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II. — On Paracyanogen and the Paracyanic Acid. By James F. W. Johnston, 

 A. M., F. R. S. E., Professor of Chemistry and Mineralogy in the University 

 of Durham. 



Bead 4th April 1836. 

 INTRODUCTION. 



The history of the newer sciences presents many instructive examples of the 

 progress of the human mind in developing the germs of natural knowledge, and 

 building on a single observation entne departments of science. Few pursuits in- 

 deed are more interesting, even to the student of immaterial nature, than in the 

 perusal of such a history to trace the footsteps of the several investigators, and to 

 mark how far, and by what means, — whether by new methods or by greater pa- 

 tience of research, — each successive observer advanced the inqiiuy. We see the hu- 

 man mind, as it were, set free from the trammels of time, and developing its resources 

 on a large and continuous scale, not limited by the powers of one intellect, the length 

 of one life, or the means of one station. We see, at the same time, what varied 

 gifts and opportunities are necessary for the elucidation of a single subject ; how 

 these gifts, though not all imparted to one man or to one generation, are yet 

 wisely and beneficently bestowed on the entire species ; and how all ai-e thus ena- 

 bled to co-operate, either in unfolding abstract truth, or in drawing forth those 

 practical results Avhich directly conduce to the amelioration and comfort of all. 



These reflections are particularly suggested by the history of that branch of 

 chemical science, to which the facts contained in the following paper are intended 

 to form a small addition. 



Early in the last centiuy, about 1710, a solution of potash which had been 

 employed by Dippel in the purification of his animal oil, and afterwards calcined, 

 was accidentally mixed with a solution of sulphate of iron. A beautiful blue pre- 

 cipitate was the result, and by the repetition of the experiment a pigment Avas 

 obtained, since known in commerce by the name of the Prussian blue. This sin- 

 gle observation gave rise to a multitude of researches. Woodward, Macquer, 

 MoRVEAU, Lavoisier, and Bergman, successively experimented on the blue sub- 

 stance with little success. Upwards of seventy years elapsed before any Ught 

 was thrown upon its true nature; — when in 1782 Scheele obtained from it the 

 hydrocyanic or prussic acid. Five years later, his results were verified and ex- 



