Jena Glass Working Silica 117 



desired i.e. rotation should be rather more rapid than with 

 soda-glass. 



Jena glass should not be brought into an oxy-coal gas flame 

 too rapidly care should be taken that it is heated up slowly 

 and regularly, finally with great care advancing it to the hot- 

 test portion of the flame. 



It may be well to remind students that while Jena glass is 

 refractory so far as the action of acids and alkalies is con- 

 cerned, and while it resists rapid changes of temperature, the 

 glass is not for these reasons specially strong in the ordinary 

 acceptation of the term. The placing of a litre Jena flask full 

 of water upon a grain of sand on a table will frequently result 

 in the shattering of the flask into very small pieces. More- 

 over, these flasks are unable to resist much difference between 

 internal and external pressure. It would appear that the glass 

 is under considerable molecular stress, is consequently rather 

 more dangerous to work with than ordinary glass, and requires 

 therefore more care. Its many advantages, however, more 

 than compensate for these disabilities. 



Fused Silica. This material is prepared by the frequent 

 fusion and quenching in water of " rock crystal," or quartz, the 

 purest kind only of which is used. It is very refractory, but 

 being light, transparent, practically unacted upon by acids, and 

 able to withstand great and sudden fluctuations in temperature, 

 it is rapidly displacing Jena glass-ware, and even ordinary glass 

 and porcelain vessels from the best laboratories. 



Silica is worked in the oxy coal-gas flame, and will be found 

 capable of manipulation in much the same way as glass, but 

 the eyes need the protection of blue glasses during work. 



Tubes and rod of this material are now supplied by most 

 dealers in apparatus, and owing to the great demand for the 

 ware, its manufacture has been commenced on a large scale, 

 and the price of the material has been considerably reduced of 

 late. Silica rod, pulled out into very fine threads by means of 

 a rotating wheel, or an arrow shot from a bow, produces quartz 

 fibres which are therefore now within the reach of the smallest 

 laboratory. 



Broken apparatus of fused silica may be repaired by fusion 



