CUTTING AND BENDING GLASS TUBES. 57 



bend the tube before it is thoroughly hot and soft enough, other- 

 wise it is likely to snap, or at least to give an ugly and misshapen 

 bend ; also not to draw the ends asunder, as this will thin the 

 glass at the bend and make it more fragile there. If, on the 

 other hand, the tube is wanted to be tapered off instead of bent, 

 as is sometimes the case, the glass is heated till soft and then the 

 ends pulled asunder by a slow steady motion, the drawn-out part 

 being cut by means of a file when cold. In the same way thin 

 tubes may be drawn out from thicker ones if required. 



The process of making glass tubes in the first instance is carried 

 out in much the same way. A glass-blower collects a lump of 

 hot soft nearly molten glass at the end of a hollow " blow-pipe," 

 and blows into this so as to swell out the glass into a sort of 

 enormously thick bubble ; another workman sticks a second rod 

 on to the end of the bubble by means of a little melted glass 

 adhering to the rod, and the two then rapidly move asunder so as 

 to pull out the bubble into a long comparatively thin tube ; the 

 glass cools quickly as it is pulled out, so that when the long tube 

 is complete the glass is sufficiently nearly solid to retain its shape. 



Like most other operations in shaping molten glass, a consider- 

 able degree of practice and dexterity is here requisite to obtain a 

 good result. 



CHAPTEK IV. 



DIRECT PASSAGE FROM SOLID TO VAPOROUS STATE 

 AND VICE VERSA. 



When a vapour is suddenly chilled to a temperature not only 

 below the boiling-point of the body in the liquid condition, but 

 also below the freezing-point of the substance, the vapour often 

 condenses directly into the solid condition without ever becoming 

 visibly liquefied. Thus on a cold autumn or winter's night, the 

 aqueous vapour in the air becomes chilled by contact with cold 

 bodies and condenses as hoarfrost (Expt. 44) or particles of solid 

 ice ; whilst snow is similarly producible when a current of warm 

 moist air meets with another current of intensely cold air, the 

 condensed aqueous vapour making its appearance as a crystallised 

 solid instead of liquid particles of mist or rain. 



Many other substances behave in similar fashion ; thus in the 



