218 Dr. Thomson on [Sept. 



one grain of the salt was heated to redness with peroxide of 

 copper, I obtained vohimes of carbonic acid gas and of azotic 

 gas which bore to each other nearly the proportion of 2^ to 1- 

 From this 1 was disposed to conclude that the carbon and azote 

 in the ferrochyazic acid bore to each other the same ratio as in 

 hydrocyanic acid ; namely, two atoms of carbon to one atom of 

 azote ; but in a paper which Mr. Porrett inserted in the Annals 

 of Philosop/ii/ for '&ept. 1818, he suggested the probability that 

 the atoms of carbon and of azote in ferrochyazic acid bore ta 

 each other the ratio of 4 to 1 ; and he stated an experiment 

 which corroborated that notion. This naturally led me to recon- 

 sider my experiments, and to endeavour to detect the cause of 

 the deficiency of carbonic acid in all my trials. I repeated my 

 experiments with an improved apparatus, but still found the 

 proportions of carbonic acid gas and azote to each other nearly 

 as at first. 



After the publication of Mr. Porrett's new paper on Ferrochy- 

 azic Acid in the Annals of Philosophy for October, 1819, in 

 which he gives experiments, from which he deduces that the 

 volume of carbonic acid gas evolved is four times that of the 

 azotic gas ; but admits the quantity of iron to be no greater than 

 I had found, I repeated the experiments again with all the 

 requisite care; but my results were still two volumes of carbonic 

 acid for one volume of azote very nearly ; nor was I able to 

 obtain so great a proportion of gas from a given weight of ferro- 

 chyazate of potash as Mr. Porrett got. I have been unable, there- 

 fore, to verify Mr. Porrett's conclusions, and consequently must 

 still adhere to those which I gave in my first paper. On coming 

 to London, Dr. Prout informed me that he had made many experi- 

 ments on the analysis of the ferrochyazates, and had always ob- 

 tained with his new apparatus two volumes of carbonic acid gas 

 for one volume of azote. I repeated my experiments with his appa- 

 ratus, which is susceptible of greater precision than my own. The 

 result was as follows : five grains of crystallized ferrochyazate of 

 potash gave out eight cubic inches of gas, when heated with 

 peroxide of copper. Of these, 4-6 cubic inches are absorbed by 

 potash, and consequently are carbonic acid gas ; while 3-4 cubic 

 inches remain unabsorbed, and consequently are azotic gas. 

 Now if we take into our estimate the carbonic acid that must 

 remain united to the potash of the salt, we shall find that the 

 quantities of gas evolved were as follows : 



Carbonic acid gas .... 6-8 cubic inches or 2 volumes 

 Azotic gas 3'4 cubic inches or 1 volume 



It seems decided from these experiments that the carbon and 

 azote exist in ferrochyazic acid exactly in the same proportions 

 as in cyanogen. As to the hydrogen and the iron, I have not 

 made new experiments to determine their proportion. 



I am still at as great a loss as ever to form an adequate idea 



