Academy of Sciences in Paris. 371 



or less thickness, which appears scarcely to contain any liquid, 

 although a considerable quantity may be obtained from it by distil- 

 lation. Bromide of silicium, when distilled, is nearly colourless ; 

 it emits thick vapours in the air. When cooled in a frigorific com- 

 position, it becomes solid at from 12 to 15 degrees below zero ( 10 

 or 5 F.) ; partaking, in this respect, of the property of bromine. 

 It is raised to a state of ebullition at from 148 to 190 (300 374 

 F.) ; its density is greater than that of sulphuric acid, for it falls 

 rapidly through that liquid, in which it is decomposed but slowly, it 

 not being until after the lapse of several days that it is entirely con- 

 verted into silica and bromine ; the latter being the result of the re- 

 action of the sulphuric acid on the hydrobromic acid. Potassium 

 acts violently on bromide of silicium at a very slight elevation of 

 temperature, producing a loud detonation which constantly fractures 

 the tube. The following are the principal points of difference be- 

 tween the chloride and the bromide of silicium. 1. The chloride 

 boils at 50 C., the bromide not until 150. 2. The chloride, which 

 sinks in water, has less density than sulphuric acid ; it remains on 

 the surface, and is there decomposed into silica and hydrochloric 

 acid. The bromide, on the contrary, is, as we have before seen, 

 heavier than sulphuric acid. 3. Potassium suffers no sensible altera- 

 tion from chloride of silicium in a state of ebullition, whereas a very 

 slight heat is sufficient to produce a violent action of that metal on 

 the bromide ; which may be accounted for thus : The potassium 

 becomes fused before the ebullition of the bromide ; the chloride, on 

 the contrary, boils before the fusion of the potassium can take place. 

 Indeed, if potassium which, from having been exposed to the air, is 

 beginning to liquefy, be dropped into bromide of silicium, the deto- 

 nation will take place instantly. 4. The chloride of silicium may 

 be cooled below 20 C., without losing its liquid form ; whereas we 

 have seen that the bromide becomes solid at from 12 to 15 below 

 zero. The bromide of magnesium may be obtained in the same 

 manner, by a mixture of carbon and carbonate of magnesia, &c. ; 

 but it is difficult to procure it pure, because it is not volatile, and 

 does not fuse under a red heat ; then, as fast as it is formed, a part 

 is carried off by the gas into the tube and the ball, which are ob- 

 scured by it under the form of a greyish powder, a mixture of chlo- 

 ride of magnesium, magnesia, and carbon ; and another part remains 

 at the bottom of the porcelain tube, and in the first part of the tube 

 which corresponds with it, under the form of a molten mass, more 

 or less pure, whitish, and crystalline. Bromide of magnesium has 

 a powerful attraction for the humidity of the atmosphere, and acts 

 strongly on water, producing detonation and development of heat. 



GEOLOGY. 



Pyrenean Chain. On the 26th of September, M. Neboul read an 

 interesting series of * Observations on the Structure of the Pyrenees,' 



