IQ2 HISTORY OF CHEMISTRY. [LECTURE X. 



chooses the formulae (and therefore the molecules) of sub- 

 stances so that they represent two volumes (H= i = i vol.) in 

 the state of vapour. In doing so he is obliged, however, to 

 admit certain exceptions to which he draws attention. Thus it 

 was known from Bineau's investigations 52 that the formulae 

 NH 4 C1, for sal ammoniac, and SO 4 H 2 , for sulphuric acid, 53 

 correspond to four volumes, but, in spite of this, these quan- 

 tities are regarded by Laurent as representing their molecular 

 weights. There were definite grounds, in these instances, which 

 appeared to make this assumption necessary. The isomorphism 

 of sal ammoniac with potassium chloride excluded the formula 

 NjHgClj; the dibasic character of sulphuric acid, which Laurent 

 looked upon as proved, demanded a molecular weight at vari- 

 ance with Avogadro's hypothesis. Even if this hypothesis, 

 therefore, was regarded as the chief standard in the fixing of 

 formulae, still the results obtained were subject to modification 

 by chemical reactions, and by physical properties such as 

 specific heat, specific volume, crystalline form, etc. Further, 

 the law of the even number of atoms, which had been outlined 

 for special cases by Gerhardt 54 in 1843, played an important 

 part in these determinations. Laurent now states this law, to 

 the effect that in all compounds the sum of the atoms of hydro- 

 gen, chlorine, bromine, nitrogen, etc., must always be an even 

 number. The law becomes of increased significance from the 

 fact that Laurent applies it in order to prove that the molecules of 

 these elements, which he calls dyads, 55 consist of two atoms. 



Gerhardt's ideas were greatly elucidated by Laurent, who 

 made them more generally accessible and comprehensible by 

 laying greater weight upon the terms he employed, and by 

 defining these terms precisely. As a result of this, an 

 important advance was effected, because the separation of 

 atom, molecule, and equivalent was now really accomplished ; 

 and hence it again became possible to employ Avogadro's 

 hypothesis (thirty-five years after its promulgation) as the 



52 Ann. Chim. [2] 68, 416. 53 Compare ibid. [3] 18, 289. 54 Ibid. 

 [3] 7, 129. 55 Ibid. [3] 18, 266; compare also Laurent, Methode de 

 Chimie. 77 ; E. 62. 



