PROM THE CHALK OF HERTFORDSHIRE. 
131 
of lime or calcium carbonate, a chemical combination of carbon 
dioxide and calcium oxide or lime in the proportion of one equiva¬ 
lent of each, and is insoluble in water ; but the addition of a second 
equivalent of carbon dioxide converts it into bicarbonate of lime, 
which is soluble in water. Water which has percolated through 
chalk, and thus holds bicarbonate of lime in solution, is termed hard. 
By adding to such hard water a certain amount of lime-water, pre¬ 
pared by driving off by heat the carbon dioxide in chalk, to convert 
the chalk into calcium oxide, or quick-lime, and then slaking this 
by adding water, the bicarbonate of lime is again converted into 
a carbonate, the previously added equivalent of carbon dioxide for¬ 
saking the bicarbonate, which it leaves as a carbonate (chalk), for 
the lime in the lime-water, which it converts back again into the 
carbonate (chalk) from which it was made. This is the reaction 
upon which is founded Clark’s process of softening hard water, the 
chalk thus re-formed sinking by gravitation, and leaving in the 
water only a very small quantity of bicarbonate of lime. 
Pure water is perfectly colourless; every ray of light falling 
upon it is either reflected from its surface or absorbed, so that if we 
could look upon an infinite depth of water absolutely free from 
matter in suspension, it would appear as black as ink. A very 
slight admixture of solid matter in fine suspended particles gives it 
a deep indigo-blue colour; a little more, an ultramarine ; while, as 
the quantity increases, the colour changes to a bluish and then to 
a yellowish green. This is caused by the gradual absorption of 
heat by the water. The invisible thermal rays are first absorbed, 
then at greater and greater depths the rays which are both thermal 
and visual,—red, orange, yellow, green, and blue in succession,—the 
light we see being that reflected from matter in suspension, from 
deeper and deeper water as such matter is less and less in quantity. 
Professor Tyndall has shown"* that this is the cause of the varying 
colour of the sea, and it also accounts for the beautiful blue and 
bluish-green colour of the water in the subsidence tanks used in 
Clark’s softening process. 
Water passes from the fluid to the solid or the gaseous state with 
comparatively moderate changes of temperature and pressure. It 
is at its greatest density at 39°-34 Pahr. It dilates uniformly from 
this point with increase and decrease of temperature, so that at 
46°‘68 it has the same volume as at 32°. At this latter point 
a slight movement will cause it to freeze, when it suddenly expands 
with an enormous force, but if kept perfectly still it may be cooled 
down to from 10° to 20° without congelation. TJnder a barometric 
pressure of 30 inches it usually boils at 212° Pahr. ( = 100° Centi¬ 
grade), but here again perfect stillness will retard the ebullition. 
If the pressure of the air were doubled, it would not boil until its 
temperature rose to 240° ; if the pressure were altogether removed, 
it would boil at 67°. The reduction in pressure on very high 
mountains makes cooking by boiling almost impossible, the water 
flying off as steam at too low a temperature. 
* ‘ Fragments of Science,’ vol. i, pp. 218-226 (6th Ed. 1879). 
