20 Effects of Heat modified by Compression. 



been shown, by comparing together a varied series of ex- 

 periments, that the mutual action between the lime and the 

 porcelain was occasioned entirely by the presence of the 

 carbonic acid, since, when it was absent, no action of this 

 kind took place. The fusion of our carbonates cannot, 

 therefore, be ascribed to the porcelain. 



Being convinced, however, by many observations, that 

 the fusibility of the carbonate did not depend upon impurity, 

 I have exerted myself to remove, by fresh experiments, every 

 doubt that has arisen on the subject. In order to guard 

 Sgainst natural impurities, I have applied to such of my 

 friends as have turned their attention to chemical analysis 

 (a branch of the science to which I have never attended) to 

 furnish me with carbonate of lime of undoubted purity. To 

 obviate the contamination arising from the contact of the 

 porcelain tubes, I determined to confine the subject of ex- 

 periment in some substance which had no disposition to 

 unite with the carbonate. I first tried charcoal, but found 

 it very troublesome, owing to its irregular absorption of 

 water and air. 



I then turned my thoughts to the construction of tubes 

 or cups of platina for that purpose. Being unable readily 

 to procure proper solid vessels of this substance, I made use 

 of thin laminated plates, formed into cups. My first me- 

 thod was, to fold the plate exactly as we do blotting paper 

 to form a filter (fig. 26) ; this produced a cup capable of 

 holding the thinnest liquid ; and being covered with a lid, 

 formed of a similar thin plate, bent at the edges, so as to 

 overlap considerablv (fig. 28), the carbonate it contained 

 was secured on all sides from the contact of the porcelain 

 tube within which it was placed. Another convenient de- 

 vice likewise occurred : I wrapt a piece of the plate of pla- 

 tina round a cylinder, so as to form a tube, each end of 

 which was closed by a cover like that just described (fig. 27 

 and 29). (In fig. 26 and 27 these cups are represented upon 

 a lame scale, and in 28 and 29, nearly of their actual size). 

 This last construction had the advantage of containing eight 

 or nine grains of carbonate, whereas the other would only 

 hold about a erain and a half. On the other hand, it was 



not 



