216 ; Mr. Porrett on the [Sept. 



products from five grs. of ferruretted chyazate of potash when 

 decomposed by ignited peroxide of copper, it will be found 

 that there should be produced 



Cubic Inches. 



Carbonic acid 7-12 



Azote 1-78 



Total gas. 8-90 



But Dr. Thomson only obtained, 



Carbonic acid 5-205 



Azote 2-420 



Total gas 7-625 



I can only account for this difference by supposing that a 

 little atmospheric air, previously contained in the apparatus and 

 carried over with the produced gases, must have formed part of 

 the 2-420 cubic inches of azote, and that some of the ferruretted 

 chyazic acid (before the oxide of copper acquired a red heat) 

 must have yielded some other product which escaped observa- 

 tion ; possibly a little prussic acid, or cyanogen. ' If I am wrong 

 in these suppositions, Dr. Thomson will, I dare say, excuse me 

 for making them; he must be as well aware as I am how exceed- 

 ingly difficult it is to obviate every source of error in experiments 

 of this delicate nature. 



It is a great satisfaction to me to find that Dr. Thomson has 

 abandoned the opinion which he entertained, that the ferruretted 

 chyazic acid contained no hydrogen, and was a compound of 

 cyanogen and of iron only, an opinion which induced him to 

 name it in his System of Chemistry the ferro-cyanic acid, and 

 its salts ferro-cyanates. I was perfectly convinced from many 

 circumstances that occurred during my first experiments that 

 this opinion was erroneous, and should have combated it when 

 it appeared in his System had I been fond of controversy, or 

 been able to find time for carrying on such a course of experi- 

 ments as would, perhaps, have been requisite to produce convic- 

 tion in others ; as it was I contented myself with expressing to 

 my chemical friends my dissent from Dr. Thomson's opinions on 

 this subject ; and I can venture to assure him that whenever he 

 makes experiments with the sulphuretted chyazic acid, he will be 

 convinced that it also contains hydrogen, and that the names 

 sulpho-cyanic acid and sulpho-cyanates are quite inappropriate ; 

 equally so are the names proposed by Dr. Henry, of ferro-prussic 

 and sulphuretted prussic acids, as these names imply that the 

 prussic acid is contained in these compounds, instead of being 

 merely the result of a new play of affinities when they are 

 decomposed. 



I shall .conclude this communication with acquainting you, for 



