164 HISTORY OF CHEMISTRY. [LECTURE IX. 



Laurent's theories. 28 The facts were still wanting, which showed, 

 in a definite and decisive manner, the analogy between the 

 original substance and the final product. Our science cannot 

 advance by means of ideas alone : it is only when an opinion 

 is called forth, and to a certain extent necessitated, by experi- 

 ment that a further development is involved in it. It was not 

 Dumas' position and name alone that now procured favour 

 for the theory which, a year before, had scarcely been taken 

 notice of. The chemists of that period had no such respect for 

 authority. The discovery of chloracetic acid lies between the 

 promulgation of the nucleus theory and that of the theory of 

 types ; and even if " a theory can be made up of words " still, 

 in chemistry, greater value is, fortunately, placed upon a decisive 

 experiment than upon daring speculations. 



An analogy between acetic and chloracetic acids could not 

 remain unobserved ; and especially after Berzelius, who had his 

 reasons for not admitting any similarity between them, had 

 given prominence to their differences ; and, with a certain 

 amount of irony, inquired for their kindred relationships. 29 

 Dumas shows the reactions which they undergo by the in- 

 fluence of potash, and points out their similarity. 30 



We have 



C 4 H 2 H 8 4 C 2 4 + C,H 2 H, 31 



Besides carbonate of potash, there is formed, in the one 

 case, marsh gas, and in the other, chloroform ; i.e., two sub- 

 stances which again exhibit the same difference in composition, 

 the one from the other, as the two acetic acids do ; and of 

 which the latter, as Dumas also particularly shows, 32 can be 

 obtained from the other by the action of chlorine. 



By the discovery of trichloracetic acid a basis is furnished 



28 Annalen. 25, I. 29 Berzelius, Jahresbericht 1840, 367 etc. 



30 Annalen. 33, 179. 31 This latter reaction appears to have been dis- 

 covered previously by Persoz (Introduction a 1'Etude de la Chimie mole- 

 culaire), as was pointed out by Pelouze and Millon (Annalen. 33, 182). 

 a2 Annalen. 33, 187 and 275. 



