THE VACUUM THERE 's SOMETHING IN IT WHITNEY 203 



represent a crude " eye " which is connected to a so-called " nerve " 

 leading to a " brain." The nerve is merely a long box having elec- 

 trical capacities and suitable amplifiers or three-electrode tubes 

 within. The capacities serve to slow down the apparent rate of flow 

 of the feeble current from the artificial eye so that indicating lamps 

 along the top of the box or " nerve " light up, one after another, as 

 the impulse from the "eye" passes along that path, or the nerve. 

 After the last lamp is thus lighted by these amplified currents an 

 electric bell rings to indicate reception at the " brain " end of the 

 circuit. This is not offered as a reliable replica of the real nervous 

 system, but as an application of the vacuum tube which amplifies 

 the slight energy available and necesary for the experiment. With 

 such slight energy it becomes practicable to show the delayed trans- 

 mission and reception which is necessary for an illustration of 

 nerve action. Nerve impulses travel much slower than electricity 

 usually does, and this low speed was one objection to visualizing 

 nerves as electrical conductors until Crehore and Williams showed 

 that nerves might be naturally so constructed as to transmit slowly. 



X RAY 



In order to show another vacuum product (the X-ray tube) an 

 effect of X rays may be demonstrated which is not usually thought 

 of in connection with X rays. Three rubber balloons, suspended 

 close together by long cords, are first charged electrically by fric- 

 tion. They then repel each other and stand stationary in space as 

 at the corners of a large triangle (Plate 3, Figure 3). As soon, 

 however, as a feeble beam of X rays is projected towards them in the 

 manner shown in Plate 4, Figure 1, they quickly discharge and fall 

 to their original position in contact with one another. X rays 

 ionize air or make it conducting so that the balloons can not re- 

 tain their mutually repelling charges. This is the basis of a method 

 for measuring the intensity of beams of X rays. 



SPECTRA 



Another application of X rays is in the study of internal structure 

 of crystalline chemical compounds and elements. Cathode rays, cur- 

 rents of negative ions in vacuum, when speeded up by high voltage, 

 produce by their impact X rays which are characteristic of the ma- 

 terial on which they impinge; one may say characteristic of the 

 mass of the atom of the substance of the target. It is through this 

 fact that the X-ray spectra of the elements considered as to wave- 

 lengths are arranged in the same order as the atomic masses in the 

 periodic table of Mendeleeff, and by this very method the newest 

 known element, hafnium, has been recently added to the known 



