VOL. LXXXl.] PHILOSOPHICAL TRANSACTIONS. 51 



constituent parts, with as mucli certainty as by that mode of proof alone it is 

 possible to obtain. But as vital air has a stronger attraction for charcoal than 

 for any other known substance, the decomposition of fixed air has not hitherto 

 been attempted. By means, however, of the united force of 2 attractions Mr. 

 T. has been able to decompose fixed air, and thus to determine its constituent 

 parts in consequence of their separation. 



It has long been known, that when phosphoric acid is combined with calca- 

 reous earth, it cannot be decomposed by distillation with charcoal : for though 

 vital air is more strongly attracted by charcoal than by phosphorus, yet in this 

 compound it is retained by 2 attractions, by that which it has for phosphorus, 

 and by that which the phosphoric acid has for lime, since the vital air cannot be 

 disengaged unless both these attractions are overcome. As these attractions are 

 more powerful than that which charcoal has for vital air, if phosphorus is applied 

 to fixed air and calcareous earth, the vital air will unite with the phosphorus, and 

 the charcoal will be obtained pure. These substances, in order to act on each 

 other, must be brought into contact when red-hot ; and this may be easily 

 effected in the following manner. Into a glass tube, closed at one end, and 

 coated with sand and clay to prevent the sudden action of the heat, a little phos- 

 phorus should be first introduced, and afterwards some powdered marble. The 

 experiment succeeds more readily if the marble is slightly calcined, probably 

 because that part which is reduced to lime, by immediately uniting with the 

 phosphorus, detains it to act on the fixed air in the other part. After the in- 

 gredients are introduced, the tube should be nearly, but not entirely, closed up ; 

 by which means so free a circulation of air as might inflame the phosphorus is 

 prevented, while the heated air within the tube is suffered to escape. When the 

 tube has remained red-hot for some minutes it may be taken from the fire, and 

 must be suffered to cool before it is broken. It will be found to contain a black 

 powder, consisting of charcoal intermixed with a compound of lime and phos- 

 phoric acid, and of lime united with phosphorus. The lime and phosphoric 

 acid may be separated by solution in an acid and by filtration, and the phosphorus 

 by sublimation. 



Charcoal, thus obtained from fixed air, appears in no respect to differ from 

 the charcoal of vegetable matters. On deflagrating a little of it in a small retort 

 with nitre, fixed air was immediately reproduced. — Since therefore charcoal, by 

 its separation from fixed air, is proved to be one of its constituent principles, it 

 can hardly be doubted that this substance is present whenever fixed air is pro- 

 duced ; and that those experiments, from which it is supposed that this acid may 

 be formed without the aid of charcoal, have not been conducted with the requi- 

 site caution. 



As vital air is attracted by a compound of phosphorus and calcareous earth 

 more powerfully than by charcoal, Mr. T. was desirous of trying their efficacy 



H 2 



