116 
priety be called the prismatic axis. So far as I am aware, 
the formation of fibrous rock-salt has not been explained. 
Having the well known fact before me, viz., “ the influence 
exerted upon the crystal form of a body crystallising out 
of its solution by the presence in that solution of a small 
quantity of a foreign body,” I thought it probable that a 
similar effect might be produced upon the growth of sodium 
chloride crystals by the presence of a foreign body or 
bodies. In order to ascertain this I made a strong saturated 
solution of ordinary table-salt, filtered it, and precipitated 
out most of the salt by passing hydrochloric acid gas into 
the solution. The liquid was drained off from the preci- 
pitated salt and allowed to evaporate very slowly for several 
days in a narrow-necked flask and then laid aside to cool' 
I then observed amongst the mass of ordinary salt cubes 
numerous long, well-developed prisms. Some of these 
prisms were an inch or more in length and sometimes ter- 
minated by a broad cube; others again were twinned 
according to the usual law, viz., “the twin plane a face of 
the octoliedron.” I observed that many of the cubical 
crystals were somewhat bent and resembled closely the bent 
barytes crystals of the prism and basal terminal plane in 
combination. Nearly all the crystals (prismatic and cubical) 
were opaque or almost so, and of a very white colour. In 
addition to free hydrochloric acid, I found in the solution a 
quantity of magnesium sulphate and magnesium chloride, 
and it is to the peculiar retarding action of these substances, 
either together or singly, that I ascribe the general forma- 
tion of fibrous rock-salt ; for they could easily occur natu- 
