XVI. 



THE ROXTGEX OR "X" RAYS. 



By Thomas M.^nsell, F.R.M.S., F.R.P.S. 



A Lecture delivered at Watford, l&th January, 1897. 

 (Abridged.) 



In the history of science there has never been so much research 

 concentrated on one particuhir field of investigation as at the 

 present time, when in all the principal physical laboratories of 

 Europe inquiries are being made into the effects and causes of 

 the new species of radiation called by one of the early discoverers 

 "X" rays, the name having been given by Professor Rontgen 

 because their nature is unknown, and in mathematical problems 

 X represents the unknown quantity. 



"We all know that any ordinary source of electricity, whether 

 a battery, dynamo, or friction machine, has two terminals, called 

 positive and negative. When these are connected by means of 

 a wire we form what is called a circuit. If we even cut the wii'e 

 the current still ti'ies to pass, and tends to leap across the gap in 

 the form of sparks. This is called the "spark-gap," and the 

 length of spark is a rough measure of the voltage or pressure of 

 electricity. 



If when we are producing the spark it is surrounded by a partial 

 vacuum, the electric force is more easily able to pass. To make 

 such a vacuum we have to fuse wires into the opposite ends of 

 a closed tube of glass and exhaust the air by means of an air-pump. 

 As the exhaustion proceeds, the sparks pass so rapidly that one 

 thinks there is a continuous string of light through the tube. On 

 further exhaustion the string of light widens until it appears like 

 a luminous glow ; and in the end the light in the tube almost 

 disappears, while the tube itself commences to shine with a 

 fluorescent light. 



Many years ago Professor "William Crookes showed that, when 

 the exhaustion in a vacuum-tube becomes about one-millionth 

 of an atmosphere, the molecules of the residual gas have a free 

 path in which they can move, without causing a visible glow in 

 the tube. At this stage the molecules are attracted to the 

 negative pole or "cathode." According to Professor Lodge, if we 

 reduce the atmospheric pressure to one-hundred millionth of an 

 atmosphere, there still remains in eveiy cubic inch of the tube 

 something like 1,540 billion complete molecules. We call this 

 radiant matter " cathode " rays, and it will be seen that when 

 the current is turned on through a vacuum-tube and a magnet is 

 brought near the tube, the beam will be deflected, that is to say, 

 the cathode rays are deflected by magnetism. Tliis deflection of 

 the cathode rays is one of the peculiarities which distinguish them 

 from the "X" rays of Rontgen. 



I have no doubt that many of my audience do not know the 



