Effects of Heat modified hj Compression. 15 



These two last experiments are rendered still more in- 

 teresting by another set which I made soon after, which 

 showed that one essential precaution in a point of such 

 nicety had been neglected in not previously drying the car- 

 bonate. In several trials made in the latter end of the same 

 month, I found, that chalk exposed to a heat above that of 

 boiling water, but quite short of redness, lost 0-34 per cent., 

 and in another similar trial 0-46 per cent. Now, this loss 

 of weight equals within 0-01 per ceut., the loss in the last- 

 mentioned experiment, that being 0-4? ; and far surpasses 

 that of the last but one, which was but 0-074. There is 

 good reason, therefore, to believe, that had the carbonate, 

 in these two last experiments, been previously dried, it would 

 have been found during compression to have undergone no 



]oS3. 



The result of many of the experiments lately mentioned 

 seems fully to explain the perplexing discordance between 

 my experiments with porcelain tubes and those made in 

 barrels of iron. With the porcelain tubes, ] never could 

 succeed in a heat above 2S°, or even quite up to it ; yet the 

 results were often excellent: whereas the iron barrel's have 

 currently stood firm in heats of 41° or 51°, and have reached 

 even to 70° or 80" without injury. At the same time, the 

 results, even in those high heats, were often inferior, in 

 point of fusion, to those obtained by low heats in porcelain. 

 The reason of this now plainly appears. In the iron barrels 

 it has always been considered as necessary to use an air-tube, 

 in consequence of which some of the carbonic acid has been 

 separated from the earthy basis by internal calcination : what 

 carbonic acid remained has been more forcibly attracted 

 according to M. Berthollet's principle, and, of course, more 

 easily compressed, than when of quantity sufficient to satu- 

 rate the lime: but, owing to the diminished quantity of the 

 acid, the compound has become less fusible than in the na- 

 tural state, and, of course, has undergone a higher heat with 

 less effect. The introduction of water, by furnishing a re- 

 acting force, has produced a state of things similar to that 

 in the porcelain tubes ; the carbonate sustaining little or no 



loss 



