90 REPORT— 1891. 



(Hey cock and Neville). The result of these experiments is to 

 show that at all events the majority of metals resemble mer- 

 cury in having monatomic molecules, and thus the impression, 

 founded on vapour-density experiments at high temperatures 

 with zinc and cadmium is confirmed. On the other hand, it 

 may be mentioned that the belief in the complexity of the 

 molecules of some of the non-metallic elements under certain 

 conditions, such as phosphorus and sulphur, on which doubts 

 have been cast by recent observations of vapour-density, has 

 been strengthened by the evidence afforded by these new 

 methods. 



I have hinted at the existence of difficulties in the way 

 of the advocates of van't Hoff's gaseous theory of solution. 

 Difficulties — and great ones — were met with at the outset. A 

 law to which there are a large number of unexplained ex- 

 ceptions has very little right to the name of law ; and those 

 who seek to clothe the law in a theory must make it large 

 enough to include the exceptions or give up the attempt. 

 Avogadro's law, as applied to gases, was long in suspense ; 

 for did not ammonium salts, phosphorus pentachloride, and 

 other substances exhibit "abnormal" vapour-density? But, 

 as we know, the conception of dissociation came to the rescue, 

 by which the law was not only saved from destruction, but 

 strengthened and greatly enriched. And exactly so has it 

 been with van't Hoff's .extension of Avogadro's law to matter 

 in solution ; only that the difficulties in this case were obvious 

 from the first, and it was not so hard to find the means of 

 conquering them. 



Since direct experiments on osmotic pressure were dif- 

 ficult, and not many had been made, the new theory had to 

 be tested chiefly by recorded experiments on the freezing- 

 points of solutions, on which much work had been done, 

 chiefly by de Coppet and Eaoult. Here the exceptions were 

 met with. Among inorganic salts especially, abnormal results 

 were obtained where one might have least expected them. 

 Some salts and acids gave practically twice the molecular 

 reduction of freezing-point that should have been, while others 

 gave more or less apparently erroneous results. The inference 

 one would naturally draw at first sight would be that the 

 gaseous laws do not really hold good in solutions. But ex- 

 amination of the facts brought out a striking result : these 

 apparent exceptions were one and all electrolytes. This sug- 

 gested that the explanation was to be found in a theory of 

 electrolytes put forward first by Arrhenius — an explanation 

 which is in accord with many other phenomena not hitherto 

 understood, and which has led already to most important 

 developements. The new hypothesis may be put thus : The 

 doctrine of dissociation, known to be an essential adjunct of 



