MOLECULES AND ATOMS 821 



Besides these Ceases of apparent discrepancy from the law of 

 Ayogadro-Gerhardt there is yet a third, which is the last, and is very 

 instructive. 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 be normal that is, if D = M/2 it often serves as a proof 

 of the purity of the substance, i.e. of its freedom from all foreign 

 matter. If it be abnormal that is, if D 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 import- 

 ant significance of the law it becomes clear that there is some error in the 

 observation, or that the density was determined under conditions in 

 which the vapour does not follow the laws of Boyle or Gay-Lussac, or 

 else that the substance has not been sufficiently purified, and contains 

 other substances. The law of Avogadro-Gerhardt in that case furnishes 

 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 chemistry. We will 

 cite one instance. In the case of pyrosulphuryl chloride, S 2 O 5 C1 2> 

 M = 215, and consequently D should = 107 '5, instead of which Ogier 

 and others obtained 53'8 that is, a density half as great ; and further, 

 Ogier (1882) demonstrated clearly that the substance is not dissociated 

 by distillation into S0 3 and S0 2 C1 2 , or any other two products, and 



one. In the case of bromine it is not much greater, and is far from being equal to 

 that for iodine. 



As in general we very often involuntarily confuse chemical processes with physical, 

 it may be that a physical process of change in the coefficient of expansion with a change 

 of temperature participates with a change in molecular weight, and partially, if not 

 wholly, accounts for the decrease of the density of chlorine, bromine, and iodine. Thus, 

 1 have remarked (Comptes Rendus, 1876) that the coefficient of expansion of gases 

 increases with their molecular weight, and (Chapter II., Note 26) the results of direct 

 experiment show the coefficient of expansion of hydrobromic acid (M = 81) to be 0'00386 

 instead of 0-008C7, which is that of hydrogen (M = 2). Hence, in the case of the vapour 

 of iodine (M = 254) a very large coefficient of expansion is to be expected, and from this 

 cause alone the relative density would fall. As the molecule of chlorine Cl ? is lighter ( = 71) 

 than that of bromine ( = 160), which is lighter than that of iodine ( = 254), we see that 

 -the order in which the decomposability of the vapours of these haloids is observed corre- 

 sponds with the expected rise in the coefficient of expansion. Taking the coefficient of 

 expansion of iodine vapour as 0-004, then at 1,000 its density would be 116. Therefore 

 the dissociation of iodine may be only an apparent phenomenon. However, on the other 

 hand, the heavy vapour of mercury (M = 200, D = 100) scarcely decreases in density at 

 ft temperature of 1,500 (D = 98, according to Victor Meyer) ; but it must not be forgotten 

 that the molecule of mercury contains only one atom, whilst that of iodine contains 

 two, and this is very important. Questions of this kind which are difficult to decide by 

 experimental methods must long remain without a certain explanation, owing to the 

 difficulty, and sometimes impossibility, of distinguishing between physical and chemical 

 changes. 



