Wilhelm Jacob s'Gravesande

The vicissitudes of the theory of heat is the most amazing in the history of natural philosophy.

The true hypothesis was accepted for a century by a succession of illustrious men.

• It was abandoned by their successors in favour of a false, absurd conception.

s’Gravesande explained that in combustion, a heated body causes the light-corpuscles to interact with the material particles.

• Some constituents become separated and carried away with the corpuscles as flame and smoke.

This view harmonizes with the theory of calcination developed by Becher and his pupil Stahl at the end of the 17th century.

• It says that metals were composed of their calces and an element phlogiston.

Combustion causes a metal to change into its calx.

• It was interpreted as a decomposition, in which the phlogiston separated from the metal and escaped into the atmosphere.

The conversion of the calx into the metal was regarded as a union with phlogiston.[18]

s’Gravesande attributed electric effects to vibrations induced in effluvia, which he supposed to be permanently attached to such bodies as amber.

The English translator of s’Gravesando’s work played a considerable part in the history of electrical science.

Jean Théophile Desaguliers (b. 1683, d. 1744) was an Englishman only by adoption. His father had been a Huguenot pastor who escaped France after the revocation of the Edict of Nantes. He brought away the boy from La Rochelle concealed in a tub.

The young Desaguliers was afterwards ordained, and became chaplain to that Duke of Chandos who was so ungratefully ridiculed by Lope.

• He formed friendships with some of the natural philosophers of the capital such as Stephen Gray, an experimenter and pensioner of the Charterhouse of whom little is known.

In 1729, Gray wrote to Dr. Desaguliers his recent discovery “that the Electrick Vertue of a Glass Tube may be conveyed to other Bodies so as to give them the same Property of attracting and repelling light Bodies when excited by rubbing. This attractive Vertue might be carried to Bodies that were many Feet distant from the Tube.”

This was very important. Previous workers had known of no other way of producing the attractive emanations than by rubbing the body.[21]

It was found that only a limited class of substances, among which the metals were conspicuous, could act as channels for the transport of the electric power.

Desaguliers continued the experiments after Gray’s death in 1736. He called them non-electrics or conductors.

After Gray’s discovery, it was no longer possible to believe that the electric effluvia are inseparably connected with the bodies from which they are evoked by rubbing.

These emanations have an independent existence, and can be transferred from one body to another.

This became known as “electric fluid”, one of the substances of which the world is constituted.

The imponderability of this fluid did not prevent its admission by the side of light and caloric into the list of chemical elements.

Was the electric fluid was an element sui generis? Or was another manifestation of that principle whose operation is seen in the phenomena of heat?

Those who held the latter view urged that the electric fluid and heat can both be induced by friction, can both induce combustion, and can both be transferred from one body to another by mere contact.

Moreover, that the best conductors of heat are also in general the best conductors of electricity.

On the other hand it was contended that, the electrification of a body does not cause any appreciable rise in its temperature; and an experiment of Stephen Gray’s brought to light a yet more striking difference.

In 1729, Gray made 2 oaken cubes:

• one solid
• the other hollow

He showed that when electrified in the same way they produced exactly similar effects. He concluded that it was only the surfaces which had taken part in the phenomena.

Thus while heat is disseminated throughout the substance of a body, the electric fluid resides at or near its surface.

In the mid-18th century it was generally compared to an enveloping atmosphere.

“The electricity which a non-electric of great length (for example, a hempen string 800 or 900 feet long) “receives, runs from one end to the other in a sphere of electrical Effluvia,” says Desaguliers in 1740[25] and a report of the French Academy in 1733 says:[26]

“Around an electrified body there is formed a vortex of exceedingly fine matter in a state of agitation, which urges towards the body such light substances as lie within its sphere of activity. The existence of this vortex is more than a mere conjecture; for when an electrified body is brought close to the face it causes a sensation like that of encountering a cobweb."[27]

The report from which this is quoted was prepared in connexion with the discoveries of Charles-François du Fay (b. 1698, d. 1739), superintendent of gardens to the King of France.

Du Fay accounted for the behaviour of gold leaf when brought near to an electrified glass tube by supposing that at first the vortex of the tube envelopes the gold-leaf, and so attracts it towards the tube.

But when contact occurs, the gold-leaf acquires the electric virtue, and so becomes surrounded by a vortex of its own. The two vortices, striving to extend in contrary senses, repel each other, and the vortex of the tube, being the stronger, drives away that of the gold-leaf.

“It is then certain,” says du Fay,[29] “that bodies which have become electric by contact are repelled by those which have rendered them electric; but are they repelled likewise by other electrified bodies of all kinds?

Do electrified bodies differ from each other in no respect save their intensity of electrification? An examination of this matter has led me to a discovery which I should never have foreseen, and of which I believe no one hitherto has had the least idea.”

He found, in fact, that when gold-leaf which had been electrified by contact with excited glass was brought near to an excited piece of copal,[30] an attraction was manifested between them.

“I had expected,” he writes, “quite the opposite effect, since, according to my reasoning, the copal and gold-leaf, which both electrified, should have repelled each other.” Proceeding with his experiments he found that the gold-leaf, when electrified and repelled by glass, was attracted by all electrified resinous substances, and that when repelled by the latter it was attracted by the glass.

“We see, then," he continues, “that there are two electricities of a totally different nature — namely, that of transparent solids, such as glass, crystal, &e., and that of bituminous or resinous bodies, such as amber, copal, sealing-wax, &e.

Each of them repels bodies which have contracted an electricity of the same nature as its own, and attracts those whose electricity is of the contrary nature.

We see even that bodies which are not themselves electrics can acquire either of these electricities, and that then their effects are similar to those of the bodies which have communicated it to them.”

du Fay called the 2 kinds of electricity as vitreous and resinous, by which they have ever since been known.