8o Laboratory Arts 



will be pale green to dark green, not to be confounded with 

 the distinct opalescent blue of Jena tube or the hard " white " l 

 of combustion or ordinary hard glass. At the same time some 

 very good soda-glass has a distinctly pink colour, and again, 

 short pieces show practically no colour, so that little satisfaction 

 is to be gained in judging by colour alone. 



Owing to its lower melting point, however, soft glass has 

 a more perfectly transparent appearance, and a smoother 

 surface than hard glass, and is usually quite free from the 

 longitudinal streaky appearance found in the harder glasses. 

 Occasionally one sees a piece of soft soda glass with a streak 

 down it, but such a piece should be rejected. The streaks in 

 soda-glass are mostly due to air bubbles, and these will be 

 found very inconvenient during the working. 



While in the flame, soda-glass gives its characteristic yellow 

 colour almost at once ; hard glass does not show this yellow 

 colour for some time, though, as sodium is present in most 

 glass, it may always be expected to show more or less yellow 

 in the flame. 



In addition to indicating the presence of sodium almost 

 instantly in this way, soda-glass melts with great ease, and 

 shows a peculiar greenish translucent tint when ready for 

 blowing. By this it is most easily recognized, though it is 

 almost impossible to describe the appearance. 



The ease with which soda-glass melts in the blowpipe flame 

 will be the cause of some little trouble to the beginner, who 

 usually collects too much melted glass for his purpose. After- 

 wards he frequently collects too little. 



II. TOOLS REQUIRED IN GLASS BLOWING 



The Blowpipe. No article of equipment is more im- 

 portant than the blowpipe used in glass working, and no 

 article is more difficult to obtain. It is quite the exception 

 to enter a strange laboratory and find in it a blowpipe suitable 

 for glass working. Blowpipes of the old-fashioned " Herapath " 

 type are quite useless, as are, more or less, most of the " fancy " 



1 " White" is a trade name it should be, of course, colourless. 



