5<D ANSWERS TO PRACTICAL QUESTIONS 



with terrific violence, commenced their experiments with 

 prayer, and placed on their crucibles the sign of the cross 

 hence the name crucible from crux (gen. crucis), a cross. All 

 these manifestations were supposed to be the work of invisible 

 spirits, to whom the name gahst or geist, a ghost or spirit, 

 was applied. The miners were in special danger from these 

 unseen adversaries, and it is said that their church service con- 

 tained the petition, ''From geists, good Lord, deliver us!" 

 The names spirits of wine, nitre, &c., are a relic of the 

 superstitions of that time. 



3. If the air iv ere pure O, what bodies would escape com- 

 bustion in a conjlagration ? 



The stones, mortar, &c., which being already combined 

 with O and other elements, and having their chemical affinities 

 satisfied, are hence termed "burnt bodies." 



4. Why will lime added to hard water often soften it ? 

 The lime will combine with the free carbonic acid absorbed 



by the water. This renders the water incapable of holding in 

 solution as much carbonate of lime as before, which is then 

 precipitated, and the water thus partly softened. 



5. Why will stirring a wood fire quicken the flame, but a 

 coal fire, will deaden it ? 



Stirring a fire lets in more O, which quickens a wood fire 

 but reduces the temperature of a coal fire below the point of 

 union between O and coal. It is really based on the fact that 

 a higher temperature is requisite to burn coal than wood. 



6. Why does blowing on a fire quicken the flame, but on a 

 lamp extinguishes it ? 



The same principle applies as in the last question. In ad- 

 dition, the force of our breath often drives the flame off the 

 wick mechanically. 



7. Why will oyster-shells placed on the grate of a coal Jire 

 prevent the formation of clinkers? 



The lime of the shells forms a flux with the silicates con- 

 tained in the coal, and thus renders them more fusible. 



