APPENDIX II 



THE PERIODIC LAW OF THE CHEMICAL ELEMENTS 



BY PROFESSOR MENDELEEFF 



FARADAY LECTURE DELIVERED BEFORE THE FELLOWS OF 

 THE CHEMICAL SOCIETY IN THE THEATRE OF THE ROYAL INSTITUTION, 

 ON TUESDAY, JUNE 4, 1889 



THE high honour bestowed by the Chemical Society in inviting me to pay a 

 tribute to the world-famed name of Faraday by delivering this lecture has 

 induced me to take for its subject the Periodic Law of the Elements this 

 being a generalisation in chemistry which has 1 of late attracted much 

 attention. 



While science is pursuing a steady onward movement, it is convenient 

 from time to time to cast a glance back on the route already traversed, and 

 especially to consider the new conceptions which aim at discovering the 

 general meaning of the stock of facts accumulated from day to day in our 

 laboratories. Owing to the possession of laboratories, modern science now 

 bears a new character, quite unknown, not only to antiquity, but even to the 

 preceding century. Bacon's and Descartes' idea of submitting the mechanism 

 of science simultaneously to experiment and reasoning has been fully realised 

 in the case of chemistry, it having become not only possible but always 

 customary to experiment. Under the all-penetrating control of experiment, 

 a new theory, even if crude, is quickly strengthened, provided it be founded 

 on a sufficient basis ; the asperities are removed, it is amended by degrees, 

 and soon loses the phantom light of a shadowy form or of one founded on 

 mere prejudice ; it is able to lead to logical conclusions, and to submit to ex 

 perimental proof. Willingly or not, in science we all must sttbmit not to what 

 seems to us attractive from one point of view or from another, but to what 

 represents an agreement between theory and experiment ; in other words, to 

 demonstrated generalisation and to the approved experiment. Is it long 

 since many refused to accept the generalisations involved in the law of Avo- 

 gadro and Ampere, so widely extended by Gerhardt ? We still may hear the 

 voices of its opponents ; they enjoy perfect freedom, but vainly will their 

 voices rise so long as they do not use the language of demonstrated facts 



