30 Mr. Smith and Dr. Brett's Additional Remarks 



tion of a well-luted and covered Berlin crucible, inclosed in 

 another covered and luted crucible, for the glass, iron and 

 copper tubes employed by him, — a variation surely not of a 

 nature to affect the product: then comes the remark, " that 

 this process is so difficult of performance," which, as we have 

 before shown, was not even distantly alluded to in the oi'iginal 

 memoirs, and which we have therefore a right to consider an 

 ex post facto objection, and as such inadmissible by us: this is 

 followed by the volunteer of the opinion, that " I do not think 

 Mr. Smith employed the same material as I used in this ex- 

 periment." Perhaps not, but at any rate it was a substance 

 prepared, as before noticed, with a slight, and, apparently to 

 us, most trivial variation of apparatus, in the mode prescribed 

 by Dr. Brown, and possessing the properties assigned by 

 him to paracyanogen. 



To gratify Dr. Brown, and "to make assurance doubly sure," 

 we prepared some paracyanogen by inclosing bicyanide of 

 mercury in a tube of German glass, drawing it out to a ca- 

 pillary, placing this in an iron tube with a plug, and exposing 

 the apparatus to a heat visible only in the dark, for a consi- 

 derable time over a coal fire. Dr. Brown directs a " coke 

 fire" to be employed; we trust this accidental departure from 

 "the very letter of the text" will not entirely vitiate the result 

 and " render the experiment of no value," for we did not no- 

 tice it until the experiment was terminated: the paracyanogen 

 thus obtained resembled that described by Dr. Brown as em- 

 ployed by him, and possessed the acknowledged properties of 

 paracyanogen ; 5 grs. of it mixed with 20 grs. of carbonate 

 of potash and ignited, in a closed platinum crucible placed in 

 an exterior covered and luted one, and all the precautions 

 taken directed in p. 236 of the Transactions, at a yellow heat, 

 which appears to be Dr. Brown's white heat, when withdrawn 

 from the fire afforded a white saline residue, which examined 

 in the ordinary and well-known way for silica, afforded none. 

 —(J. D. S.) 



This experiment was exactly repeated, only substituting 

 what Dr. Brown terms ptire paracyanogen (precipitated from 

 the sulphuric acid solution of the common product), i.e. that 

 obtained from the bicyanide of mercury, for the paracyanogen 

 previously employed, and in this case also no trace of silicic 

 acid "was obtained. — (J. D. S.) 



The objections raised against our experiments on the al- 

 leged formation of metallic siliciurets, are the paracyanogen 

 employed, and that they " were not performed according to 

 my direction." The first has already been disposed of, the 

 second is wholly uncalled for ; the difference being, that we 



