828 FORMYL. 



crystallizes below 32 in brilliant plates ; it boils at 212; its 

 density is 1.2353. The vapour of the boiling acid is inflam- 

 mable and burns with a blue flame ; this hydrate combines 

 with a second atom of water forming a definite compound of 

 which the boiling point is 222.8 (106 centig.), which does not 

 freeze at 5, and of which the density is only 1.1104 at 59. 

 Both of these hydrates are highly corrosive. 



To prepare the dilute acid, a capacious glass retort or copper 

 still is employed, of which the capacity must be at least 10 times 

 greater than the volume of the materials to be employed, namely 

 1 part of starch, 4 parts of peroxide of manganese, 4 parts 

 of water, and 4 parts of sulphuric acid. The materials with- 

 out the acid are first introduced into the retort and heated 

 to 104 (40 centig.), and then the sulphuric acid is added by 

 small portions. A violent effervescence with swelling up of 

 the materials occurs, from the disengagement of carbonic acid 

 gas ; when this ceases and all the acid is added, the capital is 

 applied to the still, and the distillation continued, for which a 

 heat a few degrees above 212 is sufficient, till 4J parts of the 

 liquid are found in the receiver. The product is a dilute acid, 

 of which the density is about 1.025 ; the last portions often 

 contain sulphurous acid, and the product has always an aro- 

 matic odour from the presence of a small quantity of a volatile 

 oil formed in the process. The nature of the decomposition 

 which yields formic acid has not been exactly traced. To 

 purify the crude formic acid it is neutralised by milk of lime, 

 which forms an insoluble compound with sulphurous acid ; the 

 excess of lime is precipitated by a stream of carbonic acid and 

 the formiate of lime evaporated to dryness. By distilling 

 10 parts of formiate of lime with 8 parts of oil of vitriol diluted 

 with 4 parts of water, 9 parts of a pure acid are obtained of 

 specific gravity 1.075. 



Formic acid is entirely decomposed by an excess of sul- 

 phuric acid, without being charred, being resolved into carbonic 

 oxide which comes off with lively effervescence, and water 

 which remains in combination with the sulphuric acid. The 

 acid in question may easily be recognised by that property and 

 also by its action upon the oxides of silver and mercury. When 

 heated upon these oxides formic acid is completely destroyed, 

 producing carbonic acid gas which is disengaged, water and 



