260 BAXTER, WEATHERILL AND SCRIPTURE. 



always carried out in ruby light. In using the nephelometer all the 

 precautions noted by Richards and 'Wells 24 were observed, such as 

 preparing the comparison tubes under as nearly as possible identical 

 conditions of temperature, concentration, and time, allowing the tubes 

 to come to constant ratio by standing for an hour or more, and taking 

 the average of several readings of each ratio. 



The Preparation of Silicon Tetrabromide. 



To prepare silicon tetrabromide, dry nitrogen saturated with pure 

 bromine at a temperature not far below the boiling point of bromine 

 was passed over silicon at red heat in an all-glass apparatus very 

 similar to that used for preparing the tetrachloride. Since the layer 

 of silicon was long, very little bromine passed through the reaction 

 tube into the receiver. About six hundred grams were secured in the 

 single preparation which was made. 



The bromide was then subjected to fractional distillation for re- 

 moval of the impurities which experience with the chloride led us to 

 expect. Since the boiling point of the tetrabromide, 153°, is far 

 higher than that of the chloride, the distillations were carried out by 

 heating the still instead of by cooling the receiver. Furthermore, 

 Hempel columns were employed in most of the distillations. 



The boiling points of the probable volatile impurities to be expected 

 are as follows : 



bpt. bpt. 



SiBr 4 153° HBr -69° 



Si 2 Br 6 240° CBr 4 190° 



TiBr< 230° 



Chlorobromides were not to be feared, since the bromine used was 

 chlorine-free. 



The general type of apparatus used in the distillations is shown in 

 Figure 5. The material in the bull) A, which has been filled from a 

 previous distillation and sealed off at A', is warmed in a water bath and 

 is distilled through the Hempel column D into the bull) B, which is 

 cooled with ice, leaving a residue of a few cubic centimeters in A. 

 The bulb A is then separated by sealing the capillary G. B terminates 

 in a special joint J through which it may be attached to another similar 

 system. The residues left in A and B may then be distilled into the 

 small bulbs a and b by inclining the bulbs to a suitable angle. The 

 complete course of the tetrabromide distillation is shown in Figure 6. 



24 Richards and Wells, Amer. Chem. Jour., 31, 235 (1904); 35, 510 (1906). 



