1819.] Vauquelin on Cyanogen and Hydrocyanic Acid. 429 
a fixed ratio between their sines and cosines. But in the first 
place, these angles can only be approximations ; the measures 
from which they have been deduced having but an indefinite 
degree of precision. Further, suppose in the valuation of these 
measures we neglect every thing beyond a certain quantity, Such 
as the minute or second, the numbers representing the sines and 
cosines will always exhibit a series» of decimals, which has no 
termination ; so that we must still neglect something in order to 
submit them to calculation. In my mode of operating the 
occurrence of a simple ratio, which presents itself to our view, 
points out the term at which we ought to stop ; so that if different 
observers are directed by the same rule, they will agree about the 
choice of the fixed point in question. If, on the other hand, we 
suppose them to set out from measures taken with different 
instruments in their possession, they will necessarily vary in the 
choice of the limit at which they ought to remain. 
Thus the measures of the angles, which have been published, 
though valuable in themselves, are hitherto nothing more than 
isolated observations, which nobody has attempted to bring 
under the requisite form to make suitable to the theory. It is 
the business of the philosophers who have given us these mea- 
sures to complete their work by giving us the manner of deducing 
from them the fixed data for the solution of problems relative to 
the geometry of crystals. But I think I can affirm, that these 
data will do nothing more than displace a little the term from 
which the theory must set out, and that without any other 
assistance than that of the ordinary goniometer, it has at present 
all that is requisite to arrive at its principal object, by a route 
equally certain and easy. 
: Articre III. 
Memoir on Cyanogen and Hydrocyanic Acid. By M.Vauquelin.* 
Prussic acid, in consequence of its singular nature, may be 
reckoned among the number of bodies which more particularly 
captivated the attention of the most celebrated chemists. The 
annals of the science recall the numerous experiments tried in 
vain by Geoffroy, Macquer, and Bergmann, to separate the 
colouring principle of prussian blue. It was.reserved for Scheele 
to make that important discovery, which afterwards received 
from Berthollet all the development consistent with the then 
state of chemical science. For decet the continual progress 
which chemistry made from day to day soon enabled us to per- 
ceive great blanks in our knowledge of the properties of prussic 
* Translated from the Journal de Pharmacie, Noy, 181%, p. 435. 
