232 Royal Society, 



a full confirmation of the existence of periodical laws regulating the 

 disturbances of the Inclination and Total Force corresponding to 

 those which he had previously deduced from the disturbances of the 

 other magnetic Element, viz. the Declination. 



Feb. 21.— The Lord Wrottesley, President, in the Chair. 

 The following communication was read : — 

 "On the Bromide of Titanium." By F. B. Duppa, Esq. 

 A comparison of the boiling-points of corresponding chlorine and 

 bromine compounds, led Prof. Kopp to the interesting discovery, that 

 on the average their boiling-points rise 32° C. for every equivalent of 

 bromine which is substituted for an equivalent of chlorine. 



^ Boiling-point. Difference. 



Chloride of ethyle, C^ H^ CI . . . . 1 1° C. 1 ,.^ 

 Bromide of ethyle, C4 H., Br . . . . 4 1° C. / •^"• 

 Dichlorinated ethylene, C4 H4 CL 67° C. Xan — o^'^'i 

 Dibrominated ethylene, C4 H^ Br^ 1 33°-6 C. / ^^"''^ ^ '^"^• 

 Terchloride of phosphorus, P Cl.^ . 78° C. \ q7_ o w 09 1 

 Terbromide of phosphorus, P Br.^ . 1 75° C. / ^ ^ - '^ >< -^^^ 3 • 

 If this difference be constant for all chlorine and bromine com- 

 pounds, it becomes obvious that very important inferences in respect 

 to the atomic constitution of these substances may be derived from 

 the determination of their boiling-points. This result has, in fact, 

 been happily applied by Prof. Kopp, as a criterion to determine the 

 equivalent of silicium, a matter of such uncertainty as to have led to 

 the admission of not less than three formulse for silica — 



SiO 

 SiOg 

 SiOg. 

 From the difference between the boiling-points of chloride (59° C.) 

 and that of bromide (153° C.) — a difference which amounts to 

 94=3 X 31i — Kopp derives the formulae 



SiCla and SiBrg, 

 as representing the atomic constitution of the chloride and the 

 bromide of silicium, and he accordingly fixes the equivalent of 

 silicium at 21 '3. 



In order, however, to prove the general validity of Kopp's obser- 

 vations, it was necessary to re-examine the boiling-points of cor- 

 responding chlorine and bromine compounds which exhibited dis- 

 crepancies, and to extend the inquiry to as great a number of new 

 compounds as possible. 



The bromine-compound of titanium was unknown. Mr. Duppa 

 has produced this substance by passing a current of bromine over an 

 intimate mixture of pure titanic acid and carbon. The reaction 

 takes place at a bright red heat, and furnishes a brown hquid, which 

 solidifies in the receiver to a crystalline mass. Distilled with an 

 excess of mercury, which removes any free bromine that may be 

 present, the bromide of titanium presents itself as an amber-yellow 

 compound, exhibiting a magnificent crystalline structure ; it attracts 

 moisture with the greatest avidity, and is converted into titanic and 

 hydrobromic acids. Bromide of titanium has a specific gravity of 



