on the Atomic Weight of Carbon. 221 



barometer and the proportion of humidity in the atmosphere, 

 exert no influence on this experiment, and the silver which 

 remains behind is not hygroscopic. 



The only precaution in these experiments, to which the 

 greatest attention must be directed, is, of course, the rigid 

 purity of the salt, and consequently the entire absence of all 

 hygroscopic water. 



It is at all times a difficult task to obtain a chemical com- 

 pound in a state of absolute purity ; to do this in our experi- 

 ments, was, however, very important, as even the most minute 

 admixture of a foreign substance must increase the found 

 weight of the atom of carbon. 



We were soon convinced that there are but few silver salts 

 available in determinations of this kind ; most of them are ob- 

 tained in the form of caseous, or pulverulent precipitates, which 

 enter into combination with a part of the precipitant. The 

 quantity of foreign matter which adheres to these precipitates 

 is so small, that it does not generally render the estimations of 

 atomic weights incorrect, but, as before observed, we are here 

 obliged to avoid every source of fallacy. We selected from 

 those silver salts which are perfectly crystalline, which retain 

 no water when dried at the ordinary temperature, which are 

 not hygroscopic, and which (and this is a character of great 

 importance) do not sputter when heated. The salts em- 

 ployed should moreover leave behind no carburet of silver 

 after calcination. 



There are, as stated above, but few silver salts which are 

 not liable to one or other of these sources of error. 



The cyanide of silver, for instance, is easily obtained in a 

 pure state, by precipitating the nitrate of that metal by means 

 of hydrocyanic acid; it can even be obtained in large shining 

 tables, by allowing to cool slowly a hot mixture of hydro- 

 cyanic acid, with a dilute ammoniacal solution of silver; care- 

 fully washing the crystals, thus obtained, with solution of 

 ammonia, and drying them at 120° C. By the latter treat- 

 ment, and even at the ordinary temperature, all the ammonia 

 is got rid of, and the crystals become opake milk-white, with- 

 out suffering any other change ; they are nevertheless in 

 practice not fitted to be employed in our experiment, for they 

 do not appear to be entirely decomposed by heat. 



When the cyanide of silver is first heated, it melts without 

 evolving any gas ; by increasing the temperature cyanogen is 

 given off, and a basic cyanide is formed ; at a certain point 

 beyond this, flame is developed, nitrogen is then evolved, the 

 flame being at the same time as it were extinguished; and 

 fused carburet of silver, of a dull white colour, remains be- 



