112 Mr William Nicol, on the 



from Cheshire, a considerable number of very small irregular ca- 

 vities were observed dispersed through certain parts of it. 

 These were all filled with a fluid, and some of thera had also a 

 minute globule of air. Such of the cavities as had no globule, 

 constantly acquired one on the application of a moderate degree 

 of heat ; but the globule never appeared until after the heat be- 

 gan to diminish. 



On heating a piece of salt containing a globule of air, the 

 globule diminishes in bulk as the heat increases, and in general 

 entirely disappears, even before the heat becomes so great as to 

 be painful to the touch. As the heat diminishes, the globule 

 re-appears, and increases in bulk until the temperature be redu- 

 ced to that of the atmosphere. 



On the application of a heated wire to that side of a cavity 

 which was opposite to the place of a globule of air, the globule 

 in no instance had the slightest tendency to move from its 

 place ; and when a rent was made to extend from the surface 

 into a cavity, the globule sustained a very slight enlargement, 

 but was unable to expel any part of the fluid through the rent 

 to the surface. In respect of elasticity, the globule of air in the 

 fluid cavities of rock-salt, is therefore greatly inferior to that of 

 the globule of air in the fluid cavities of fluor-spar and sulphate 

 of barytes. 



When a direct opening is formed into a cavity of the salt, the 

 fluid remains in the cavity, but shews no tendency to crystal- 

 lise, even in those states of the atmosphere when a saturated so- 

 lution of muriate of soda is rapidly crystaUising. If heated, 

 however, the fluid submits to the laws of crystallization, and as- 

 sumes the form of extremely slender acicular crystals ; but these 

 rapidly deliquesce even in the driest states of the air. 



From this circumstance it evidently appears, that the fluid is 

 not a solution of common salt, and although the apphcation of a 

 very few chemical agents enables us to indicate the materials of 

 which the fluid is composed; yet, on account of jthe smallness of 

 all the cavities I have had an opportunity of operating on, I am 

 unable to determine their relative proportion. 



When a solution cf nitrate of silver is applied to the fluid, a 

 copious precipitate ensues, which indicates the presence of mu- 

 riatic acid. As muriate of barytes gives no precipitation, it is 



