MODIFIED by COMPRESSION. 137 



likewife, fhewed the entire internal folidity of themafs. Un- 

 fortunately, this fpecimen has fuffered much by the fame de- 

 cay to which all of them are fubject which have loft any con- 

 fiderable weight. The part next the outward furface alone 

 remains entire. I have never been able to explain, in a fatis- 

 factory manner, this difference of durability ; the laft-men- 

 tioned refult having loft more in proportion to its weight than 

 this. 



About the beginning of June, I received from Mr Hatchett 

 fome pure carbonate of lime, which he was fo good as to pre- 

 pare, with a view to my experiments ; and I have been conftant- 

 ly employed with it till within thefe few days. 



My firft experiments with this fubftanee were peculiarly un- 

 fortunate, and it feemed to be lefs eafily acted upon than any 

 fubftanee of the kind I had tried. Its extreme purity, no doubt, 

 contributed much to this, though another circumftance had 

 likewife had fome effect. The powder, owing to a cryftallization 

 which had taken place on its precipitation, was very coarfe, 

 and little fufceptible of clofe ramming; the particles, therefore, 

 had lefs advantage than when a fine powder is ufed, in acting up- 

 on each other, and I did not choofe to run any rifk of contami- 

 nation, by reducing the fubftanee to a finer powder. Whatever 

 be the caufe, it is certain, that in many experiments in which 

 the chalk was changed to marble, this fubftanee remained in a 

 loofe and brittle ftate, though confifting generally of clear and 

 fhining particles. I at laft, however, fucceeded in obtaining 

 fome very good refults with this carbonate. 



In an experiment made with it on the 18 th of June, in a 

 ftrong heat, I obtained a very firm mafs with a laline fracture, 

 moulded in feveral places on the platina, which was now ufed 

 in the cylindrical form. On the 23d, in a fimilar experiment, 

 the barrel failed, and the fubject of experiment was found in 

 an entire ftate of froth, proving its former fluidity. 



Vol. VI,— P.I. S On 



