DISCOVERY AND PROPERTIES OF RADIUM 147 



" The essentials in determining the correct equivalent of an 

 element are : 



" (1) A pure compound of the element and sufficient evidence 

 of purity. 



" (2) An advantageous transformation in which the weight of 

 the element or elements combined with the one of which the 

 equivalent is to be determined is as large as possible. 



" (3) If possible no transference, and no operation which 

 necessitates the use of reagents which can convey into the 

 solution matter which may be absorbed. 



" (4) A quantity sufficient in amount to make it possible with 

 the balance at disposal to determine its weight to, at least, 

 1 part in 20,000. 



" (5) Resistant vessels which will not themselves give up any 

 material to the substance and so make its purification difficult. 



" Determinations of the equivalent of radium have been 

 made by Madame Curie, by Sir Edward Thorpe, and by 0. Honig- 

 schmid. Madame Curie's first determination, made in 1902, 

 may be taken as avowedly only a rough approximation. Using 

 90 mgrm. of chloride she found the atomic weight to be 225, 

 assuming, no doubt with justice, that radium is a diad. Her 

 second determination employed the same method, viz., pre- 

 cipitation and weighing of silver chloride from a known weight 

 of anhydrous radium chloride. Madame Curie, in her earlier 

 work, proceeded to the ultimate atomic weight progressively, 

 raising the number from 140 to 146, then 174, then greater than 

 220, and in 1902 to 223-3 ; finally, with 90 mgrm., she obtained 

 the figures 225-5, 226-0, and 224-2 ; mean 225-2, which she re- 

 garded as accurate within a unit. 



" The method of crystallisation described in her later paper 

 is merely indicated. . . . The samples were tested spectro- 

 scopically for barium. . . . The amount taken was about 0-40 

 grm. After deducting the weight of the filter ash the figures 

 226-62, 226-31, and 226-42 were obtained, the values for Ag= 

 107-93 and 01=3545 having been taken. Substituting 107-88 

 and 35-46 the figures are less by 0-09 or 226-53, 226-22, and 

 226-33 ; the mean of these is 226-36. . . . 



" In his Bakerian Lecture for 1907, Sir Edward Thorpe 

 described experiments on the equivalent of radium. His raw 

 material, placed by the courtesy of the Austrian Government 

 at the disposal of the Royal Society, was ' about 500 kgrm.,' or, 



