HOLWELL CAVERN. 23 
of the cliff and just below the soil ; accordingly I reasoned 
thus.—The rain-water penetrating the soil moistened the 
sulphuret of iron, and decomposed it, the oxigen of the 
water converting the sulphuret to the sulphate, and the 
sulphate of iron being a soluble salt, passed through the 
clay, and was slowly admitted into contact with the 
surfaces of the limestones and shells. A local electric 
action was excited, in which the limestones and seashells 
became negative, whilst the upper stratum of sulphuret of 
iron was positive. The sulphate of iron and carbonate of 
lime suffered each a decomposition, and sulphate of lime 
was produced in a crystallized form upon the negative 
surfaces of the limestones and shells, carbonic acid gas 
being liberated ; moreover, this iron being deprived of 
its sulphuric acid, absorbed oxigen, and was converted 
into red oxide of iron, which was abundantly precipitated 
around the base of the erystals of sulphate of lime, in a 
powdery form. In order to prove the correctness of this 
theory, on my return home, I took a large basin, half filled 
it with pipe-clay and kneaded up with water to the con- 
sistence of moist putty, and imbedded in the clay some 
pieces of limestone and some seashells. I next formed a 
stratum upon the clay of powdered sulphuret of iron, and 
then filled the basin with common water, and put it aside 
in a dark cupboard for a twelvemonth. At the end of 
this period I brought it into the light and examined it 
with no small anxiety ; but was delighted to find that every 
piece of limestone and seashells which had been embedded 
in the clay, when taken out, washed and dried, was covered 
with prismatic crystals of sulphate of lime, exactly similar 
to those found in the cliff at Weymouth, but of course 
they were small, though perfect. Such are the effects of 
what I term LOCAL ELECTRICITY. Observe that here no 
