of catchi ng them by means of Wild Horses. ^9 
the electric shock takes place only at the will of the aniinal. 
Two persons, one of whom holds the tail, and the other the 
head, cannot, by joining hands and forming a chain, force the 
gymnotus to dart his stroke. 
In employing very delicate electrometers in a thousand ways, 
insulating them on a plate of glass, and receiving very strong 
shocks, which passed through the electrometer, I could never 
discover any phenomenon of attraction or repulsion. The same 
observation was made by Mr Fahlberg at Stockholm. This 
philosopher, however, has seen an electric spark, as Walsh and 
Ingenhousz had done before him at London, by placing the 
gymnotus in the air, and interrupting the conducting chain by 
two gold leaves pasted upon glass, and a line distant from each 
other. No person, on the contrary, has ever perceived a spark 
issue from the body of the fish itself. We have irritated it for 
a long time during the night, at Calabozo, in perfect darkness, 
without observing any luminous appearance. 
Art. VI. — Observations mi Mr Menge's Account ofhisMinera^ 
logical Journey in Iceland. In a Letter from Sir G. S. Mac- 
kenzie, Bart. F. R. S. L. and President of the Physical Class 
of the Royal Society of Edinburgh, &c. to Professor Jameson. 
Dear Sir, 
ISTot WITHSTANDING the numerous investigations that have 
been carried on in volcanic districts by eminent observers, I am 
disposed to think, that many distinctions are yet to be made 
among volcanic productions, and in the manner of their formation, 
before we arrive at a clear developement of volcanic agency. 
We must understand perfectly what is meant by the word lava, 
the too profuse employment of which has unquestionably left us 
much in the dark. Mr Menge mentions in his letter to you, 
that in one of the Westmanna Islands and Eyafialia-Jokull (nei- 
ther of which I visited), he observed three different streams of 
lava, separated from each other by layers of trass. Now, it is 
well known, that every stream of lava that Jiows in the open air, 
assumes an appearance so rugged, that nothing is seen at all re- 
sembling what we commonly understand by a bed., which is a mass 
bounded by planes that are parallel, or very nearly so. Hence, 
VOL. II. NO. 4 . APRIL 1820 . 
R 
