The penetrating power of the X-rays varies with the vacuum of the tube, a low vacuum giving rays of small penetration, and a high vacuum rays of great penetration. Tubes are called hard or soft according to the degree of the vacuum, a hard tube having a high vacuum and a soft tube a low one. It should be remembered that the terms high and low, as applied to the vacuum of X-ray tubes, are only relative, because the vacuum must be very high to admit of the production of X-rays at all. The vacuum becomes higher as the tube is used, and after a while it becomes so high that the tube is practically useless, for the penetrating power of the rays is then so great that sharp contrasts between different substances, such as flesh and bone, cannot be obtained, and the resulting radiographs are flat and poor. The vacuum of a hard tube may be lowered temporarily by gently heating the tube, but this is not a very convenient or satisfactory process, and tubes are now made with special arrangements for lowering the vacuum when necessary. There are several vacuum-regulating devices, and Fig. 39 is a diagram of the “Standard” mica regulator used in most of the well-known “Muller” X-ray tubes. This consists of a small additional bulb containing an electrode D carrying a series of mica discs. A wire DF is attached to D by means of a hinged cap. The vacuum is lowered while the discharges are passing through the tube. The wire DF is moved towards the cathode terminal B, and kept there for a few seconds. Sparks pass between F and B, and the current is now passing through the electrode D in the regulator chamber. This causes the mica to become heated, so that it gives off a small quantity of gas, which passes into the main tube and so lowers the vacuum. The wire DF is then moved well away from B, and after a few hours’ rest the tube, now of normal hardness, is ready for further use. We have already referred to the heating of the anti-cathode caused by the bombardment of the cathode rays. Even if these rays are not focused very sharply, the anti-cathode of an ordinary tube becomes dangerously hot if the tube is run continuously for a fairly long period, and for hospital and other medical work on an extensive scale special tubes with water-cooled anti-cathodes are used. These tubes have a small bulb blown in the anti-cathode neck. This bulb is filled with water, which passes down a tube to the back of the target of the anti-cathode. By this arrangement the heat generated in the target is absorbed by the water, so that the temperature of the target can become only very slightly higher then 212° F., which is the temperature of boiling water, and quite a safe temperature for the anti-cathode. In some tubes the rise in temperature is made slower by the use of broken bits of ice in place of water. Fig. 40 shows a Muller water-cooled tube, and Fig. 41 explains clearly the parts of an X-ray tube and their names. [Illustration: FIG. 40.--Muller Water-cooled X-Ray Tube.]