IU4 I'KINCIPLKS OF CHKM1STKY 



In- disunion of compounds and the decomposition of complex mole- 

 cules. 17 



1 V-idrs tin's,- cases of apparent discrepancv from the la\v of Avogadro- 

 (lerhardt there is yet a third, which is the last, and one which is very 

 instruct ive. In the investigation of separate substances they have to 

 be isolated in the purest possible form, and their chemical and physical 

 properties, and among them the vapour density, then determined. 

 If it he normal --that is, if l') = ]\I '1 it is a pi oof of the purity of the 

 substance and of its freedom from all impurities. If it be abnormal 

 that is, if 1> be not equal to M '2 then for those who do not believe 

 in the law it appears as a new argument against it and nothing more : 

 but to those who have already grasped the important significance of 

 the law it heroines clear that there is some error in the observation, 

 or thai the density was determined under conditions in which the 

 vapour dues not follow the laws of Hoyle or Gay-Lussac, or else that 

 the substance has not been sulliciently purified, and contains impurities, 

 Arc. The law of Avogadro-Gerhardt then appeal's as a convincing 

 evidence of the necessity of a fresh and more exact research. And as 

 yet the causes of error have always been found. There are not a few 

 examples in point in the recent history of chennMrv. We will cite 

 one instance. In the case of pvrosulphurvl chloride, S.,O ;) G1.,, ]\I '2 1 ">, 

 and consequently I> should 107 '">, instead of which < >girr and others 



A It In >iiLdi at first there appeared to IK- a similar phenomenon in t lie case of chlorine, 

 I: wa- afterwards proved that if there is a decrease of density it is onh a small one. In 

 tin- ca-e of bromine it is not much ;_rreatcr. and i- far from lieim: equal to that for iodine. 

 A- \ve in general very often involuntarily confuse chemical processes with physical. 

 it may be that, a physical process ot change in the coef'ticieiit of expansion with a change 

 df temperature and molecular weight participates, if not \\hollv. at all e\ents partiiilly, 

 in tli,. matter ,,f the decrease of the density of chlorine, bromine, and iodine. Thus. I 

 ha ve remarked > ( 'ompte-, 1 1 end n-. Is7'>i that the coef'ticieiit of expansion of ^ases incri-Rses 

 uitli theii- molecular weight, and iChap. II. Note'Ji'n the results i,t direct cxjierimeiit 

 ^li,,w the coefh'eient of expansion of hydrobromic acid iM si i t , , 1 ,,- tniti.'jsc. instead of 

 ()-IMi:;|-,7 for hydn>-en iM =lii. Therefore, in the case of the \ap.Mir of iodine | M 'jr.J'i a 

 N , r\ l.ii-e coeflicient of expansion is to l.e expected, and from this cause alone the 

 ,1,.,,-itv would fall. As (he molecule ,,f chlorine C] , i:, li-hter i -71 ithau that of bromine 

 , ir.in. v.hic], i li-hter than that of indim- ( =1.",.| i, therefore the order in which the 

 decnmpo-ability of the \a]iours of the^-e haloids is nlisc-rvcd i'orr->poinls \\ ilh the ex]iected 



;1 -. o-iiiii. the,, : .t IIIIMI it - den-iiy would already be 111 1 ,. Therefore it may be that, the 

 ,li-- ocial 1011 of io( line i. only an ap]iarent phenomenon. However, on t he other hand, 

 the hea\ y \ aponr- of mercury iM Jim. I> lu(l) scarcely decrea-e in den-it v at a t em- 

 I , , -,,t in -e i ,| I .Mm .[) '.is. according to Victor Me\eri; but it mii-1 not lie forgotten that 

 ,!,,. molecule nf inercur\ contain- only one at . mi. u h i 1 -t that of iodine contains two, which 

 v .,,,,l(l !_ nil's much. (.ine.Co,,-^ i thi- kind uhich are difficult )o decide by experimental 

 m , ,,,, ',. pecially the accurate determination of /i mii-t remain l<>icj without bein^ defi- 

 nitelv explained byrea on of the dif'ticultv. and -omet imes impo--.ibility, of separating 

 I,;-, .i,-;,] from chemical chan-,-. 



