366 METALLIC OXALATES. 



heat. In fact, this reaction does not take place ; at least, not 

 without a strange complication. 



It would seem at first sight that oxalic acid is in the same 

 position ; but this is only true when we start from the acid in 

 a solid state. In fact, the acid partly assumes the gaseous 

 state, at the moment of decomposition ; for observation proves 

 that a portion is always volatilised under these conditions. 

 However, this volatilisation of the solid acid must from analogy 

 absorb about 8 to 12 Cal. Taking this quantity into considera- 

 tion, we see that oxalic acid, when gaseous, is on the confines of 

 an exothermal decomposition, which explains its instability. 

 When the acid is in solution, the decomposition is in reality 

 exothermal, for 



H 2 + C 2 + 4 + water = H 2 C 2 4 liberates + 1947, 

 while 



2C0 2 (gas) + water = 2C0 2 (dissolved) + 199-2. 



The difference," + 4'5 Cal., represents the heat liberated in the 

 reaction. Copper oxalate is also on the confines, and even 

 beyond, its decomposition being exothermal. Finally, that of 

 mercuric and silver oxalates is positively exothermal. 



5. Nevertheless, as regards mercuric oxalates the heat 

 liberated is limited, from a certain temperature, by the vola- 

 tilisation of the mercury, which absorbs 15 '4; but this 

 restriction does not exist in the case of silver oxalate ; and, in 

 fact, this compound is very explosive. It explodes very 

 energetically when subjected to a shock or when heated to 

 about 130. At 100 and lower it decomposes slowly and 

 progressively. 



We see from these facts how thermo-chemistry explains the 

 explosive properties of certain metallic oxalates, and also the 

 difference which exists between the conditions of decomposition 

 of these and other oxalates. 



