Now bring the tube into the blowpipe flame gradually, and rotate it, while heating, at the place where it is to be closed. Allow the glass to soften and commence to run together until the diameter of the tube is reduced to about half its original size. Remove from the flame and draw the ends apart, this should give a long, thick extension as shown by _f_, Fig. 4. If any liquid is to be introduced, it may now be done by inserting a thin rubber or other tube through the opening and running the liquid in. A glass tube should be used with caution for introducing the liquid, as any hard substance will tend to scratch the inside of the glass and cause cracking. The final closure is made by melting the drawn-out extension in the blowpipe flame; the finished seal being shown by _g_, Fig. 4. If the sealed tube has to stand internal pressure, it is desirable to allow the glass to thicken somewhat more before drawing out, and the bottom seal should also be made thicker. For such a tube, and especially when it has to stand heating, as in a Carius determination of chlorine, each seal should be cooled very slowly by rotating it in a gas flame until the surface is covered with a thick layer of soot, and it should then be placed aside in a position where the hot glass will not come in contact with anything, and where it will be screened from all draughts. _Joining Tube._--We will now consider the various forms of join in glass tubing which are met with in the laboratory. First, as being easiest, we will deal with the end-to-end joining of two tubes of similar glass. _a_, _b_, and _c_, Fig. 5, illustrate this. One end of one of the tubes should be closed, a lip should be turned out on each of the ends to be joined, and both lips heated simultaneously until the glass is thoroughly soft. Now bring the lips together gently, until they are in contact at all points and there are no places at which air can escape; remove from the flame, and blow slowly and very cautiously until the joint is expanded as shown in _b_, Fig. 5. Reheat in the flame until the glass has run down to rather less than the original diameter of the tube, and give a final shaping by re-blowing. The chief factors of success in making such a join are, thorough heating of the glass before bringing the two tubes together, and avoidance of hard or sudden blowing when expanding the joint. The finished work is shown by _c_, Fig. 5. [Illustration: Fig. 5] To join a small glass tube to the end of a large one, the large tube should first be sealed, a small spot on the extreme end of the seal heated, and air pressure used to expand the heated spot as shown in _d_. This expanded spot is then re-heated and blown out until it bursts as shown in _e_, the thin fragments of glass are removed and the end of the small tube turned out as shown in _f_. After this the procedure is similar to that used in jointing two tubes of equal size.