Chemistry and Physics. 285 
rises or falls when, in any compound, chlorine is replaced by bro- 
mine, or vice versd, bromine by chlorine. After determining the num- 
ber of degrees which express this difference for the substitution of each 
atom of chlorine or bromine, it is possible, on the other hand, to con- 
clude, from the difference between the boiling-points of a chlorid and 
* the corresponding bromid, as to the number of atoms replaced, Now 
it results, from the comparison of the boiling-points of several bromids 
and c is, that the substitution of 1Cl by IBr raises the boiling- 
point 32° Cent. ; of 2Cl by 2Br, 2X 32=64°; of 3Ci by 3B, 3x 32—96"; 
while the boiling-point falls in the same proportion when, on the con- 
trary, bromine is replaced by chlorine. Compare, for example, the 
boiling point of the following substances :— * 
Boiling-point. 
C,H,Cl — Chlorethyle, . . . . . -F1I°, Pierre. 
C,H,Cl —Chloracetyle, . . . . . —18° to 15°, Regnault. 
Cl, Chlorid of phosphorus . . 78°, Dumas, Pierre. 
Boiling-point found. Calculated. 
Br —_ Bromethyle » «Mls APR Sy See 
Br Bromacetyle . . . . . Ord.temp. 14° to17°. 
PBr, Bromid of phosphorus. . . 175°, Pierre. 174°. 
Several other comparisons enumerated by Kopp lead to the law above 
- announced respecting the change in the boiling-point in substitutions of 
r and chlorine. It consequently follows, as above stated, that 
according as the boiling-point of a bromid, on comparison with that of 
ils corresponding chlorid, is situated at 32, 64 or 96 degrees higher 
than in the chlorine compound, this latter must be regarded as contain- 
ing 1, 2, 3 atoms of chlorine replaced by bromine. ‘The boiling-points 
of the chlorid of silicon and of the bromid of silicon have been deter- 
mined by Pierre, a most accurate observer, the first to be 59°, the lat- 
ter to be 153°; the difference is 94°; whence it follows that in the 
bromid of silicon 3 atoms of bromine are substituted for 3 atoms of 
chlorine in the chlorid of silicon; that the first is SiBr,, the latter 
SiCI,, and that silica is therefore SiO, ; and consequently we must ad- 
mit the atomic weight of silicon to be 21°3, H being assumed —I. 
13. On the Extraction of Mannite from the. Dandelion; by Messrs. 
Smiru; with an Analysis of the Mannite, by Dr. Srennouss. Com- 
municated by Dr. Georce Witson, (Proc. R. Soc. Edinb., ti, 223. )— 
essrs. Smith stated that they had extracted from the dandelion, a large 
«Amount of a crystalline sweet substance, having all the physical char- 
‘hae acters of mannite. It was analysed by Dr. Stenhouse, and foun 
Contain carbon, hydrogen, and oxygen, in the proportions which char- 
acterize the accepted formula for mannite ; viz.,C,H,O,, so that it 
Certainly was the substance it was supposed to be. .. 
Messrs. Widumann and Frickhinger, it is stated, had anticipat 
a 
Messrs. Smith in the separation of mannite from the dandelion juice, 
and were led to believe that the mannite did not preéxist ready formed 
a 
