Dr. Thomson on Oxalic Acid. 
shews traces of the presence of ammonia, changing vegetable 
blues to green, and smoking when brought near muriatic acid ; 
but this minute portion of ammonia is probably only accident- 
ally present. All the oxalates which I decomposed by distil- 
lation, were obtained by double decomposition from oxalate of 
ammonia ; and though they were washed with sufficient care, 
yet 1 think it not unlikely that a minute portion of oxalate of 
ammonia might continue to adhere. Practical chemists know 
the extreme difficulty of removing every trace of a salt with 
which another has been mixed. 
The carbonic acid remains partly combined witli the base, 
which always becomes a carbonate, and partly makes its escape 
in the form of gas. 
The carbonic oxide and carbureted hydrogen make their 
escape in the form of gas : the charcoal remains in the retort 
mixed with the base, to which it communicates a grey colour : 
the quantity of it depends in some measure upon the heat. If 
the oxalate was exposed to a very violent heat, no charcoal 
at all remains. Hence it probably acts upon the carbonic acid 
united to the base, converting it into carbonic oxide, as hap- 
pens when a mixture of a carbonate and charcoal are heated, 
3. I was induced to examine this decomposition with con- 
siderable attention, because I conceived that it would furnish 
the means of estimating the composition of oxalic acid; and I 
pitched upon oxalate of lime, as the salt best adapted for the 
purpose I had in view. A determinate quantity of this salt 
was put into a small retort, and gradually heated to redness. 
This retort was connected with a pneumatic trough by means 
of a long glass tube, having a valve at its extremity which 
allowed gas to issue out, but prevented any water fronj 
L 2 
