359 
organ: the firft, of irritabiliry augment- 
ed, naturally or artificially; and the 
fecond, of diminifhed irritability. Thefe 
two ftates, which he terms ,pa/itive and 
negative, are neverthelefs, as he’ remarks, 
only different degrees of irritability, and 
not phenomena effentially diftinét from 
each other. 
In individual beings, fenfible by 
nature, the effets produced by alkaline 
folutions, by the muriatic oxygenate acid, 
by the folution of oxide of arfenic, are 
avery feldom found to be of the fame 
intenfity. eae 
In the cafe of irritability augmented, 
mufcu'ar motions take place without the 
application of metal or carbonic matter. 
The fame motions may be obtained with 
metals, although’ no communication 
fhould fubfift between the nerve and the 
mufcle, that is to fay, there be no chain. 
They may be alfo obtained by contti- 
tuting the chain of fimilar metals. Let 
the crural nerve of an animal, by nature 
wivacions, be placed upon glafs; let 
a fmall piece of freth mufcular flefh 
be fixed on a flick of fealing-wax, 
and put into contaét with the crural 
mufcle; the refult of this will be.a vio- 
lent convulfion of the mufcle, at the in- 
frant when the chain fhall be faftened. 
A familar refult is obferved, if, in heu of 
the little niece of mufcular fleth, a piece 
detached from the crural nerve be fixed 
on the flick of fealing-wax. The chain 
then only confifts of two things: the 
nerve and mufcular fibre. How 
then, enquires M. HumsBo_tpr, in this 
fimple procefs, are we to account for the 
action of the fluid whicl 
the 
h paffles from 
the nerve into the mutcle? Ee 1s of. op - 
nion, that the fluid only becomes fiimu- 
lant. becaufe it returns from the nerve 
into the nerve, through the nedium of 
a foreign animal matter, that is to fay, 
matter not organically connected with 
the nerve.- 
The difparity of the metals which con- 
titute the chain, has been hitherto con- 
fidered as a part of the procefs effentially 
necefiary to produce Galvanic irritation : 
this hypothefis, however, is overturned - 
by the experiments of M. Humbolds. 
Tf it be true, that in the fiate of leffened 
prricability, contraction very  feldom 
occurs when fimilar metals are made ufe 
Of (as VOLTA maintains, in oppofition to 
AvpINI) this citcumftance becomes a 
matter of no-importance, in the cafe of 
irritability augmented. M. Hum- 
$OLDT put tome mercury, accurately 
Humboldt’s Experiments on Galvanifin. 
{ May, 
purified, intoa China cups this he ap- 
plied clofe to a hot frying-pan, in order 
that its whole mafs might affume an equal 
temperature ; the furface of it was alfo 
clear, without the flighteft appearance of 
oxidation, humidity, or duft; a frog’s leg 
was prepared in iuch a maaner that the 
crural nerve, anda bundle of mufcular 
fibres, of the fame length, were fulfpended 
feparately, by means of two filk threads 
above the mercury ; the refult of this 
was, that when the nerve alone came 
into contaét with the furface of the 
metal, no irritation appeared te follow; 
bur as foon as the muicular fibres and the 
nerve touched the mercury together, 
they fell into fuch ftrong convulfions, 
that the fkin became ftretched -as vio- 
lently asin the attack of a tetanus. 
We ought not to be furprifed here, at 
the precaution ufed by M. HumBoLDT 
in heating the mercury ; this is a con- 
fequence refulting from his opinion, that 
the purity of metals does not depend 
on the homogeneity * of their che- 
mical conftituent parts, but on their heat, 
polith, firmnefs, and form. 
Gold, placed between two armatures 
of zinc, produces irritation only when 
the former is moiftened by any volatile 
fluid, or by mere refpitation. 
M. Humboldt has attempted to in- 
clude all the poifible cafes of experiments 
in the following formule : 
z. In the ftate. of irritability aug- 
mented : 
(Frog—mufcular Ach. 

do. —=ZzIne——zinG: ie 
Banive do. —zinc—miufcular flef;—filver. 
es do, —zine—alver—zine. ; 
do ——mufcular flelh—filver—zine, 
| do. —-zinc—mufcular fAlefh—filver. 
L mufcular fefh—zine, 
2. Inthe flate of diminithed irritability. 
- Frog—=zinc—filver. 
do. —-zine—mutfcuiar flefh—filver—- 

Pofitive 2 
wee zinc. : 
aacsS. - ~ 
iy do. —-zinc—mufcelar flefh—fil ver. 
mufcular flelh—filver—zine. 
r Frog—zinc—zinc. 
Negative do. —zinc — muicular fi: fh—fil Ver. 
5 
cafes. ) do.—zinc--mufcular iefh—fily.— 
zinc, 
M. Humsoupr concludes his letter 
with fome obfervations on the /fhemic or 

* The word in the orginal is homogent&at ; 
but, as this term means exclufively the identity 
of the conftituent parts, it cannot be confidered 
as proper here, where the queftion relates merel 
to the identity of the kind of metal. P. 
aftbenig 
