The other channel for the oxygen is to the right, down the vertical tube shown there (the lower vertical tube on the left is also a dummy), underneath the saturator, and finally coming up into the mixing chamber from below, transforming the white flame into an intensely hot blowpipe exactly as it does with a coal gas jet. The front tap controls the supply of saturated ether to the mixing chamber, and whereas at first a good stream of oxygen is needed to pick up enough ether, by degrees as the instrument warms in the lantern, the oxygen passing through the saturator can be cut off entirely, and even then the front tap must be gradually closed down to prevent the hot ether coming off too fast.
There is a disagreeable feeling of 'sitting on the safety-valve' in doing this, but in reality the pressure is never likely to become great enough to cause danger.
Of saturators for use outside the lantern the best-known is probably the 'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to the inlet marked A; B goes
direct to the oxygen tap of any ordinary mixed-gas jet; while C, from whence issues the saturated stream, is connected to the coal gas tap of the jet. Whichever pattern is used, the essential thing is to keep a good supply of oxygen well saturated. If the lime becomes incandescent without any free oxygen, or it is found that this requires gradually turning off, it indicates that the saturation is becoming defective, and to continue is to risk the jet snapping out. In the case of an outside saturator such as the 'Pendant,' this may even blow off the connecting tubes with a loud report, though no worse accident is likely to happen, and for this reason an outside saturator should be placed as close to the jet as possible, so that the rubber tube may be kept short, and incidentally this keeps the saturator warm and accelerates vaporisation.
As ether vapour usually contains a certain amount of moisture which does not vaporise to any great extent, this gradually accumulates and the capacity of the instrument becomes reduced. It is therefore usually necessary to return a saturator to the makers every now and again for repacking.
The only real danger with a modern saturator is not in using but in filling. This should be done if possible in the open air, and at any rate never near a light. Ordinary sulphuric ether of specific gravity 720-730 is usually considered the best, and a quarter of a pint will keep an ordinary small-bore jet going for nearly two hours.
More precise directions are usually sent out by the makers, and as the various patterns of saturator in use are pretty numerous, it would be useless here to attempt more detailed instructions for working.
Oxy-Acetylene Jets.—Any good mixed gas jet may be used with acetylene instead of coal gas, provided that it is under pressure more or less corresponding to that from an oxygen cylinder, and at the present day there is no difficulty in obtaining this, in civilised countries at all events, by
means of compressed or, to speak more correctly, 'dissolved' acetylene cylinders, referred to later on.