Anode Rays to the Investigation of Isotopes. 863 



double charge, appeared. With the magnetic field at my 

 disposal it was impossible to separate lines even two units 

 apart if such had existed for calcium, so the number and 

 mass of the isotopes remains undetermined. It is, however, 

 certain that one or more of the atomic weights (39, 40, 

 and 41) were present; and as all these are already known to 

 exist as isotopes of other elements (39 and 41 potassium, 

 and 40 argon), it is' clear that calcium is an instance of a 

 so-called isobare. No previous instance had been known 

 except among the radioactive elements (see PL XXVI.). 



Sti^ontium. 

 An attempt was made to get this metal with a mixture 

 of strontium chloride (anhydrous), lithium bromide, and 

 graphite ; a good beam of lithium was obtained, but no 

 strontium could be seen on the photograph. After various 

 other unsuccessful attempts, I finally used a mixture of 

 strontium iodide, strontium bromide, and graphite. This 

 gave a purple beam showing the blue strontium line ; 

 though the beam was fairly strong the photographs were 

 very faint, but showed a line corresponding to an atomic 

 weight of 87 as measured on one plate and 85 on another, 

 a water line at 18 being taken as a standard. One plate also 

 showed a faint line at 28 (CO) [atomic weight of strontium 

 is 87*6]. There was no sign of a line at 44 corresponding 

 to a double charge, though as a line in this position might 

 also be due to C0 2 , its presence would not be conclusive 

 evidence in favour. 



Sodium and Potassium. 

 These were only used for reference lines. Sodium gives a 

 considerably greater effect both on the photograph plate and 

 in the spectrum of the rays than does potassium, but either 

 can be obtained without difficulty from a mixture of the 

 iodides with graphite. . Neither showed any trace of double 

 charges as might be expected from their chemical pro- 

 perties. 



Absence of doubly and negatively charged atoms. 



Gehrcke and Reichenheim * found that the strontium rays 

 all carried a double charge. I have failed to rind any sign 

 of this, though the rays were so weak that a small proportion 

 doubly charged would have escaped notice. It is certain, 

 however, that the majority have- a single charge. Gehrcke 

 and Reichcnheim's result depended on assuming that the 

 * Vevh. D. Phys. Gescll. vol. xi. p. 878, 



