56 Observations of a Comet. 



siate, as soon as we present to it either the black oxide, or 

 a salt with a base of black oxide, and acquires, besides the 

 advantage of crystallizing, that of being no longer decom- 

 posable by the heat of ebullition. 



This prussiate, which was the test liquor so long wanted 

 by chemists, does not give Prussian blue with solutions of 

 red oxide; but it gives them if ihey contain black oxide, be- 

 cause its acid is attached immediately to that portion of the 

 same oxide, which ought to serve as an intermedium be- 

 tween it and the red oxide. 



The triple prussiate of iron, or Prussian blue, strongly 

 heated, is reduced to ammonia, to the two gases of carbonic 

 acid and gaseous oxide, to iron (feracerc) and to charcoal. 



The prussiate of mercurv gives the same products by its 

 decomposition, besides a certain portion of oil. 



The charry lixiv'mms contain but little triple prussiate, 

 and a great deal of simple prussiate. They must not be con- 

 centrated without having in the first place strengthened the 

 constitution of the simple prussiate by an addition' of black 

 qxide, or of green sulphate. 



In order to obtain from these lixiviums the whole of the 

 Prussian blue which they are susceptible of giving, it is in- 

 dispensably requisite to use a sulphate, a portion of which at 

 least is green : without this precaution, the simple prussiate 

 contained in them could not furnish blue with a sulphate, 

 the base of which was completely red. 



To conclude, if this memoir be compared with that of 

 Scheele, it will be found that all the facts above stated were 

 perfectly well known to him : but they appeared to me to 

 require some further elucidation ; and with this view I lay 

 rny present memoir before the public. 



X. Observations of a Comet, made with a View to investi- 

 gate its Magnitude i and the Nature of its Illumination , 

 By William Hekschel, LL.D. F.R.S* 



X he comet, which We have lately observed, was pointed 

 out to me by Mr. Piggot, who discovered it at Bath the 



* From Philosophical Transactions for 1308, Part H. 



2Sth 



