660 REPORT — 1884. 



called substitution theory. Dumas, to whom we owe this theory, showed that 

 chlorine can take the place of hydrogen in many compounds, and that the resulting 

 body possesses characters similar to the original. Berzelius opposed this view, in- 

 sisting that the essential differences between these two elements rendered the idea 

 of a substitution impossible, and notwithstanding the powerful advocacy of Liebig, 

 and the discovery by Melsens of reverse substitutions (that is the re-formation of 

 the original compound from its substitution-product), Berzelius remained to the 

 end unconvinced, and that which was in reality a confirmation of his own theory 

 of compound radicals, which, as Liebig says, ' illumined many a dark chapter in 

 organic chemistry,' was looked upon by him as an error of the deepest dye. This 

 inability of many minds to see in the discoveries of others confirmation of their 

 own views is not uncommon ; thus Dalton, we may remember, could never bring 

 himself to admit the truth of Gay-Lussac's laws of gaseous volume-combination^ 

 although, as Berzelius very truly says, if we write atom for volume and consider 

 the substance in the solid state in place of the state of gas, the discovery of 

 Gay-Lussac is seen to be one of the most powerful arguments in favour of 

 Dalton's hypothesis. 



But there is another change of view, dating from the commencement of the Dumas 

 epoch, which has exerted an influence equal, if not superior, to those already named 

 on the progress of our science. The relative weights of the ultimate particles, to 

 use Dalton's own words, which up to this time had been generally adopted by 

 chemists, were the equivalent weights of Dalton and Wollaston, representing, in 

 the case of oxygen and hydrogen, the proportions in which these elements com- 

 bine, viz., as 8 to 1. The great Swedish chemist at this time stood almost alone 

 in supporting another hypothesis ; for, founding his argument on the simple laws of 

 volume-combination enunciated by Gay-Lussac, he asserted that the true atomic 

 weights are to be represented by the relations existing between equal volumes of 

 the two gases, viz., as 16 to 1. Still these views found no favour in the eyes of 

 chemists until Gerhardt, in 1843, proposed to double the equivalent weights of 

 oxygen, sulphur, and carbon, and then the opposition which this suggestion met 

 with was most intense, Berzelius himself not even deigning to mention it in his 

 annual account of the progress of the science, thus proving the truth of his own 

 words : ' That to hold an opinion habitually often leads to such an absolute con- 

 viction of its truth that its weak points are unregarded, and all proofs against it 

 ignored.' Nor were these views generally adopted by chemists until Cannizaro-, 

 in 1858, placed the whole subject on its present firm basis by clearly distinguishing 

 between equivalent and molecular weights, showing how the atomic weights of the 

 constituent elements are derived from the molecular weights of their volatile com- 

 pounds based upon the law of Avogadro and Ampere, or where, as is the case 

 with many metals, no compounds of known vapour-density exist, how tbe same 

 result may be ascertained by the help of the specific heat of the element itself. 

 Remarkable as it may appear, it is nevertheless true that it is in the country of 

 their birth that Gerhardt's atomic weights and the consequent atomic nomenclature 

 hare met with most opposition, so much so that within a year or two of the present 

 time there was not a single course of lectures delivered in Paris in which thess 

 were used. 



The theory of organic radicals, developed by Liebig so long ago as 1834, 

 received numerous experimental confirmations in succeeding years. Bunsen's 

 classical research on cacodyl, proving the possibility of the existence of metallo- 

 organic radicals capable of playing the part of a metal, and the isolation of the 

 hydrocarbon ethyl by Frankland in 1849, laid what the supporters of the theoiy 

 deemed the final stone in the structure. 



The fusion of the radical and type theories, chiefly effected by the discovery in 

 1849 of the compound ammonias by Wurtz, brings us to the dawn of modern 

 chemistry. Henceforward organic compounds were seen to be capable of compari- 

 son with simple inorganic bodies, and hydrogen not only capable of replacement 

 by chlorine, or by a metal, but by an organic group or radical. 



To this period my memory takes me back. Liebig at Giessen, Wohler in Got- 

 tingen, Bunsen in Marburg, Dumas, Wurtz, and Laurent and Gerhardt in Paris, were 



