tihiii cipplhd to the Orgnns of Animals, 259 



carbonic m?.tt-er. They may be obtained with metals, though wiihouS communication be- 

 tween the nerve and the mufcle ; that is to fay, without the regular connedlion or chain. They 

 may be alfo obtained by forming the chain of fimilar metals. 



Let the crural nerve of an animal naturally tenacious of life be placed upon glafs. Let a 

 fmall piece of frefli mufcidar flefli be fixed on a flick of fealing-wax, and then brought into 

 contafl: with the crural mufcle. The refult will be a violent convulfion at the inftant w'hcn 

 the chain of communication is completed. The fame thing happens if, inftead of the fmall 

 piece of mufcular flefh, a detached piece of the crural nerve be fixed on the ftick of fealing- 

 wax. The connexion is therefore formed of two things, nerve and mufcular fibre. How 

 in this fimplc cafe can the fluid which partes from the nerve into the mufcle caufe it to be 

 contracted ? Mr. Humboldt thinks that it becomes ftimulant, merely bccaufe it returns from 

 the nerve into the nerve by a foreign animal matter •, that is to fay,- not organically conneftcd 

 with the nerve. 



The difparicy of the metals forming the chain has hitherto appeared as a neceflary con- 

 dition to produce Galvanic irritation. This hypothefis, however, is overturned by the ex- 

 periments of Mr. Humboldt. If it be true that, in the (late of lefs irritability, there is very 

 rarely contraction with fimilar metals (as Volta affirms, contrary to Aldini), this circumflance 

 becomes indifferent in the cafe of increafed irritability. Mr. Humboldt put into a china cup 

 fome mercury exactly purified ; he placed the whole near a warm flove, in order that the 

 entire mafs might affume an equal temperature: the furface was clear, without the appear- 

 ance of oxidation, humidity, or dud. A thigh of a frog, prepared in fuch a manner that a 

 crural nerve and a bundle of mufcular fibres of the fame length hung down feparately, was 

 fufpended by two filken threads above the mercury. When the nerve alone touched the 

 furface of the metal, no irritation was manlfeflcd ; but as foon as the mufcular bundle and 

 the nerve touched the mercury together, they fell into convulfions fo brifk tliat the fkin was 

 extended as in an attack of tetanus. 



We ought not to be furprifed at the precaution here taken by Mr. Humboldt to heat the 

 mercury. This is required in confequence of the opinion which he announces, that the 

 parity of the metals does not depend on the homogeneity of their chemical conftituent parts, 

 but of their heat, polifh, hardnefs, and form. 



Gold, placed between two armatures of zinc, produces irritation only when the gold is 

 moiflencd by fome volatile fluid, or by the moiflufe of refpiration. 



Laflly, Mr. Humboldt has attempted to include all the cafes in the following formula: : 



I. In the State of increafed Irritability. 



'Frog — mufcular flefli. 



Frog — zinc — zinc. 



Frog — zinc — mufcular flefli — filvcr. 



Frog — zinc — filver — zinc. 



Frog— mufcular flcfh — filvcr — zinc, 

 .l-'rog — zinc — mufcular tlelh — filver — mufcular flcfh— zinc. 



2. In the State of diminifhed Irritability. 



f Frog — zinc — filver. 

 Pofitlvc cafes. 7 Tog — 7'nc — mufcular flcfh — filver — zinc. 



IFrog— zinc — mufcular flefli— filver— mufcular flefli — filvcr — zinc. 



L 1 2 Negative 



Pofitive cafes. < 



