196 HISTORY OF CHEMISTRY. [LECTURE X. 



iodine, nitrogen, phosphorus, arsenic, and antimony, always 

 occur in even numbers only ; and this rule, if it is to hold for 

 the molecules of the elements as well, renders the existence of 

 single atoms in the free state impossible. Besides this, Laurent 

 brings forward the well-known effects of the nascent state, and 

 explains them by the assumption that at the moment of the 

 separation of the elements from their compounds the single 

 atoms are isolated, and therefore combine far more easily with 

 others than in those cases where it is a question of molecules 

 or of atomic groups which must first be decomposed before 

 reaction can take place. 



Laurent and Gerhardt, with their far-reaching reforms, met 

 with almost no immediate recognition ; on the contrary, it 

 seems as if the conception of the equivalent, in its first uncer- 

 tain form, had now found more adherents than formerly, and 

 as if Gay-Lussac's law of volumes appeared to chemists to be 

 less adapted than ever to constitute the basis of a system. 

 For this reason there was not, in general, the slightest ten- 

 dency exhibited to assume, with Laurent, the divisibility of the 

 molecules of the elements. It is no doubt true that urgent 

 grounds, 'and especially, chemical grounds, were still wanting. 

 It was a very happy idea that Laurent and Gerhardt had hit 

 upon when they stated that the formulae of substances must 

 represent comparable quantities ; but the standard was still 

 wanting. The spaces occupied by the gases were known only 

 in relatively few cases, and even amongst these there were 

 some cases where the molecular weights deduced were un- 

 serviceable because they were at variance, or at least they 

 appeared to be at variance, with the chemical properties. A 

 series of facts which confirmed these ideas, and eventually pro- 

 cured for them general recognition, was still wanting. We 

 are indebted for our knowledge of them to Williamson, who 

 showed how to find the molecular weight by chemical methods, 

 and thereby rendered a service to our science which cannot be 

 too highly estimated. Even although he did not instigate the 

 reform of chemistry, still it was his investigations which first 

 made its accomplishment necessary and possible. 



