VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 5-17 



transparency. When urged with a stronger fire, it melts and unites to the cru- 

 cible, but does not become caustic* 



I buried it in charcoal-dust in a covered crucible, and then exposed it to a 

 pretty strong heat ; but it did not part with its air. May we not conjecture 

 then, that as caustic lime cannot unite to fixed air without the intervention of 

 moisture, and as this spar seems to contain no water in its composition, that it 

 is the want of water which prevents the fixed air assuming its elastic aerial state ? 

 This supposition becomes still more probable, if we observe that when the solu- 

 tion of the spar in an acid is precipitated by a mild alkali, c 1,2, some water 

 enters into the composition of the precipitate, for it weighs the same as before 

 it was dissolved, and yet contains only '20 oz. measures of fixed air, while the 

 native spar contained 25 oz. measures ; so that there is an addition of weight 

 equal to that of 5 oz. measures of fixable air, or 3f grs. to be accounted for, 

 which can only arise from the water ; and this precipitate, thus united to water, 

 readily loses its aerial acid in the fire, e 1. 



4thly. Professor Bergman supposes the terra ponderosa to be a metallic earth ; 

 its entire separation from an acid by means of the phlogisticated alkali (d 5) cer- 

 tainly favours such a supposition ; but, if it be so, it is evident from experiments 

 H 1, 2, that other means than those commonly employed must be used to effect 

 its reduction. 5thly. The precipitate made by the caustic vegetable alkali d 4 

 taking some of the alkali down with it, and thus forming a substance neither 

 soluble in water nor in acids, is a very curious phenomenon. I afterwards varied 

 the experiment by adding the terra ponderosa lime-water e to caustic vegetable 

 and caustic fossil alkali. In both cases this insoluble compound was immediately 

 formed ; but not so when caustic volatile alkali was used. 



6thly. As it appears from experiments d 1, 2, 3, 4, that fixed alkalis, both 

 mild and caustic, separated the terra ponderosa from marine acid, I was at a loss 

 to know why Professor Bergman, in his admirable table of simple elective attrac- 

 tions, placed the terra ponderosa caustica immediately under the vitriolic, nitrous, 

 and marine acids, and consequently above the caustic alkalis. I was interested 

 in the reality of the facts, because I had so seldom seen reason to doubt the ob- 

 servations of that very excellent chemist, and therefore made the following ex- 

 periments. To different portions of terra ponderosa salita and terra ponderosa 

 nitrata I added, drop by drop, caustic vegetable, caustic fossil, and caustic vola- 

 tile alkalis. In every case the earth was thrown down ; and I have so often re- 

 peated these experiments to satisfy myself and others, that I am persuaded the 



* If the crucible is of clay ; but Dr. Hope, professor of chemistry at Edinburgh, expelled the 

 carbonic acid from this fossil and obtained the barytic earth in a caustic state, by subjecting it, in a 

 black-lead crucible, to the heat of a forge. 



4 A 2 



