498 
DR. PLAYFAIR ON THE NITROPRUSSIDES, 
24' 14 grs. gave 6'92 grs. peroxide of 
iron and 6'70 
grs. oxide 
zinc. 
9'43 grs. gave 7*10 grs. 
, carbonic acid and 0'335 
gr. water. 
Calculated. 
Iron 
20'07 
5 
140 
20-11 
Zinc 
22-26 
5 
160 
22-98 
Carbon .... 
20-53 
24 
144 
20-69 
Hydrogen . . 
0-39 
2 
2 
0-28 
Nitrogen I 
36-75 
15 
210 ■) 
35-94 
Oxygen J 
5 
40 J 
100-00 
696 
100-00 
This analysis would lead to the formula FegCyjg 3NO Zn 5 + 2 HO. 
Section III. — Changes experienced hy certain Nitroprussides when their solutions 
are heated or hept. 
18. Several of the nitroprussides, especially nitroprussic acid, nitroprussides of 
ammonium, barium and calcium, deposit either prussian blue or oxide of iron when 
their solutions are heated or are kept for sometime. The residual liquid, after evapo- 
ration, yields crystals of the same shape and exactly of the same properties as before. 
Analysis however shovvs that some change has resulted in their composition, for the 
iron or electro-negative metal is now in greater than atomic proportion to the electro- 
positive metal. The proportion of carbon is also somewhat different. Still the differ- 
ence in composition is not very considerable, although decidedly marked ; it is not 
however sufficient to cause any obvious alteration in their general properties. In 
fact, there is an attached impurity, probably a cyanide of iron, which cannot now be 
removed by crystallization, precipitation, digestion with nitric acid, or any of the 
ordinary means of purification. This impurity, if it be one, remains so obstinately 
attached that all methods of purification have quite failed to remove it. This circum- 
stance, before it was understood, had thrown the greatest difficulties in the way of 
the inquiry, and protracted it to a most tedious length by preventing the attainment 
of accordant results. It is to prevent the like inconvenience to those who repeat these 
experiments that this section of the paper is specially devoted. Attention has pre- 
viously been drawn to the fact, that the nitroprussides form chemical compounds 
with the cyanides of iron. This seems to be a case of the same kind, but of more 
ultimate union. The impurity or chemically attached cyanide in this case appears 
to be Fe Cyg, or perhaps FeCy+H Cy, judging from analysis only, for its separation 
has not been accomplished. The proportion in which it is present is very small, 
generally only 2(Fe Cyg) to 7 equivs. of a nitroprusside, or if it be a chemical com- 
pound, 7 (Fe 5 Cyi 2 3NO-l-5R)-{-Fe2 Cy 4 . Still as the crystalline form and all the 
properties of the nitroprussides remain unchanged, we can scarcely view its presence 
in any other light than as an impurity. Several of the nitroprussides, viz. nitroprussic 
