381 
as slates. Those rocks which are commonly called the 
metamorphic rocks are quite excluded from the present 
consideration. 
Is there any evidence that a polar force of any kind has 
determined the conditions of the crystalline rocks without 
entering upon the question of magnetic and dia-magnetic 
forces so fully and beautifully developed by the delicate 
investigations of Faraday, Plucker, and Tyndale ? I would 
briefly state an experiment or two of my own, which appear 
to me to indicate the line along which investigation should be 
pursued. 
Upon the poles of an electro-magnet, (which, when con- 
nected with the voltaic battery, exerted a lifting power of 
about 2 cwt.,) was placed a glass dish. Into this was poured a 
solution of the salt which was under examination ; and it was 
allowed to crystallise by spontaneous evaporation, the magnet 
being maintained in full action. If we examine the order of 
the magnetic radiations, or, as Faraday calls them, lines of 
magnetic force, we shall find them producing curves. See 
Fig. 1. 
If the crystallising salt belongs to the magnetic class, its 
magnetic polarity is indicated by the arrangement of the 
crystals. Thus, the salts of iron are found to place them- 
selves in such a manner that these lines correspond with 
their longer axes. The order of arrangement, on the 
contrary, of a salt of the dia-magnetic class, amongst which 
all the earthy and alkaline salts are included, is such that 
their axes are at right angles to those lines. In several 
experiments with such salts the result has been the formation 
of curves of crystals. See Fig. 2. 
Connecting such results as these with such as have been 
discovered by the philosophers already named, it certainly 
appears that magnetism may be regarded as a powerful 
directing agent in the formation of our crystalline rocks. 
