322 Rev. W. V. Vernon on the 



tion of quantities, and I added to it caustic soda ; a precipi- 

 tate fell which had a greenish tinge ; the liquid also changed 

 its colour to a yellow green, and being boiled let fall a further 

 green precipitate. These precipitates consisted of the prot- 

 oxide of chrome, together with some lead and lime. 



The liquid after the separation of the last precipitate re- 

 sumed its yellow colour, which led me to imagine that it 

 might contain chromic acid. The method which I adopted 

 for ascertaining this point, was founded upon the property 

 possessed by certain vegetable acids of abstracting oxygen 

 from the acid of the chromates, a property not belonging to 

 the acetic or prussic acids, but which I have found in the oxalic 

 and citric as well as in the tartaric. The two latter of these, 

 though they convert the chromic acid into protoxide of chrome, 

 do not furnish a ready method of separating the chrome, be- 

 cause a part of them remaining undecomposed, they form 

 soluble triple salts with the oxide and with whatever alkali 

 might be employed to throw it down ; but if the oxalic acid 

 be used, though in this case also the chrome cannot be thrown 

 down by ammonia, it may, with the assistance of heat, be pre- 

 cipitated by soda. 



I therefore added oxalic acid to the liquid, and after boil- 

 ing it perceived a change in the colour : having then neutra- 

 lized it with soda, I obtained a green precipitate ; this was 

 separated and re-dissolved in oxalic acid, to clear it from any 

 lime that might have fallen with it ; a small quantity of lead 

 still adhering to it was I'emoved by sulphuretted hydrogen, it 

 was neutralized with soda ; and of the precipitate which took 

 place a particle was heated on platina foil with nitrate of pot- 

 ash : the yellow salt thus obtained gave with the nitrates of 

 silver and lead the crimson and yellow precipitates by which 

 the chromates are distinguished. 



It is evident from these experiments that chromic acid ex- 

 isted in the nitric solution of the mineral ; and since nitric acid 

 does not, in the circumstances above described, acidify the prot- 

 oxide of chrome, it may be concluded that the orange phos- 

 phate of lead contains chromic acid, and the colour of the 

 mineral is thus sufficiently accounted for. 



A good reason can now be assigned for the circumstance 

 noticed by Klaproth, that when the muriate of tin is poured upon 

 this phosphate it removes the colour, — a phaenomenon which 

 induced him to suppose that the lead contained in it is in a 

 state of superoxidation. We see also how it happens that 

 when the crystals of this substance are heated by the exterior 

 flame of the blowpipe, or out of contact with inflammable 

 matter, the colour is unaltered, growing only darker during 



ignition ; 



