246 Atomic Weight of Boracic and Tartaric Acids. [April, 



tartaric acid 8 375. These numbers being probably derived 

 from my experiments on these acids in the New Series of the 

 Annals (vol. ii. p. 131 and 138), it may be proper to state, that 

 by subsequent experiments I have satisfied myself that the true 

 atomic weights of these two acids are as follows : 



Boracic acid 3*0 



Tartaric acid 8*25 



1. By turning to my experiments on boracic acid, you will 

 find that I was not quite satisfied of their accuracy. I was 

 anxious, therefore, to find some method which would be suscep- 

 tible of greater precision, and found it last summer in fluoboric 

 acid, which is a compound of 



Fluoric acid 1*25 



Boracic acid 3*0 



4-25 



And its atomic weight is 4*25. I had previously determined the 

 atomic weight of fluoric acid to be 1*25, and I knew from my 

 old experiments that the atomic weight of boracic acid was at 

 least as high as 2*75. It is obvious from this that fluoboric 

 acid is a compound of an atom of each of its constituents, and 

 consequently that an atom of boracic acid is 3. 



Davy's analysis of the hydrated boracic acid must be nearer 

 the truth than those of Berzelius and my own. I have repeated 

 them again with the same result as before. No doubt some of 

 the boracic acid had made its escape during the application of 

 the heat. 



2. My experiments on tartaric acid were made with the crys- 

 tals of that acid. I began to suspect that these crystals, which 

 are usually large, might contain some water mechanically lodged 

 between their plates. I, therefore, had recourse to tartrate of 

 potash, which contains no water of crystallization, and which 

 may be made anhydrous by exposure to a heat of about 212° for 

 a sufficient time. 14*25 grains of this anhydrous salt were dis- 

 solved in water and mixed with a solution of 2075 grains of 

 nitrate of lead. After the tartrate of lead had precipitated, the 

 supernatant liquid was tested by tartrate of potash, and by nitrate 

 of lead, but was not affected by either. Tartrate of lead is very 

 slightly soluble in water. The consequence of this is, that sul- 

 phate of soda when dropped into the supernatant liquid in the 

 above experiment occasions a sensible precipitate. 



The crystals of tartaric acid contain 1 atom of water united to 

 1 atom of acid; hence their true atomic weight is 9*375, and 

 not 9*5 as I stated formerly. 



I am, dear Sir, yours truly, 



Thomas Thomson. 



