2 8 Introduction 



The total weight of each solution was 3 pounds 10 ounces and 12 

 pennyweights, or 1116 pennyweights Troy. The quantity of each sub- 

 stance when reduced to pennyweights is in every case very nearly the 

 equivalent weight of that substance in the system adopted at present, in 

 which the equivalent weight of hydrogen is taken as unity*. 



Now these experiments were made in 1777, and it is difficult to see 

 from what source, other than determinations of his own, he could have 

 derived these numbers. Wenzel's Lehre von den Verwandschaften was pub- 

 lished in 1777. I have not been able to consult the work itself, but from 

 the account of it given in Kopp's Geschichte der Chemie, the equivalent 

 numbers seem to have been larger than those used by Cavendish. Richter's 

 Anfangsgrunde der Stochyometrie was not published till 1792. 



It is difficult to account for the agreement not only of the ratios but 

 of the absolute numbers given by Cavendish with those of the modern 

 system, in which the equivalent weight of hydrogen is taken as unity. 

 I can only conjecture from several parts of his paper on Factitious Airs 

 (Phil. Trans. 1766), that Cavendish was accustomed to compare the 

 quantity of fixed air from different carbonates with that from 1000 grains 

 of marble. Now the modern equivalent weight of marble is 100, so that if 

 Cavendish took 100 pennyweights as the equivalent weight of marble, the 

 equivalents of other substances would be as he has given them. This I 

 think is more likely than that he should have selected inflammable air 

 as his standard substance at a time when even his own experiments left 

 it doubtful whether inflammable air was always of the same kind. 



In his journal, Cavendish writes down these equivalent weights just 

 as a modern chemist might do, without a hint that a list of these numbers 

 was not at that time one of the things which every student of chemistry 

 ought to know by heart. It is only by comparing the date of these re- 

 searches with the dates of the principal discoveries in chemistry, that we 

 become aware, that in the incidental mention of these numbers we have 

 the sole record of one of those secret and solitary researches, the value 

 of which to other men of science Cavendish does not seem to have taken 

 into account, after he had satisfied his own mind as to the facts. 



I take this opportunity of expressing my thanks to the many friends 

 who have given me assistance in preparing this edition, and in particular 

 to Mr C. Tomlinson, who gave me valuable information about the manu- 

 scripts; to Mrs Sime, who lent me a manuscript book of letters, &c., 

 relating to Cavendish, collected by her brother, the late Dr George Wilson; 

 to Mr W. Garnett, of St John's College, Cambridge, who copied out Arts. 

 236-294; and Mr W. N. Shaw, of Emmanuel College, who took the photo- 

 graphs from which the facsimile figures were executed; to Mr H. B. 

 Wheatley, who furnished me with information connected with the history 



* See Note 34. 



