The "traditional" way of cutting glass tubing is by scoring and snapping it. This technique works well for small-diameter tubing and long lengths of large-diameter tubing where you have sufficient leverage on the tube to break it. However, it's pretty useless for cutting the likes of test-tubes into short lengths, as was required for the Houskeeper seal. The other way of cutting tubing which is widely described is to make a scratch right round the tube and then heat it using a heated wire. This does work, but it requires a hefty current source to heat the wire quickly enough, and it has problems with uniform heat transfer from the wire to the glass.
A method which overcomes these problems is to make the scratch on the lathe and then use a micro torch flame to sharply heat the scratch as it's rotating. I have used this method to cut lots of 1" test-tube glass into a variety of lengths, some as short as ¼". It works with both soda-lime and borosilicate glass. I made a video showing the process, see below. The torch used is the little micro-torch I made and is using an oxy/propane mixture.
22/11/10: I later discovered a much better way of scoring the glass on the lathe. Instead of using the point of the carbide tool, I used the corner edge of the long 1/8" square carbide blank. This still made a perfect scratch around the tube, in fact probably a better scratch, and the edge of the carbide blank lasts a lot longer than the point. It's really just like a proper carbide knife which is what's properly used for this job!
03/05/11: I recently discovered the company Adams & Chittenden Scientific Glass, from California. They have an absolutely wonderful website at http://www.adamschittenden.com/, with loads of information and photos of various glasswork. More relevant to the content on this page, they also have a great video of cutting large-diameter (over 30cm!) glass tubing using the scratch-and-heat method. Check it out here (MOV file): http://www.adamschittenden.com/downloads/Scribe-cut%20300mm%20tubing.mov.
16/05/11: Here is a better video showing how to use the edge of the carbide blank, as described above: