SCIENCE OF COMMON THINGS. 195 



Volatile substances. What is distillation ? 



Because the current of air (which is always passing 

 over a heated surface) drives it along. 



1290 Why does a laundress put a little saliva on aflat-iron to know if 

 it be hot enough ? 



Because when the saliva sticks to the iron and is 

 evaporated, she knows it is not sufficiently hot ; but 

 when it runs along the iron, it is. 



1291 Why is the flat-iron hotter if the saliva runs along it, than if it 

 adheres till it is evaporated? 



Because when the saliva runs along the iron, the heat 

 is sufficient to convert the bottom q the drop into vapor 

 but if the saliva will not roll, the iron is not sufficiently 

 hot to convert the bottom of the drop into vapor. 



1292 To what substances do we apply the term " volatile ?" 



To those which have a great tendency to assume the 

 gaseous form. 



1293 To what substances do we apply the term "fixed," or "non-vola- 

 tile?" 



To those in which the tendency to assume the gaseous" 

 form is small. 



1294 Do vapors occupy much more space tlian the substances from which 

 they were produced f 



They occupy a much greater space ; water, in passing 

 from its point of greatest density into vapor, expands to 

 sixteen hundred and ninety-six times its volume. 



1295 Under what two heads does the subject of vaporization divide 

 itself? 



Into ebullition and evaporation 



1296 What is distillation? 



A process by which one body is separated from an- 

 other by means of heat, in cases where one of the bodies 

 assumes the form of vapor at a lower temperature than 

 the other ; this first rises in the form of vapor, which is 

 received and condensed in a separate vessel. 



1297* How is the process of simple distillation effected ? 



A peculiar-shaped vessel called a retort (Fig. 43) is 

 half filled with a volatile liquid and heated ; the steam, 

 as it forms, passes through the neck of the retort into a 



