Lenses and Waves

1690 - TRAITÉ DE LA LUMIÈRE 239
subtle and lithe and that is agitated by an extremely swift movement.” 85 The
behavior of this subtle matter may explain gravity, the effect of gunpowder,
flames, magnetism, elasticity of solid bodies and of air. Huygens did not
make clear what united these phenomena in his view. Was it because they all
arise from vortices, streams of subtle matter as Descartes had proposed
them? This might explain why light was not mentioned among the
phenomena subtle matter would explain, as according to his (later) views it
was not a vortex but an action propagated through the ether. Or did the
phenomena Huygens mentioned constitute a different class? The ensuing
discussion at the Académie might suggest that they did, namely, a group of
hidden forces in nature that called for mechanistic explanation. It might also
suggest that his recent move to Paris somehow occasioned Huygens to make
these notes on a subject he had not considered before. He may have been
reacting – as was often the case with him – to some discussions going on at
the Académie. For the cases of gravity and magnetism Huygens elaborated
the idea of vortices in papers he read before the Académie in 1669 and 1680
respectively. The “Traité de l’aimant” of 1680 is mainly an effort to
determine the exact way in which turbulences of subtle matter stream on the
basis of observations of patterns of filings of iron. 86 I will not discuss this
paper.
Huygens considered his ideas on gravity further in some notes he made
probably between February and May 1668. 87 Besides remarks about various
materials like iron, lead and water, he laid down what he considered its main
properties. Gravity works towards the center of the earth; the subtle matter
causing it easily penetrates all bodies; weight is proportional to the quantity
of matter in a body. In these notes he discussed some of Descartes’ claims,
including an experiment to simulate the effect of vortices. It described a
vessel containing rotating water in which pieces of lead pushed pieces of
wood towards the center. Because lead is heavier than wood, Huygens
thought that “this experiment does not serve to show the cause of gravity,
…”. 88 Gravity ought to be explained by movement alone so there should not
be a difference in weight between the various materials in the vessel.
In the paper he read at the Académie on 29 August 1669, Huygens
described an alternative experiment. In a vessel filled with water, two strings
allow a small sphere to move along the diameter. Turning the vessel around
its axis will make the water rotate. If it is suddenly made to stop, the water
85
OC19, 553. “Qu’il y a une matiere tres subtile et deliée et qui est agitée d’un mouvement extremement
viste.”
86
OC19, 575-581. Huygens appears to have modified his interpretations afterwards, laying particular stress
on the pores of the magnet and the direction of the streams: OC19, 591-603.
87
OC19, 625-637. See also: Westfall, Force, 185-186.
88
OC19, 626. “Cette experience ne sert point a faire voir la cause de la pesanteur, …” Descartes had
described the experiment in a letter to Mersenne of 16 October 1629, AT2, 593-594.