IV. Means to prevent over-heating- of the antikathode are various:

(a) Heavy metal antikathodes, as in Gundelach’s tube, illustrated here, serve well, the increased mass of metal taking much longer to become injuriously hot than the ordinary thin discs employed. For the first twenty or thirty excitations such tubes are disappointing, since they soften rapidly, and to a marked degree. This is due evidently to the occluded gases unavoidably present in the metal, but after a time these become exhausted. Then the tube works steadily, and will stand long runs with heavy currents very satisfactorily.

We would suggest that a new tube of this type be used for therapeutic purposes for a time, its condition being carefully noted meanwhile before reliance is placed on it for radiographic work.

(b) Water-cooled tubes have the antikathode and stem surrounded by a water-jacket, which retards the heating of the target. Special attention must be paid to the position of the tube, so as to have the water in the jacket always in contact with the target. Thus the range of use of such tubes is limited. Fig. 11 shows in diagram a design allowing adjustment of plugs to suit the position of the tube.

(c) Air-cooled tubes of similar principle are a recent modification, and, in our experience, seem to be a very efficient arrangement for the purpose, being very con- stant in action and capable of being used in any position. Their price is so far somewhat prohibitive for ordinary use, and more experience of their working is desirable before finally judging them. A good example of this tube is the ‘ Tantalum/ as shewn in Fig. 12A.

V. The metal parts have been modified in various ways, as mentioned earlier, to avoid disintegration and damage from over-heating. Since the main cost of a tube is in the labour of manufacture—and this is the same for whatever material be employed—it is obvious that the best suitable material should be insisted upon in all tubes, however their design or construction may be modified to lessen their price. The purity of metals employed is of prime importance, especially in the case of the kathode and its supporting stem, for which aluminium has proved to be pre-eminently suitable.

Much of the value of a tube depends upon the construc- tion of the antikathode, since the quality of the resultant radiation seems to depend largely upon the * stopping power ‘ of the material opposed to the kathode rays. Probably in no case are the kathode rays all stopped by their first impact on the surface molecules of the target, but some rays penetrate to deeper molecules before being stopped and giving origin to the resultant X rays. Thus radiations are produced, as it were, from successive rows of molecules ; and this would seem to explain the origin from one tube of a collection of rays of differing qualities. Those rays produced by the stoppage of kathode rays at the first row of molecules are of the highest degree of penetration, and probably also actinic effect. Platinum, of all the metals tried, possesses the highest stopping power, and the heavy anodes of pure platinum employed in some of the most expensive tubes give beautiful effects. But pure platinum is very liable to over- heat, and probably the best all-round antikathodes are now made of platinum alloyed with a proportion of pure nickel, thus combining stopping power with resistance to over- heating.

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