jlpbil — sept. 1858.] Notes and Queries. 



143 



is almost entirely composed, the iron being casually present and 

 the excess of base being due to the gradual liberation of carbonic 

 acid, occasioned by the act of deposition and the presence of silicic 

 acid in a soluble form. The solubility of the alkaline earths is due 

 to excess of carbonic acid which as it is liberated leaves the sili- 

 cates of potash and magnesia with some carbonate and iron, in the 

 plastic form, at first, but becoming hard with lime. 



As a whole the substance thus hardened is insoluble in cold, 

 and but slightly so in water of higher temp. At 212°, however, 

 there is sensible action after a time. In diluted hydrochloric acid, 

 solubility ensues hastened by increased temp. Solution is attend- 

 ed by slight effervescence, some carbonic acid being liberated. 



The natural processes by which the changes just noticed are 

 brought about are probably the following. 



Water from rivulets, mountain torrents or streams always con- 

 tains some carbonic acid, although it may be a mere trace, or a 

 considerable quantity. This being mainly dependent on the 

 amount of vegetable matter, leaves, woody fibre, &c, over which it 

 has traversed, thus impregnated, the water becomes a powerful 

 solvent of alkaline earths, and other matters presented to it, in its 

 course through, or ever the various striata of the earth, and in this 

 way we find it holding silica alkalies, lime and magnesia as well 

 as iron in solution, chiefly in the form of bi-carbonates after water 

 has thus become a means of dissolving the substances named and 

 subsequently a mechanical agent for their transport from one loca- 

 lity to another. It remains but to show under what conditions the 

 substances dissolved are again deposited. These are numerous and 

 various, but to make them fully appreciated it will perhaps be the 

 simplest course to refer to a broad principle universally acknow- 

 ledged and established. I allude to the degree of force with which 

 atoms are held in combination and especially to the difference in 

 force by which a second atom of similar nature to the primary one 

 combined, is retained, thus as an illustration we may take one atom 

 of an earthy base or oxide, which may be Lime — i. e. oxide of cal- 

 cium, now if this atom of base meet with an atom of carbonic 

 acid, they will directly combine forming one atom of the salt 

 we commonly term chalk or more correctly carbonate of lime, 



