Il ANSWERS TO PRACTICAL QUESTIONS 



8. Upon what fact depends the formation of stalactites ? 



Water containing carbonic acid in solution will dissolve 

 carbonate of lime freely, but when, on exposure to the 

 air, the gas escapes, the carbonate is deposited. 



9. Why is HF kept in gutta-percha bottles ? 



Because it will dissolve silica, and so destroy a glass 

 bottle. 



10. Explain the use of borax in softening hard water? 



It softens hard water by uniting with the soluble salts 

 of lime or magnesia, and making insoluble ones which 

 settle and form a thin sediment in the bottom of pitchers 

 in which it is placed. 



1 1 . How are petrifactions formed ? 



Certain springs contain large quantities of some alka- 

 line carbonate ; their waters, therefore, dissolve silica 

 abundantly. If we place a bit of wood in them, as fast 

 as it decays, particles of silica will take its place atom 

 by atom and thus petrify the wood. The wood has 

 not been changed to stone, but has been replaced by stone. 



12. In what part of the body, and in what forms , is 

 phosphorus found ? 



As a phosphate if is the principal earthy constituent of 

 the bones. It is also a never-failing ingredient of the 

 brain and nervous system. The susceptibility of phos- 

 phorus to oxidation especially adapts it to the rapid 

 changes incident to the structure and offices of the 

 brain.* 



* Phosphorus is an element which can imperceptibly and quickly pass from 

 a condition of great chemical activity to one of equal chemical inertness. In 



