Part 14 (1/2)

The longitude may in some places be found by the variation of the needle; and Mr Churchiventhe observations of others, and many of his own, thinks that he has found a ree of certainty, in netic poles of the earth, one to the north and the other to the south, at different distances fro in different times; and from the combined influence of these two poles he deduces rules for the position of the needle in all places of the earth, and at all tinetic pole, he says, makes a complete revolution in 426 years, 77 days, 9 hours, and the south pole in about 5459 years In the beginning of the year 1777 the north4 itude fro south latitude, and 140 deg east from Greenwich

LECTURE xxxIV

_Of Electricity_

Electricity is a property belonging to, or capable of being communicated to, all substances whatever; and whereas by soreat ease, and by others with much difficulty, they have been divided into two classes, and denominated _conductors_ or _non-conductors_ of electricity Also the latter receiving this power by friction, and other means, are termed _electrics_, and the former _non-electrics_

Metals of all kinds, and water, are conductors, though in very different degrees; so also is charcoal All other substances, and also a perfect vacuum, are non-conductors of electricity But lass, resin, baked wood, and perhaps all the rest on which the experiment can be made in this state, are conductors

It is the property of all kinds of electrics, when they are rubbed by bodies different froht substances of all kinds, to exhibit an appearance of _light_, attended with a particular _sound_, on the approach of any conductor; and if the nostrils are presented, they are affected with a _smell_ like that of phosphorus

This attraction isthat electricity is produced by a fluid exceedingly elastic, or repulsive of itself, and attracted by all other substances

An electric exhibiting the appearances above mentioned, is said to be _excited_, and some of the and cooling, as well as by friction It appears, however, that excitation consists in theof electricity froreat source of electricity is in the earth On this account it is necessary to the considerable excitation of any electric, that the substance against which it is rubbed (hence termed _the rubber_) have a communication with the earth, by means of conductors; for if the rubber be _insulated_, that is cut off from all communication with the earth by means of electrics, the friction has but little effect

When insulated bodies have been attracted by, and brought into contact with, an excited electric, they begin to be repelled by it, and also to repel one another; nor will they be attracted again till they have been brought into contact with so with the earth; but after this they will be attracted as at first

If conductors be _insulated_, electric powers may be communicated to them by the approach of excited electrics, or the contact of other electrified bodies They will then attract light bodies, and give sparks, &c like the excited electrics thely communicated to insulated animal bodies, the pulse is quickened, and perspiration increased; and if they receive, or part with, their electricity on a sudden, a painful sensation is felt at the place of communication But what is more extraordinary, is, that the influence of the brain and nerves upon the muscles seems to be of an electric nature

This is one of the last and most important of all philosophical discoveries I shall, therefore, give the result of all the observations that have hitherto been made on the subject, in a _series of propositions_, drawn up by an intelligent friend, who has given much more attention to it than I have done

1 The nerve of the li laid bare, and surrounded with a piece of sheet lead, or of tinfoil, if a communication be forcontractions will be produced in the limb

2 If a portion of the nerve which has been laid bare be armed as above, contractions will be produced as powerfully, by for the communication between the armed and bare part of the nerve, as between the armed part anda nerve and si the armed part of the nerve with the metallic conductor

4 Contractions will take place if a muscle be armed, and a communication be for nerve The same effect will be produced if the communication be formed between the aruous to it

5 Contractionsthe pieces of metal into contact with each other at some distance from the limb, provided the latter make part of a line of communication between the two metallic conductors

The experiplaced upon a table, let the operator hold with one hand the principal nerve, previously laid bare, and in the other let him hold a piece of zinc; let a small plate of lead or silver be then laid upon the table, at some distance from the limb, and a communication be formed, by means of water, between the li If the operator touch the piece of silver with the zinc, contractions will be produced in the limb the moment that the metals come into contact with each other The same effect will be produced if the two pieces of metal be previously placed in contact, and the operator touch one of theer This fact was discovered by Mr William Cruikshank

6 Contractions can be produced in the a the two metals into contact with each other at a small distance froh, and excited contractions in, one lih, and excite contractions in, another li this experi circu be taken; let one of them be laid upon a table, and its foot be folded in a piece of silver; let a person lift up the nerve of this limb with a silver probe, and another person hold in his hand a piece of zinc, hich he is to touch the silver including the foot; let the person holding the zinc in one hand catch with the other the nerve of the second limb, and he who touches the nerve of the first limb is to hold in his other hand the foot of the second; let the zinc now be applied to the silver including the foot of the first limb, and contractions will immediately be excited in both limbs

8 The heart is the only involuntary muscle in which contractions can be excited by these experily, the farther the coating is placed froin of the nerve

10 Animals which were almost dead have been found to be considerably revived by exciting this influence

11 When these experiments are repeated upon an animal that has been killed by opiuht contractions are produced; and no contractions whatever will take place in an animal that has been killed by corrosive sublimate, or that has been starved to death