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dwell in marble halls, but whether we live in palaces or small 

 suburban villas, in baronial castles or thatched cottages, whether 

 the walls are of brick or of stone, mortar made with lime is prac- 

 tically universal. Important as is this use of lime it is not the only 

 one, and we now go on to mention a few of the others. As I have 

 already said, lime properly so called, i.e., the oxide of the metal 

 calcium, is very infusible. Heated to a very high temperature by 

 a flame of hydrogen or coal gas fed with oxygen, lime glows with 

 a pure white light. This is the lime-light that Mr. Thomas, the 

 Society's lanternist, manipulates so well. As lime is infusible at 

 the temperature of the oxy-hydrogen flame, platinum can be 

 melted in a cavity excavated in a block of it. A couple of pounds 

 was melted in this way as far back as 1847, and since then the 

 methods have been improved and much larger amounts can now 

 be dealt with. In the electric furnace even higher temperatures 

 can be obtained than in the oxy-hydrogen one. Here, again, the 

 infusibility of quick-lime has rendered it of the greatest service. 



Carbon dioxide gas, which is given off from chalk when 

 strongly heated, can also be obtained by treating the chalk with 

 an acid. It is in this way that most of the gas used by the manu- 

 facturers of soda water, lemonade, and other effervescing drinks 

 is obtained. Such beverages are often called mineral waters and 

 the mention of this name suggests another and most important use 

 of lime, namely, the softening of naturally hard water. Although 

 I gave some account of this in my earlier lecture I can hardly 

 pass it over without further reference now. As I have already 

 said so many times that you will be weary of hearing it, limestone 

 and chalk are chemically identical substances composed of lime 

 and carbonic acid, or, more strictly, carbon dioxide. Now lime 

 as such is soluble in water. One equivalent of lime combined with 

 one equivalent of carbonic acid is the insoluble carbonate, but 

 one equivalent of lime with two equivalents of the acid is the 

 soluble bicarbonate. Water from springs and wells in chalk or 

 limestone contains this bicarbonate in greater or less amount. 

 When much is present the water is unpleasantly hard, tends to 

 waste of soap, and is in various other ways objectionable. In 

 order to soften it the proportion between the equivalents of lime 

 and carbonic acid must be altered from 1:2, giving the soluble 

 bicarbonate, to 1: 1, giving the insoluble carbonate. The neces- 

 sary alteration in the proportions can be brought about either by 

 reducing the amount of carbonic acid or by increasing the amount 

 of lime. When the hard water we are speaking of is boiled it 

 is softened in the former of these ways. Carbonic acid is expelled 

 as gas and the resulting carbonate separates as a crust or fur on 

 the bottom or sides of the vessel. Such a plan for softening water 

 is admirable for a pint or two or even a few gallons, but is 

 obviously impossible to apply to the water supply of a town. In 

 such a case we can fall back upon our second method. Instead 

 of bringing the proportion between the equivalents of lime and 

 carbonic acid from 1 : 2 to the desired value of 1 : 1 by decreasing 



