to be softened. In Clark's 

 original process, as adopted at the Colne Yalley Waterworks, the 

 chalk is allowed to subside to the bottom of a settling- tank ; 

 in the modification of it known as the Porter-Clark process, as 

 adopted at the Southampton Waterworks (and many others), it is 

 mechanically filtered away. 



We cannot boil hard water, wash in it, wash our clothes in it, 

 or cook oar food in it, without softening it, and at considerable 

 expense ; we boil it in a closed boiler, such as is used for heating 

 water for baths, at the risk of an explosion from a choked-up 

 pipe ; we cannot effectually cleanse our skin with it ; and our 

 clothes are sometimes rather washed away than cleansed by its use. 



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