662 POPULAR SCIENCE MONTHLY. 



relieatino- of the tube is very apt to lead to a disastrous cracking 

 of the o-lass. He must take the utmost precautions against un- 

 equal heating and sudden cooling, and he must, above all, have 

 phenomenal patience. 



Fig. 1 shows the evolution of the Crooke's tube which is used 

 to produce the X rays. The first form of tube was barely larger 

 than a goose's egg. The size has been gradually increased, and at 

 present it is three or four times larger than the original form. 

 The interior arrangement has not been materially changed, and 

 consists, as we have said, of a concave mirror, which constitutes 

 the negative electrode, and an inclined sheet of platinum, from 

 which the X rays seem to emanate. 



The later forms of tube have accessory chambers, filled with 

 certain chemicals, which, on being slightly heated, reduce the 

 vacuum to the desired point. Certain forms of tubes have merely 

 an additional chamber which, on being heated, reduces the vacuum 

 in the main vessel. The latest form of tube, devised by Dr. Wil- 

 liam Kollins, of Boston, has a hollow anode tube {B C, Fig. 1), 

 through which a current of water can circulate in order to save the 

 tube from breaking. The end of this anode tube is small, in order 

 to form a sharp radiant point of light. One of the platinum wires 

 (P) inserted in the tube projects outside some distance. When the 

 vacuum becomes too high in the tube, this platinum wire is slightly 

 heated in a gas flame; then the flame is blown out and the hydro- 

 gen is allowed to flow against the heated wire. A sufficient amount 

 of the gas is absorbed by the heated wire to reduce the vacuum in 

 the tube. This tube stands very powerful electrical discharges, and 

 is the most scientifically designed tube at the command of the 

 experimenter. 



There are three methods of generating the electrical discharge 

 which produces the rays. The commonest method is that in which 

 the Ruhmkorf coil is used. This coil is what is now known as a 

 transformer, and consists of one coil of a few turns of coarse wire, 

 which is connected to a battery or other source of electricity, and 

 of another coil surrounding the first of a great number of turns 

 of fine wire. Any sudden change of the battery current produces 

 an electric pressure or electro-motive force at the ends of the fine 

 coil of wire. By this simple arrangement of two coils we can thus 

 exalt a current of low pressure to one of high electro-motive force. 

 A battery current which can barely produce an electric spark of 

 one hundredth of an inch at the ends of the coarse coil can cause 

 a spark of eight inches or more at the terminals of a fine coil. 



In the second method one uses an ordinary electrical machine 

 in which the glass plates are supplanted by rubber ones, which are 



