:v2i 



The rotation, already somewhat smaller at 80°, decreases slowly 

 and at a nearly linear rate up to 280° after which it falls off very 

 rapidly and vanishes apparendy at about éGO"". The coarse of the 

 observations was as follows. The mirror was heated gradually up 

 to 300°. At 340^ it began to tarnish. The freshly polished mirror 

 was then heated as rapidly as possible, since the "tarnish colours" 

 form only slowly, up to 305°. Here a few test readings were taken 

 which were satisfactorily continuous with the previous ones. At 

 400°, howevei', the mirror began to discolour again, the rotation 

 increasing gradually. Nevertheless, I was able to estimate the mini- 

 mum value of the rotation at about 4'. The behaviour on cooling 

 could not be determined on account of the tarnish. The temperature 

 curve is given by Table 3. 



TABLE 3. 

 f = ff{t) >l=r615,'//t Ferrosilicon (saturated) Tammann. 



Ferrocobalt ; (Fe.Co, Wwss and von Fkkldenreich, 6*, = 987°). This 

 substance was investigated by Preuss ^) ; its intensity of magnetization 

 at saturation, as is well known, is about 10°/^ greater than that 

 for pure iron. It shows, accordingly, throughout the entire spectrum 

 the most powerful rotation yet observed and a clearly distinguishable 

 ellipticity. The curve has a sharp minimum in the violet, then rises 

 rapidly in the green and less rapidly in (he red where (he uiaximum 

 rotation amounts to about — 34'. 



^) A. Preuss, Dissert Techn. Hochschule. Zurich 1912. 



