220 Dr. Thomson on [Sept. 



azate of potash is heated in the open air. Let us now see what 

 happens when the pigment is heated in vessels excluding the 

 action of the external air. The experiments which I am going 

 to relate were frequently repeated ; but I conceive it to be suffi- 

 cient to give a single example, as the results in all the experi- 

 ments were entirely similar. Into a copper tube, about 14 inches 

 in length, and about 0"35 of an inch in internal diameter, close 

 at one end, and having a tube partly of brass and partly of glass, 

 fitted to the open end by grinding, 1 pat 60 grs. of ferrochyazate 

 of potash, constituted, as stated in a preceding part of this paper. 

 The pigment was secured in its place by filling the empty part 

 of the tube with asbestos. The internal capacity of the whole appa- 

 ratus, when empty, was 2*18 cubic inches ; but the glass tube 

 which constituted the greatest partof that capacity (being blown 

 out towards the middle into a hollow ball) was partly filled with 

 dry muriate of lime ; so that the whole common air which 

 existed in the apparatus did not exceed one cubic inch. The 

 copper tube was surrounded with burning charcoal, which was 

 urged by a pair of bellows so as to bring the pigment as speedily 

 as possible to a strong red heat ; and in this state it was kept for 

 two hours. 



After the conclusion of the process, I allowed the tube to get 

 quite cold. I then shook out the whole residual matter from 

 the 50 grs. of pigment. It still acted feebly as a pyrophoru^, 

 catching fire of its own accord upon coming in contact with the 

 air, and giving out the smell of ammonia ; so that even a red 

 heat of two hours' continuance did not decompose the pigment 

 completely. It has been long known, I presume, that when 

 ferrochyazate of iron is exposed to a red heat in a close vessel, 

 and then exposed to the air, it burns strongly as a pyrophorus. 

 I am disposed to ascribe this property to a small residue of pot- 

 ash which the pigment always retains, and which by the reaction, 

 of the other constituents, is converted into potassium. The 

 pyrophorous quahty, however, is considerably impaired when 

 the red heat is kept up for two hours. 



The weight of this residue after the spontaneous combustion 

 was over amounted to 24 grs. When dissolved in muriatic acid, 

 this residue did not leave any prussian blue, but only a small 

 quantity of black matter, Avhich possessed the usual character* 

 of animal charcoal. 



NowoOgrs. of ferrochyazate of iron contain (as willbe seen above) 

 18"9 grs. of peroxide of iron ; and the iron in the ferrochyazic 

 acid present amounted to 6-&1 grs. We see from this that part 

 of the iron must have been driven off' by the heat; for 18*9 + 

 6-6 1 = 25*5 1 grs. which exceeds the whole weight of the residue by a 

 grain and a half. I am disposed from this to suspect that part 

 of the ferrochyazic acid is driven off without undergoing decom- 

 position. 



The muriate of lime increased in weight during the continu- 



