208 SMITH'S INTERMEDIATE CHEMISTRY 



Thus, when glass is covered with melted paraffin to protect the 

 surface, and marks or letters are made by removing the paraffin 

 with a sharp instrument, hydrogen fluoride will decompose the 

 glass at the parts thus exposed (test for fluorine) . In this way the 

 graduation on thermometer stems and lettering on glass are 

 frequently made. The vapor gives rough, easily visible depres- 

 sions, the solution smooth, glossy ones. 



On account of this property, hydrofluoric acid is used for re- 

 moving adhering sand from castings and for cleaning the outsides 

 of granite and sandstone buildings. 



THE HALOGENS AS A FAMILY 



The reader is recommended to compare carefully the properties 

 of the several halogens and their compounds with hydrogen. 

 It will be found that, while very striking similarities exist 

 throughout the whole halogen family in the case of nearly 

 every physical and chemical property, there is always a 

 regular gradation in properties in the order of the atomic 

 weights, namely F, Cl, Br, I. A few examples are noted 

 below; the student should tabulate the rest himself for his 



own convenience. 



aaexxyvx^uia 



Color in liquid state: F 2 yellow; C1 2 orange-yellow; Br 2 brown; 

 I 2 deep violet. 



Boiling-point: F 2 -187; C1 2 -34; Br 2 59; I 2 184. 



Action on hydrogen: F 2 very rapid action in cold, without 

 light; C1 2 action rapid in cold, only with strong light; Br 2 

 action rapid only when heated; I 2 action slow and incom- 



plete even when heated. -~ * ^ JL 



As an aid to the memory, such a table is exceedingly valuable. 

 But it is really not necessary to attempt to memorize all the prop- 

 erties of each halogen and of each halogen compound separately. 

 The properties of bromine, for example, are all intermediate 

 between those of chlorine and iodine. Indeed, when bromine 



-U* 



es 

 { 



