Cathode Rays and some analogous Rays. 



481 



the difference between the value of the mechanical equivalent found 

 by Rowland, and that found by Griffiths and by Schuster and Gannon, 

 using electrical methods, may not be due to an error in the experi- 

 ment itself, but to some error in the electrical standards of resistance 

 or of electromotive force," 



A happy result of this agreement (arrived at by such devious 

 paths) concerning the changes in the capacity for heat of water, is 

 that we can now utilise the results recently published by Professor 

 Reynolds regarding the capacity for heat of water over the range 0° 

 to 100° C. A hurried calculation indicates that if we accept Row- 

 land's corrected values, the mean thermal unit over the range 0° to 

 100° is in close equality with the " thermal unit at 19°." 



" Cathode Rays and some Analogous Rays." By SiLVANUS 

 P. Thompson, D.Sc., F.R.S. Received May 10,— Read 

 June 17, 1897. 



(Abstract.) 



1. The size of the cathodic shadow of an object depends upon its 

 own electric state, as already found by Crookes.* If it is negatively 

 electrified the shadow expands. If it is positively electrified the 

 shadow contracts. The position, as well as the size of a cathodic 

 shadow, may be affected electrostatically ; the rays which cast the 

 shadow being repelled from a neighbouring body if the latter is 

 negatively electrified. In some cases the contraction of the shadow 

 of a narrow object that is made positively electrical (anodic) may go 

 so far that the luminous margins approach and even overlap, giving 

 the appearance of a bright or negative shadow in place of a dark 

 one. The enlargement of a shadow when the object is made cathodic, 

 and the diminution of the shadow when the object is made anodic, 

 both depend upon the degree of exhaustion of the tube ; and both 

 are augmented up to a certain point by raising the degree of 

 exhaustion. They are also unequal, the enlargement when the 

 object is made cathodic vastly surpassing the diminution when the 

 object is made anodic, other things remaining equal. The conclusion 

 is reached that cathode rays are capable of being deflected electro- 

 statically, being apparently strongly repelled from a neighbouring 

 cathodic surface ; and less strongly attracted towards a neighbour- 

 ing anode. Incidentally it was observed that two cathode beams 

 from two small disk cathodes can cross through or penetrate one 

 another without interfering with one another. 



2. The electrostatic deflection of cathode rays by an electrified 

 object is found to be dependent upon the surface of that object as 



* f Phil. Trans.,' 1879, Part II, p. 648. 



