CLOSED TUBES FOR HEATING UNDER PRESSURE

(_Carius method for determination of the halogens and sulphur._) In this case the tubing used must have thick walls (usually about 3/32 inch) to withstand the pressure. Its external diameter is usually about 3/4 inch.

One length will usually make two tubes of standard length for the cannon furnace. Especial care must be taken in heating and cooling it on account of the thick walls. A length is gradually warmed in the center, finally heated at that point until soft, drawn out, cut apart and annealed. Taking one of the pieces, the cone is carefully heated and shrunk, as in Exercise 4, until its walls are as thick as those of the main tube. A flame with a little tinge of yellow should be used for this operation to prevent devitrification (page 2), as the thick gla.s.s shrinks slowly. The tail is now drawn off and the whole end heated and gently blown several times to make a rounded end, like a test-tube, with walls as thick as those of the main tube. This must be carefully annealed. It is more important that the walls be thick than that the end be nicely rounded: it may indeed be left somewhat conical in shape.

At a point about two inches from the open end of the tube, it is slowly warmed and finally heated to the softening point. Grasping the open end with a pair of crucible tongs, it is cautiously pulled out, a little at a time, usually during rotation in the flame, to make a constriction of moderate wall-thickness, but of sufficient internal diameter to admit the tube containing the substance. After annealing this, cooling and cleaning the tube, the acid and salt are introduced (the former by means of a long-stemmed funnel) and the tube is inclined and rotated about its axis so that the acid wets its surface about half way up from the bottom. The substance is now weighed out in a piece of thin-walled gla.s.s tubing, closed at one end, and about two inches long. Inclining the large tube at a suitable angle, the small one is introduced, closed end first, and allowed to slide down the walls of the large tube until it reaches the place where the acid has wet the tube. Here it will stop, and if the tube is kept inclined during the rest of the operation it will roll around inside the tube at this point and thus not get down where any acid is likely to get into it and produce any pressure by decomposing it before the open end of the tube is sealed. Now the tube is held in an inclined position, taking care that the acid does not reach up to the substance, the constricted portion cautiously warmed and shrunk. It is finally shrunk and drawn out into a somewhat elongated cone, with walls as thick as the rest of the tube, and when this is accomplished the end of the cone is sealed and the waste piece drawn off. Anneal with great care, and cool in such a position that the acid cannot reach the hot gla.s.s. The shrinking of this cone takes a good deal of patience, and is one of the most important parts of the process. If the walls are left too thin, the tube may burst when heated, and the whole labor is lost. If care is taken, the same tube can be used for a number of determinations, until it becomes quite short.