1820.] Ferrocki/azate of Iron. 217 



that the pigment is essentially a compound of one atom of ferro- 

 chyazic acid and one atom of peroxide of iron, supposing the 

 weight of an atom of peroxide of iron to be 5. The pigment 

 contains hkewise a certain quantity of water, from which it 

 cannot be entirely freed without undergoing at least partial 

 decomposition. Probably there is a certain temperature (about 

 212°) in which, if it be kept, it would be reduced to a compound 

 of one atom acid + one atom peroxide + one atom water. On 

 that supposition its constituents would be as follows : 



1 atom acid == 6-750 52-42 



1 atom peroxide. . . = 5-000 38-8o 



1 atom water = 1-125 8-75 



12-875 100-00 



The quantity of water in the pigment which I subjected to 

 analysis exceeded the above theoretical quantity by about two 

 per cent, doubtless because I had not been at the requisite pains 

 to dry it sufficiently. 



I hinted in a preceding immber of the Ajuiah of Philosophy at 

 some of the phenomena produced by the exposure of this pig- 

 ment to a red heat ; but they are so curious and so instructive 

 that I think them well deserving a more particular detail, and 

 shall, therefore, make them the subject of the present paper. It 

 will be recollected that the ferrochyazate of iron, which was the 

 subject of the following experiments, was composed of 



Ferrochyazic acid 51-0 or 25*5 



Peroxide of iron 37-8 18-9 



Water 11-2 5-6 



100-0 60-0 



Before proceeding to detail the effects produced on this salt 

 by a red heat it may be necessary to lay before the reader the 

 present state of my knowledge of the constituents of ferrochy- 

 azic acid. In vol. xii. of the A/uials of Philosophij, p. 102, will 

 be found an account of a set of experiments which I made on 

 purpose to determine the composition of this acid. My analysis 

 of the ferrochyazate of potash gave me the following quantities : 



Acid/^^°" ^^'^1-45-Q 



^^^^ t Volatile matter 30-9 / - ^^ ^ 



Potash =41-6 



Water = 13-0 



100-5 



From this analysis it appears that nearly one-third of the acid 

 consists of iron; and that the weight of the otlier constituents 

 of the acid in a grain of the salt amounts to 0*309 gr. When 



