THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
75 
acid gas dissolved in water) present. The soluble substance 
formed by the action of carbonic acid on a carbonate is usually 
called a bicarbonate. On boiling the water, any carbonic acid 
gas present is slowly driven off, and the carbonates of lime or 
magnesia previously dissolved by it thrown down as a preci¬ 
pitate—the fur of the tea kettle, and the incrustation of the 
steam boiler—and the water is to that extent softened; hence 
the term “ temporary ” to hardness arising from the presence 
of carbonates. A process adopted on a larger scale for 
softening water is that known as Clark’s process ; it is carried 
out by adding a sufficient amount of lime to the water to take 
up all the carbonic acid gas present. The two substances 
form carbonate of lime, which is thrown down, together with 
the carbonates previously dissolved by aid of the carbonic 
acid now otherwise engaged. 
Permanent hardness is due chiefly to the presence of the 
sulphates of lime and magnesia in water, and in this case also 
it is the lime salt that is most commonly present. There 
being no workable process for removing the sulphates from 
water, the hardness arising fiom their presence is termed 
permanent. Chloride of calcium may sometimes contribute 
to the permanent hardness. 
The total solid residue obtained on evaporating a water may 
be made up of a number of substances, only part of which give 
to water the property of hardness. The others may or may 
not indirectly contribute to the hardness, or be injurious 
themselves. 
The question of hardness and total solids deserves some 
little consideration, because the Marlstone water is certainly 
rather hard, and that previously supplied to the town always 
contained about fifty grains of solid matter per gallon, an 
amount which it is usually considered should not be exceeded 
in a good drinking water.* From my own observations I have 
no doubt that the water was good and wholesome, and not 
injurious to the public health, though one or two instances 
came under my notice where the water appeared to produce 
some derangement of the digestion with persons using it, for 
the symptoms ceased when they boiled the water before 
drinking. The general opinion appears to be that within 
considerable limits there is no difference between hard and 
soft waters as regards health, and that habit has a great deal 
to do with the matter ; a change from one kind of water to 
* For a considerable amount of evidence on this and other similar 
matters by Doctors Frankland, Lyon Playfair, Parkes, Miller, Ac., see 
“ National Water Supply,” a pamphlet published by the Society of 
Arts in 1878. 
