AT HIGH PEESSURES BY OPTICAL METHODS. 
137 
sulphur crystals, but it has not been known that the crystallization of one crystal 
phase out of one other crystalline phase may take place in exactly the same way. 
These anisotropic crystal germs appear at any point inside the isotropic crystal grains 
or at their border. They are very few at the melting-point pressure, and are scarcely 
seen to grow at all, but if the pressure is lowered they begin to be more numerous, 
and grow more and more rapidly. The lower pressure given in Table IT. at any 
given temperature is the pressure at which the anisotropic crystal germs are seen 
only just to begin to grow with noticeable speed. The higher pressure given again 
is that at which the transition from anisotropic to isotropic form is seen to take place 
when pressure is increased. In this case it is not possible to see when transition really 
begins, and only a quite small increase of pressure is necessary in order to produce a 
very rapid transition' into the isotropic modification. The change is thus more like 
melting, and the opposite change, from isotropic to anisotropic crystal form, takes 
place in a similar way to crystallization. 
Table II. 
Temperatures 
(corrected). 
Pressures, P 3 
(corrected). 
Pressures, P 4 
(corrected). 
° 0 . 
57 • 54 
310 kg./cm .2 
340 kg./cm.“ 
70-89 
715 „ 
770 „ 
81-20 
1,055 „ 
1,090 
91-96 
1,385 „ 
1,430 „ 
95-72 
1,515 „ 
1,550 ,, 
The difference between the pressure values P;, and P 4 range between 30 and 55 kg., 
the average being 40 kg./cm.^, but this value is apparently not dependent upon the 
pressure at which the transition takes place. 
Determinations were further made at constant pressure, both at rising temperature. 
Table III., and at falling temperature. Table IV., Series A and B. 
Table III. 
Observations at rising 
temperatures, about 
Constant pressures, P 5 . 
°C. 
64-5 
400 kg./cm.2 
78-6 
900 ,, 
96*0 
1,500 
VOL. CCXII.-A. 
T 
