7 o 
Dr. Thomson on Oxalic Acid. 
exposed in a damp place, it absorbs a little moisture from the 
atmosphere. 
This salt combines with an excess of acid, and forms a 
superoxalate, long known by the name of salt of sorrel. It is 
very sparingly, soluble in water, though more so than tartar. 
It occurs in commerce in beautiful 4-sided prisms attached to 
each other. The acid contained in this salt is very nearly 
double of what is contained in oxalate of potash. Suppose 
100 parts of potash ; if the weight of acid necessary to con- 
vert this quantity into oxalate be x then 2 x, will convert 
it into superoxalate. 
6. Oxalate of soda readily crystallizes. Its taste is nearly 
the same as that of oxalate of potash. When heated, it falls to 
powder, and loses the whole of its water of crystallization. 
Soda is said to be capable of combining with an excess of acid, 
and of forming a superoxalate. I have not tried the experi- 
ment. 
7. Oxalate of ammonia is the most important of all the 
oxalates, being very much employed by chemists to detect the 
presence of lime, and to separate it from solutions. It crystal- 
lizes in long transparent prisms, rhomboidal, and terminated, 
by dihedral summits. The lateral edges are often truncated, 
so as to make the prism 6 or 8-sided. Sometimes the original 
faces of the prism are nearly effaced. 
The taste of this salt is bitter and unpleasant, somewhat 
like that of sal ammoniac. At the temperature of 6o°, 1000 
grains of water dissolve only 45 grains of this salt. Hence, 
1000 grains of saturated solution of oxalate of ammonia con- 
tain only 43.2 grains of this salt. The specific gravity of- this 
solution is 1.0186. As it may be useful to know the weight 
