334 Dr. Thomson on the true Weight of [[Nor. 



seem easy to obviate. I found the quantity of water in these 

 crystals to vary somewhat in different specimens. Besides the 

 water which enters as a chemical constituent into the salt, there 

 seems to be some water lodged mechanically between the plates 

 of the crystal, and this water I suppose may vary in quantity. 

 But the best formed, and firmest, and most solid crystals of sul- 

 phate of soda that I have been able to procure contain more than 

 10 atoms of water united to an atom of the anhydrous salt. 



Dr. WoUaston's number for the weight of an atom of protoxide 

 of lead is the same as mine, while the weight assigned by Dalton 

 is a httle too high, and that of Berzelius a little too low. My 

 number is a multiple of 0-125; for 0-125 x 112 = 14. This is 

 not the case either with the numbers of Dalton, or of Berzelius, 

 and Berzelius's number is so large and complicated that it can- 

 not easily be remembered. 



Dr. WoUaston's number for the atom of nitric acid differs very 

 little from mine. The number of Berzelius is a little too high. 

 As usual, it is complicated, difficult to remember, and not a 

 multiple of 0-125. As to Mr. Dalton's number, it deviates so 

 very far from the truth that it is not worth while to compare it 

 with that of the other chemists.* 



I shall now show by a simple and decisive experiment that 

 my numbers both for nitric acid, and for protoxide of lead, are 

 correct. 



Nitrate of lead crystallizes in octahedrons, and when properly 

 prepared, it contains no water of crystaUization. It is composed 

 of an atom of nitric acid united to an atom of protoxide of lead. 

 No-vv 



1 atom nitric acid = 6-75 



1 atom protoxide of lead =14-00 



1 atom nitrate of lead =20-75 



We have seen already that sulphate of potash is composed of 



1 atom sulphuric acid = 5 



1 atom potash = 6 



11 



Dissolve in distilled water 20-75 grs. of pure and dry nitrate 

 of lead, dissolve in another portioii 11 grs. of perfectly dry sul- 

 phate of potash. Mix the two solutions together, and agitate 

 them well. 



Sulphate of lead speedily precipitates. After the liquid has 

 become quite clear draw it off by a sucker, and divide it into 



♦ It is proper to observe, however, that Mr. DaUon conceives the nitrates to be 

 compounds of two atoms of nitric acid with one atom of base. In reality, there- 

 fore, we ought to double the number which he assigns for the weight of an atom of 

 nitric acid. This would make it 0-4256, a number which is still a great deal too 

 low, though much nearer the truth than the number in the table. 



