496 



5. Tin. When we put \'^a]c again 2 units higher, viz. 

 ]/nk = S8 . 10-2, vve calculate with bk = 265 . 10-^ 2y = 3,44 (see 

 below), >.=r 0,847: 



68,54 X 1444 



T,, = -J i^ = 3730 abs. 



26,5 



At the time (1887) Guldberg calculated the too low value 3000° 

 abs. from various data. 



For 2y we find 1 + 0,04 X 61,1 — 3,44 , and thus we have: 



3,44X118,7 408,3 , , , 



D, = ^ = = 6,87. (calculated) 



265.10-5X22412 59,4 - — ^ ^ 



At — 163°,4 Cohen and Olie (Zeitschr. f. ph. Cii. 71, 400; 1909) 

 found for white tin, 7,35 and for grey tin 5,77. The value calculated 

 by us lies near that of white (tetragonal) tin, which is stable at 

 higher temperature. Tkechmann (1880) found the value 6,5 to 6,6 

 for the density of the rhombic tin, which is stable above 161°. Our 

 value 6,9 lies, therefore, between that of tetragonal (7,3) and rhombic 

 tin (6,6)^). 



And now the vapour pressure determinations, made by Greenwood 

 in 1911 (Z.f. ph. Ch. 76, 484). Let us, however, first calculate the 

 probable value of the critical pressure. 



For this vve find : 



0,0314 X 1444.10-4 



Pk = ;:; = 650 atm. 



^ 702,25.10-8 



As Greenwood found 2270° C. =: 2543° abs. for the boiling point, 

 the ratio would be J/,: 7"^ = 1,47, which is too low in my opinion, 

 so that either the temperature is still higher than 3700°, or — what 

 in connection with what will appear for lead, is by no means 

 improbable — the boiling temperature determined by Greenwood 

 has been given too high (or the vapour pressure at that temperature 

 too low). If 2543° were correct, a value about equal to 2543 x 1,6 = 

 — 4100° abs. might be expected for Tjc, in consequence of which 

 also \/ajc would have to be raised to about 40 . 10-'^ instead of 

 38 . 10-2. 



If the value 3730°, calculated by us, is correct, the ratio Tjc : Tt,. 



•) When we take the density of liquid tin at the melting point, viz. 6,99 

 (VicENTiNi and Omodei, 1888), as standard, we might certainly expect Do to be 

 >.7. But then it is overlooked that the solid rhombic modification, which has the 

 density 7,!iJ at the melting point (232°), has the so much smaller density 6,5 a 

 6,6 at the ordinary temperature. Abnormal changes of density are, therefore, 

 also to be expected in the liquid phase at decreasing temperature, if this phase 

 could be realized below the melting point. 



