AND THE PAEACYANIC ACID. 31 



tended by Berthollet. In 1806, the characters and compounds of the new acid 

 were more fuUy detailed by Proust; and in 1815, its composition rigorously de- 

 termmed by Gay Lussac, in his admirable researches into the properties of cya^ 

 nogen. Still it was not till 1819 that the exact constitution • of the original pig- 

 ment, Prussian blue, was established by Berzelius with any degree of certainty : 

 and not tiU after the discovery of the red prussiates or ferro-cyanides by Leopold 

 Gmelin in 1822 that the last doubts were removed. Thus, from the time of its 

 accidental discovery, a period of 112 years intervened, before our knowledge be- 

 came so far extended that we could give a satisfactory reason for the various 

 steps necessary to its production. 



Yet the many more or less unsuccessful labours which this long period saw 

 were not spent in vain. Ahnost every experimenter has recorded observations 

 fitted to be the germ of new researches, and so many branches have akeady shot 

 forth from the main trunk of investigation, as to render the department which m- 

 cludes cyanogen and its Idndred compounds, by far the most complicated and dif- 

 ficult of the chemistry of the present day. 



Among the more unportant memoirs to which we are indebted for the recent 

 development of this branch of the science, may be enumerated those of Poerett 

 on the ferro and sulpho cyanic acids ; of Leopold Gmelin on the suites of com- 

 pounds formed on replacing the iron of the ferro-cyanides by otlier metals ; of 

 WoHLER on the cyanic acid ; of Howard, Liebig, and Edmund Davy on the ful- 

 minic acid ; of Sehullas on the cyanic acid, and many other interesting com- 

 pounds of cyanogen ; and of Mosander and others on the complicated combina- 

 tions which the double cyanides are capable of forming with each other. To these 

 must be added also the beautiful memoirs of Liebig and Wohler on the cyanic 

 and cyanuric acids ; of Liebig alone on mellon, and the curious compounds to 

 which, by the action of acids and alkalies, it gives rise ; and, most recently, of Leo- 

 pold Gmelin on the mellonic and hydro-mellonic acids, the investigation of which 

 still remains to a. considerable degree imperfect. 



While, therefore, the history of this one department shews what lengthened 

 and laborious research the illustration of some branches of. nature requhes, it 

 shews, at the same time, how little the failures of even a succession of experi- 

 menters affects the ultunate advancement of knowledge,— how men may grope 

 on m darkness year after year, perhaps age after age, despairing of success,— and 

 yet may be all the while unconsciously laying the foundations and storing up the 

 materials of future buildmgs, till at length accident or genius guides some pbUo. 

 sopher into a new path, or puts mto his hand a new instrument before which all 

 obstacles give way. 



