Wilhelm Jacob s'Gravesande's Light-corpuscles

Newton died in 1727. His philosophy was published in his lifetime by his followers.

The most insightful one was the Physices elementa mathematica experimentis confirmata by Wilhelm Jacob s’Gravesande (b. 1688, d. 1742), published at Leyden in 1720.

The Latin edition was translated into French and English and had a well-deserved influence on contemporary thought.

s’Gravesande supposed that light consisted in the very fast projection of corpuscles from luminous bodies to the eye, as shown by astronomical observations.

Since many bodies, e.g. the metals, become luminous when they are heated, he inferred that every substance possesses a natural store of corpuscles, which are expelled when it is heated to incandescence.

Conversely, corpuscles may become united to a material body; as happens, for instance, when the body is exposed to the rays of a fire.

Moreover, since the heat thus acquired is readily conducted throughout the substance of the body, he concluded that corpuscles can penetrate all substances, however hard and dense they be.

From the time of Boyle (1626-1691), substances perceptible to the senses were either elements or compounds or mixtures.

The compounds were chemical individuals, distinct from mere mixtures of elements.

But the substances at that time accepted as elements were very different from those which are now known by the name.

Air and the calees[5] of the metals figured in the list, while almost all the chemical elements now recognized were omitted from it.

Some of them, such as oxygen and hydrogen, because they were as yet undiscovered, and others, such as the metals, because they were believed to be compounds.

Among the chemical elements, it became customary after the time of Newton to include light-corpuscles.[6]

That something which is confessedly imponderable should ever have been admitted into this class may at first sight seem surprising. But it must be remembered that questions of ponderability counted for very little with the philosophers of the period.

3/4 of the 18th century had passed before Lavoisier enunciated the fundamental doctrine that the total weight of the substances concerned in a chemical reaction is the same after the reaction as before it.

As soon as this principle came to be universally applied, light parted company from the true elements in the scheme of chemistry.

We must now consider the views which were held at this time regarding the nature of heat. These are of interest for our present purpose, on account of the analogies which were set up between heat and electricity.

The various conceptions which have been entertained concerning heat fall into one or other of two classes, according as heat is represented as a mere condition producible in bodies, or as a distinct species of matter.

The former view, which is that universally hell at the present day, was advocated by the great philosophers of the seventeenth century.

In the Novum Oryanum, Bacon writes:

“Calor est motus expansiviis, cohibitus, et nitens per partes minores."[7]

Boyle[8] affirmed that the “Nature of Heat” consists in “a various, vehement, and intestine commotion of the Parts among themselves.”

Hooke[9] declared that “Heat is a property of a body arising from the motion or agitation of its parts.”

Newton[10] asked: “Do not all fixed Bodies, when heated beyond a certain Degree, emit light and shine; and is not this Emission performed by the vibrating Motion of their Parts?”

He suggested the converse of this, namely, that when light is absorbed by a material body, vibrations are set up which are perceived by the senses as heat.

The doctrine that heat is a material substance was maintained in Newton’s lifetime by a certain school of chemists.

The most conspicuous member of the school was Wilhelm Homberg (b. 1652, d. 1715) of Paris. He[11] identified heat and light with the sulphureous principle, which he supposed to be:

• one of the primary ingredients of all bodies, and
• present even in the inter- planetary spaces.

This was in sharp opposition to that of Newton.[12]

But a few years later, the followers of Newton began to develop their system under the influence of Homberg’s writings.

This evolution can be traced in s’Gravesande. His starting-point is the Newtonian idea that heat bears to light a relation similar to that which a state of turmoil bears to regular rectilinear motion.

Whence, conceiving light as a projection of corpuscles, he infers that in a hot body the material particles and the light-corpuscles[13] are in a state of agitation, which becomes more violent as the body is more intensely heated.

s’Gravesande thus holds a position between the two opposite camps.

• On the one hand he interprets heat as a mode of motion
• But on the other, he associates it with the presence of a particular kind of matter, which he further identifies with the matter of light.

After this, the materialistic hypothesis made rapid progress.

It was frankly advocated by another member of the Dutch school, Hermann Boerhaave[14] (b. 1668, d. 1738), Professor in the University of Leyden, whose treatise on chemistry was translated into English in 1727.

Later, it was found that the heating effects of the rays from incandescent bodies may be separated from their luminous effects by passing the rays through a plate of glass, which transmits the light, but absorbs the heat.

After this discovery, it was no longer possible to identify the matter of heat with the corpuscles of light.

The former was consequently accepted as a distinct element, under the name of calorie.[15]

In the latter part of the 18th and early-19th centuries[16] caloric was conceived as occupying the interstices between the particles of ponderable matter.

This idea fitted well with the observation that bodies commonly expand when they are absorbing heat. But it was less competent to explain the fact[17] that water expands when freezing.*

This was solved by assigning a chemical nature to the freezing and melting of water. In this way, the changes in volume would be out of scope for predictions.

The imponderability of heat did not appear to the philosophers of the 18th century to be a sufficient reason for excluding it from the list of chemical elements.

In any case, they doubted as to whether caloric was ponderable or not.

• Some experimenters believed that bodies were heavier when cold than when hot.
• Others believed that they were heavier when hot than when cold.

The century was far advanced before Lavoisier and Rumford finally proved that a body’s temperature has no sensible influence on its weight.