322 M. Karsten's Observations and Experiments 



pressure of the screws, necessary to connect the caps and keep 

 them water-tight, from chipping the glass. To one of the brass 

 caps must be fixed a well bored cylinder, for the solid piston to 

 slide in, &c. Sea-water might be used in the cylinder, with a ther- 

 mometer to show what capacity water may have to retain its ca- 

 loric when under a high pressure. Such experiments would be 

 interesting to compare with experiments which have been made 

 on the temperature of the sea at great depths ; and also the spe- 

 cific gravity of the water in the cyhnder ascertained before and 

 after the experiment, which would probably throw light on the 

 subject of increased specific gravity of water drawn from great 

 depths, and also whether the effects of pressure on water are 

 permanent, and owing to the imperfect elasticity of water. I 

 am, &c. 



James Dunlop. 

 Dalry, ^Bth Aug. 1827. 



Observations and Experiments on the different hinds of Coal. 

 By M. Karsten. Continued from p. 71. 



X HIS general manner in which coals comport themselves 

 may, however, be considerably modified by other circumstances. 

 When intermixed with mineral charcoal, as is often the case, they 

 are rendered very difficult to kindle. In good coals, whether 

 with intumesced or conglutinated coke, the obstacle which re- 

 sults from the mixture of a great quantity of mineral charcoal, 

 becomes less sensible ; but a coal with pulverulent coke, may 

 thus become altogether useless, because its mass becomes so 

 compact, that it arrests the passage of the air. 



Another obstacle is produced by the quantity of earths which 

 occurs mixed with the mass of the combustible. A coal which 

 leaves much ashes, developes but a slow and feeble heat, because 

 the ashes oppose the access of the air. The same obstacle pre- 

 sents itself in the case in which the body of the combustible it- 

 self leaves little ashes, but where the stratum is, as it were, in- 

 terlarded with clay or slate. If it be the body of the coal itself 

 that is much divided by numerous fissures or partitions, this 

 circumstance may render a coal with pulverulent coke altogether 



