572 LECTURE LIV. , ' 



quences of the rarefaction produced by the heat which is excited; thus, the 

 explosion, attending the transmission of a shock or sparii through the air, 

 may easily be supposed to be derived from the expansion caused by heat; and 

 the destruction of a glass tube, which contains a fluid in a capillary bore, 

 Avhen a spark is caused to pass through it, is the natural consequence of the 

 conversion of some particles of the fluid into vapour. But when a glass jac 

 is perforated, this rarefaction cannot be supposed to be adequate to the effect. 

 It is remarkable that such a perforation may be made by a very moderate 

 dischar<i-e, when the glass is in contact with oil or with sealing wax; and no 

 sufHcicnt explanation of this circumstance has yet been given. 



A stron"" current of electricity, or a succession of shocks or sparks, trans- 

 mitted through a substance, by means of fine wires, is capable of producing 

 many chemical combinations and decompositions, some of which may be 

 attributed merely to the heat which it occasions, but others are wholly diiFer- 

 ent. Of these the most remarkable is the production of oxygen and hy- 

 drogen gas from common water, which are usually extricated at once, in 

 such quantities, as, when again combined, will reproduce the water which 

 has disappeared ; but in some eases the oxygen appears to be disengaged 

 most copiously at the positive wire, and the hydrogen at the negative. 



When the spark is received by the tongue, it has generally a- subacid taste; 

 anil an explosion of any kind is usually accompanied by a smell somewhat like 

 that of sulfur, or rather of fired gunpowder. The peculiar sensation, which 

 the electric fluid occasions in the human frame, appears in general to be 

 derived from the spasmodic contractions 6f the muscles through wh.ich it 

 passes; although in some cases it produces pain cf a different kind; 

 thus, the spark of a conductor occasions a disagreeable sensation in the skin, 

 and when an excoriated surface is placed in the galvanic current, a sense of 

 smarting, mixed with burning, is experienced. Sometimes the effect of 

 a shock is felt most powerfully at the joints, on account of the diiliculty 

 which the fluid finds in passing the articulating surfaces which form the cavity 

 of the joints. The sudden death of an animal, in consequence of a violent shock, 

 is probably owing to the immediate exhaustion of the whole energy of the 

 nervous system. It is remarkable th!i,t a very minute tremor, communicated to 

 the most elastic partsof the body,in particular to the chest, produces an agita- 

 tion of the nerves, whicli is not wholly unlike the effect of a weak eltjctricity. 



