MOLECULES .\.\-l) ATOMS 315 



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 SO 3 and S0 2 C1 2 , or any other two products, and thus 

 the abnormal density of S 2 O 5 C1 2 remained unexplained until D. P. 

 Konovaloff (1885) showed that the previous investigators were working 

 with a mixture (containing SO 3 HCl),and that pyrosulphuryl chloride has 

 a normal density of approximately 107. Had not the law of Avogadro- 

 Gerliardt served as a guide, the impure liquid would have still passed 

 as pure ; all the more as the determination of the amount of chlorine 

 could not aid in the discovery of the impurity. Thus, by following a 

 true law of nature we are aided in the attainment of true deductions. 



All cases which have been studied confirm the law of Avogadro- 

 Oerhardt, and as by it a deduction is obtained, from the deter- 

 mination of the vapour density (a purely physical property), as to the 

 size of the molecule or quantity of a substance entering into chemical 

 reaction, therefore, this law links together the two provinces of learn- 

 ing physics and chemistry in the most powerful manner. Besides 

 which, the law of Avogadro-Gerhardt places the conceptions of mole- 

 cules and atoms on a firm foundation, which was previously wanting. 

 Although since the days of Dalton it had become evident that it was 

 necessary to admit the existence of the atom (the chemical individual 

 indivisible by chemical or other forces) of elements, and the groups of 

 .atoms or molecules of compounds indivisible by mechanical and physical 

 forces ; still the relative magnitude of the molecule and atom was not 

 defined with sufficient clearness. So, for instance, the atomic weight of 

 oxygen might be taken as 8 or 16, or any multiple of these numbers, 

 and nothing indicated a means for the acceptation of one or another of 

 these magnitudes ; 18 whilst as regards the weight of the molecules of 

 elements and compounds there was no trustworthy conception whatever. 



18 And so it was in the fifties. Some took O = 8, others O = 16. Water in the first 

 case would be HO and hydrogen peroxide HOo, and in the second case, as is now gene- 

 rally accepted, water H. 2 O and hydrogen peroxide H.^O 2 or HO. Discussion and confu- 

 sion were reigning. In 1860 the chemists of the whole world met at Carlsriihe for the 

 purpose of arriving at some agreement and uniformity of opinion. I was present at this 

 Congress, and well remember how great was the difference of opinion, and how a condi- 

 tional agreement was defended with the greatest acumen by the ranks of science, and with 

 what warmth the followers of Gerhardt, at whose head stood the Italian professor, 

 Canizzaro, followed up the consequences of the law of Avogadro. In the reign of 

 .scientific freedom (without which science would make no progress, and would remain 

 petrified as in the middle ages) and with the simultaneous necessity of scientific conser- 

 vatism (without which the roots of past study could give no fruit) a conditional agree- 

 ment was not arrived at, and ought not to have been, but instead of it truth, in the form 

 of the law of Avogadro-Gerhardt, received by means of the Congress a wider develop- 

 ment, and soon afterwards conquered all minds. Then the new so-called Gerhardt 

 Atomic weights established themselves, and in the seventies they had already become 

 .generally used. 



