﻿and some Applications to Physical Measurements. 495 

 The observations are shown in Table IV. 

 Table IV. 



Wave-length 



of Set 2 



in metres. 



Frequency 



of Set 2 



per sec. 



N. 



Frequency 

 of Set 1 

 per sec. 

 »=N/2. 



Change in 



number of 



beats per sec. 



p = '2dn. 



pfB. 



Tempera- 

 ture 

 degrees C. 





X10 5 



XlO 5 



Oryst. 



Powder. 



Oryst. 



Powder. 











XlO' 6 



XlO" 6 





352 



S-53 



426 



4-40 



440 



5"15 



5-15 



16 



375 



s-oo 



4-00 



4-10 



4-12 



5-13 



5-15 



16 



392 



7-65 



383 



391 



3-90 



5-11 



5-10 



16 



410 



7'32 



3-66 



3-74 



3-71 



511 



5-07 



16 



414 



7-25 



3-63 



3-70 



3-69 



510 



5-09 



16 



417 



7-20 



3-60 



3-68 



3-68 



511 



511 



16 





Mean p/N for both crystals and powder=5-12xl0~ 6 at 16° C. 



The susceptibility o£ the salt is thus the same in the solid 

 as in the powder form. A similar identity in the sucepti- 

 bilities of different forms of the same material was observed 

 by Wilson * using a balance method in the investigation of 

 certain iron ores. 



The volume susceptibility is 



K«= 76*6 x lO- 6 c.g.s., 



whence the mass susceptibility pf the complete salt is 

 40*4 xl0~ 6 c.g.s. The percentage mass of FeS0 4 in one 

 gram of salt is 54'6, so that, neglecting the contribution to 

 the susceptibility of the water of crystallization, we find 

 K m = 740 x 10- 6 c.g.s. at 16° C. 

 The mean of the values for K wl , all determined in the form 

 of a solution, given in the Tabellen of Landolt and Bornstein 

 is 78 x 10~ 6 c.g.s. Finkef found for K y for the complete salt 

 a value 80 X 10~ 6 c.g.s., and the present result agrees very 

 well with these. 



III. Ferrous Ammonium Sulphate. — The salt was examined 

 in the form of monoclinic crystals which proved, on estima- 

 tion, to have the composition FeS0 4 (NH 4 ) 2 S0 4 , 6H 2 to an 

 accuracy of 0*3 per cent. The density of this salt. is 

 1*813 gm. per c.c. at 15° C. A mass of 1 gin. was taken and 

 placed in a glass tube, occupying a length of about 8 cm., 



* Wilson, Proc. Pov. Soc. A. vol. xcvi. p. 429 (1919). 

 t Finke, Ann. d. Phys. (4 | xxxi. p. 149 (liHC). 



