162 Dr. Turner on the Volatility of Oxalic Acid. 



instead of remaining pulverulent, becomes covered with nu- 

 merous minute acicular crystals, and an acrid vapour rises, 

 which condenses on cold surfaces in the form of needles. This 

 vapour is accompanied with a small quantity of moisture, which 

 completes the two equivalents of water required to be with- 

 drawn, in order to constitute the sublimed acid. 



The sublimation of oxalic acid at 212, though sufficient 

 both to occasion loss in analysis, and to establish the fact of 

 volatility, is too slow for affording a supply of the sublimed 

 acid. A convenient process for this purpose is the following: 

 About half an ounce or an ounce of oxalic acid, purified by 

 repeated crystallization, is dried in a rather deep evaporating 

 basin, exposed on the sand-bath to a temperature of about 350 

 or 4-00 Fahr. : as soon as sublimation commences, the vessel 

 should be covered with a layer of smooth filtering paper, on 

 which is laid a fold of common blotting-paper, and both are 

 pressed tight upon the edge of the basin by means of another 

 and somewhat larger capsule, placed with its convexity down- 

 wards, and containing cold water or ice. During this rapid 

 sublimation some of the acid is decomposed, and the water 

 derived from this source is absorbed by the coarse outer fold 

 of paper ; while the acid is condensed on the smooth paper 

 below, and gradually falls down upon the sides of the dish. 

 At intervals of about an hour the apparatus should be removed 

 from the fire, and the sublimed portions, while still warm, be 

 brushed away with a feather, and quickly secured in a well- 

 stoppered bottle. 



Sublimed oxalic acid, as thus procured, is commonly in 

 .he form of minute shining acicular crystals; but I have oc- 

 casionally obtained it in slender prisms half an inch long, 

 possessed of considerable lustre and transparency. On ex- 

 posure to the air it becomes dull and opaque from the absorp- 

 tion of moisture, 45 parts or one equivalent of the sublimed 

 acid rapidly acquiring two equivalents of water, and thus re- 

 gaining its original constitution. This water is again com- 

 pletely expelled by a temperature of 212. The vapour of 

 the acid is very pungent, exciting cough and sneezing more 

 readily than the fumes of nitric or muriatic acid. 



Sublimed oxalic acid rises slowly, as already mentioned, at 

 212. As the temperature increases, the sublimation becomes 

 more rapid ; and if the heat does not exceed 300 or 330, the 

 acid sublimes entirely without decomposition. At 360 the 

 sublimation is very free; between this point and 400 it sub- 

 blimes rapidly; and at 4-14> it fuses and enters into brisk 

 ebullition. At temperatures exceeding 330 more or less of the 



subliming 



