

1814.] Elements of Organic Nature are comllned. 40.9 



prepared, of a large porcelain mortar filled with sand, which I had 

 previously raised to the heat of 212° or 230°. I put this hot mortar 

 under the receiver of an air-pump, along with a quantity of con- 

 centrated sulphuric acid, and immediately pumped the air out of 

 the receiver. I do not believe that many vegetable substances could 

 retain water under such circumstances ; at least I do not think that 

 any compound of them with oxide of lead could do it. 



We must never employ acetate or subacetate of lead to form 

 insoluble compounds between organic bodies and oxide of lead, 

 because a part of the acetate combines so intimately with the preci- 

 pitate that it cannot be separated by washing. Hence if we employ 

 such a combination for an analytical experiment, the result becomes 

 very incorrect, without our being able to perceive it. It' this happen 

 with the nitrate of lead, we always perceive it, because nitric acid 

 is formed, which partly mixes with the water produced by the 

 combustion, and partly passes over with the gas, which forms in 

 consequence stains of oxide on the mercury. These circumstances 

 occasioned me a great deal of trouble, for at the commencement of 

 my experiments 1 employed the acetate of lead in preference, be- 

 cause it is so easily decomposed. 



I analysed these vegetable salts with a base of lead in two ways : 



1. By means of sulphuric acid. This method has the inconve- 

 nience of depending on the dexterity of the operator. The trans- 

 vasations and filtrations of necessity occasion small errors in the 

 result. 



2. By combustion. I took a small capsule of glass, very thin. I 

 weighed it, and put into it a determinate quantity of the salt to be 

 analysed. I then heated the capsule by means of a spirit of wine 

 lamp till all the vegetable matters were burnt. 1 then weighed it. 

 The oxide of lead was then dissolved by distilled vinegar. I well 

 washed the metallic lead left undissolved by the vinegar, and I 

 weighed it anew. The weight of the lead shows how much oxygen 

 must be added to the mu^s remaining after combustion to have the 

 weight of the oxide of lead, and of consequence of the vegetable 

 substances burnt. This method gives results so uniform that not 

 unfrequently in three or four subsequent analyses 1 obtained exactly 

 the same weights. This method, however, has an inconvenience, 

 but it is not considerable. The combination is decomposed in such 

 a manner as to produce in the first place a carburet of lead, which 

 afterwards takes fire, and burns with such vivacity that a small 

 quantity of the metal is driven oil', and this causes a loss, which 

 being added to the quantity of acid occasions a double error. 



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