336 -Dr. Thomson on the true Weight of [Nov. 



1 atom chromic acid = 6*5 



1 atom oxide of lead = 14-0 



1 atom chromate of lead = 20'5 



While nitrate of lead is composed of 



1 atom nitric acid = 6*75 



1 atom oxide of lead = 14-00 



1 atom nitrate of lead = 20-75 



It is obvious from this that in order to decompose completely 

 one atom of bichromate of potash, we nmst employ two atoms of 

 nitrate of lead. Dissolve, therefore, in distilled water 19 grs. of 

 bichromate of potash which has been dried in a red heat. In 

 another portion of distilled water dissolve 41-5 grs. of dry nitrate 

 of lead. Mix the two solutions intimately together, and allow 

 the chromate of lead to precipitate ; then draw off the clear 

 liquid with a sucker. It is perfectly colourless, and is neither 

 precipitated by sulphate of soda, nor by nitrate of lead. Hence 

 it is obvious that it contains neither chromic acid, nor oxide of 

 lead. Consequently the chromic acid in 19 grs. of dry bichro- 

 mate of potash is just capable of saturating the whole oxide of 

 lead in 41-5 grs. of dry nitrate of lead. If the chromate of lead 

 formed be dried in a temperature of about 400°, and weighed, 

 its weight will be found to amount to 



2 atoms chromic acid = 13 grs. 



2 atoms of oxide of lead =28 



We see from this that chromate of lead is a compound of 



1 atom chromic acid . . . . = 6-5 or 31*7074 

 1 atom oxide of lead . . . . = 14-0 68-2926 



20-5 100-0000 



And the weight of an atom of chromic acid is exactly G'5 



If the experiment be made by mixing the weights of nitrate of 

 lead and bichromate of potash indicated by Berzelius's numbers 

 for the weights of the respective atoms ; namely, 41*4352 grs. of 

 nitrate of lead and 18*93555 grs. of bichlorate of potash, we shall 

 find that the liquid, after the whole of the chromate of lead has 

 precipitated, still retains lead in solution — a plain proof that the 

 atomic weights of Berzehus are not correct. 



If we take 12-5 grs. of pure dry chromate of potash, and mix 

 them with 20-75 grs. of dry and pure nitrate of lead, the result 

 will be precisely the same. After the precipitation of the chro- 

 mate of lead, the liquid will be colourless, and will contain no 



