1816} Experiments on Prussic Acid. 351 
It is to Macquer that we owe the first important experiments on . 
Prussian blue.* On boiling it with a solution of potash in excess, , 
that‘ skilful chemist observed that nothing remained but oxide of, 
iron, while the alkali, combined with the colouring matter... On the 
other hand, if the Prussian blue predominate, the potash is com- 
pletely saturated with the colouring matter, and loses its alkaline 
properties, just as if it were saturated with an acid. In both cases 
it has acquired the property of producing Prussian blue with, solu- 
tions of iron, by means of double affinity;; and it precipitates the 
greater number of the other metalline solutions. When Prussian, 
blue is calcined, volatile alkali is formed—a fact which had likewise 
been observed by Geoffroy—and there remains oxide of iron _at- 
tracted by the magnet, and a quantity of charcoal. From these ex- 
periments, Macquer concluded that Prussian blue is a compound,of, 
oxide. of iron with an inflammable substance, which is converted by 
calcination into volatile alkali and charcoal. iy to dT 
Twenty years after, MM. de Morveau and Bergman considered | 
the colouring matter as a particular acid, and the first. gave it, the, 
name of prussic acid. However, its true nature remained still 
unknown. . . ' sl 
Scheele, whose name is connected, with so. many. brilliant disco- 
veries, succeeded in 1782 in obtaining prussic acid in a.separate., 
state by a very ingenious process, and approached verv near to,a, 
knowledge of the true nature of its constituents ; for he ascertained 
that it is obtained by the union of ammonia witha charry matter 
rendered yolatile by heat. + 5 isd 
‘To these important experiments of Scheele succeeded, those of 
Berthollet, which are not less so. He. showed that, Macquer’s 
combination of the colouring matter and potash is a triple salt, of 
which iron constitutes the third element. .On mixing chlorine with . 
prussic acid, as obtained by Scheele, he observed that the first 
substance is changed into muriatic acid, and that the second has , 
acquired a much stronger smell, and has lost part of its.affinity for 
alkaline bases. In this new state it no longer forms Prussian blue 
with solutions of iron, but a green precipitate, which becomes blue 
when exposed to light, or when mixed with sulphurous acid. If 
potash be added, the prussic acid is completely destroyed, ammonia 
is produced, which is disengaged; and carbonic acid, which re- 
mains combined with the potash. From these results, and from 
the knowledge of the elements of ammonia, for which likewise we 
are indebted to Berthollet, he considered prussic acid as a com-. 
ound of carbon, azote, and hydrogen. He does not admit oxygeu 
into the number of its elements, and he supposes that the oxygen 
contained in the carbonic acid produced by the action of potash on— 
prussic acid altered by chlorine, is furnished by tiis last substance, 
The absence of oxygen in prussic acid not being rigorously demon- 
* Mem. de I’Acad. des Sciences, 1752, 
+ Opusc. de Scheele, vol. ii, p. 141. t Aon, de Chim, vol, i. p, 30, 
