_The bigness of the component parts of natural Bodies may be conjectured
by their Colours._
For since the parts of these Bodies, by _Prop._ 5. do most probably
exhibit the same Colours with a Plate of equal thickness, provided they
have the same refractive density; and since their parts seem for the
most part to have much the same density with Water or Glass, as by many
circumstances is obvious to collect; to determine the sizes of those
parts, you need only have recourse to the precedent Tables, in which the
thickness of Water or Glass exhibiting any Colour is expressed. Thus if
it be desired to know the diameter of a Corpuscle, which being of equal
density with Glass shall reflect green of the third Order; the Number
16-1/4 shews it to be (16-1/4)/10000 parts of an Inch.
The greatest difficulty is here to know of what Order the Colour of any
Body is. And for this end we must have recourse to the 4th and 18th
Observations; from whence may be collected these particulars.
_Scarlets_, and other _reds_, _oranges_, and _yellows_, if they be pure
and intense, are most probably of the second order. Those of the first
and third order also may be pretty good; only the yellow of the first
order is faint, and the orange and red of the third Order have a great
Mixture of violet and blue.
There may be good _Greens_ of the fourth Order, but the purest are of
the third. And of this Order the green of all Vegetables seems to be,
partly by reason of the Intenseness of their Colours, and partly because
when they wither some of them turn to a greenish yellow, and others to a
more perfect yellow or orange, or perhaps to red, passing first through
all the aforesaid intermediate Colours. Which Changes seem to be
effected by the exhaling of the Moisture which may leave the tinging
Corpuscles more dense, and something augmented by the Accretion of the
oily and earthy Part of that Moisture. Now the green, without doubt, is
of the same Order with those Colours into which it changeth, because the
Changes are gradual, and those Colours, though usually not very full,
yet are often too full and lively to be of the fourth Order.
_Blues_ and _Purples_ may be either of the second or third Order, but
the best are of the third. Thus the Colour of Violets seems to be of
that Order, because their Syrup by acid Liquors turns red, and by
urinous and alcalizate turns green. For since it is of the Nature of
Acids to dissolve or attenuate, and of Alcalies to precipitate or
incrassate, if the Purple Colour of the Syrup was of the second Order,
an acid Liquor by attenuating its tinging Corpuscles would change it to
a red of the first Order, and an Alcali by incrassating them would
change it to a green of the second Order; which red and green,
especially the green, seem too imperfect to be the Colours produced by
these Changes. But if the said Purple be supposed of the third Order,
its Change to red of the second, and green of the third, may without any
Inconvenience be allow'd.
If there be found any Body of a deeper and less reddish Purple than that
of the Violets, its Colour most probably is of the second Order. But yet
there being no Body commonly known whose Colour is constantly more deep
than theirs, I have made use of their Name to denote the deepest and
least reddish Purples, such as manifestly transcend their Colour in
purity.
The _blue_ of the first Order, though very faint and little, may
possibly be the Colour of some Substances; and particularly the azure
Colour of the Skies seems to be of this Order. For all Vapours when they
begin to condense and coalesce into small Parcels, become first of that
Bigness, whereby such an Azure must be reflected before they can
constitute Clouds of other Colours. And so this being the first Colour
which Vapours begin to reflect, it ought to be the Colour of the finest
and most transparent Skies, in which Vapours are not arrived to that
Grossness requisite to reflect other Colours, as we find it is by
Experience.
_Whiteness_, if most intense and luminous, is that of the first Order,
if less strong and luminous, a Mixture of the Colours of several Orders.
Of this last kind is the Whiteness of Froth, Paper, Linnen, and most
white Substances; of the former I reckon that of white Metals to be. For
whilst the densest of Metals, Gold, if foliated, is transparent, and all
Metals become transparent if dissolved in Menstruums or vitrified, the
Opacity of white Metals ariseth not from their Density alone. They being
less dense than Gold would be more transparent than it, did not some
other Cause concur with their Density to make them opake. And this Cause
I take to be such a Bigness of their Particles as fits them to reflect
the white of the first order. For, if they be of other Thicknesses they
may reflect other Colours, as is manifest by the Colours which appear
upon hot Steel in tempering it, and sometimes upon the Surface of melted
Metals in the Skin or Scoria which arises upon them in their cooling.
And as the white of the first order is the strongest which can be made
by Plates of transparent Substances, so it ought to be stronger in the
denser Substances of Metals than in the rarer of Air, Water, and Glass.
Nor do I see but that metallick Substances of such a Thickness as may
fit them to reflect the white of the first order, may, by reason of
their great Density (according to the Tenor of the first of these
Propositions) reflect all the Light incident upon them, and so be as
opake and splendent as it's possible for any Body to be. Gold, or Copper
mix'd with less than half their Weight of Silver, or Tin, or Regulus of
Antimony, in fusion, or amalgamed with a very little Mercury, become
white; which shews both that the Particles of white Metals have much
more Superficies, and so are smaller, than those of Gold and Copper, and
also that they are so opake as not to suffer the Particles of Gold or
Copper to shine through them. Now it is scarce to be doubted but that
the Colours of Gold and Copper are of the second and third order, and
therefore the Particles of white Metals cannot be much bigger than is
requisite to make them reflect the white of the first order. The
Volatility of Mercury argues that they are not much bigger, nor may they
be much less, lest they lose their Opacity, and become either
transparent as they do when attenuated by Vitrification, or by Solution
in Menstruums, or black as they do when ground smaller, by rubbing
Silver, or Tin, or Lead, upon other Substances to draw black Lines. The
first and only Colour which white Metals take by grinding their
Particles smaller, is black, and therefore their white ought to be that
which borders upon the black Spot in the Center of the Rings of Colours,
that is, the white of the first order. But, if you would hence gather
the Bigness of metallick Particles, you must allow for their Density.
For were Mercury transparent, its Density is such that the Sine of
Incidence upon it (by my Computation) would be to the Sine of its
Refraction, as 71 to 20, or 7 to 2. And therefore the Thickness of its
Particles, that they may exhibit the same Colours with those of Bubbles
of Water, ought to be less than the Thickness of the Skin of those
Bubbles in the Proportion of 2 to 7. Whence it's possible, that the
Particles of Mercury may be as little as the Particles of some
transparent and volatile Fluids, and yet reflect the white of the first
order.
Lastly, for the production of _black_, the Corpuscles must be less than
any of those which exhibit Colours. For at all greater sizes there is
too much Light reflected to constitute this Colour. But if they be
supposed a little less than is requisite to reflect the white and very
faint blue of the first order, they will, according to the 4th, 8th,
17th and 18th Observations, reflect so very little Light as to appear
intensely black, and yet may perhaps variously refract it to and fro
within themselves so long, until it happen to be stifled and lost, by
which means they will appear black in all positions of the Eye without
any transparency. And from hence may be understood why Fire, and the
more subtile dissolver Putrefaction, by dividing the Particles of
Substances, turn them to black, why small quantities of black Substances
impart their Colour very freely and intensely to other Substances to
which they are applied; the minute Particles of these, by reason of
their very great number, easily overspreading the gross Particles of
others; why Glass ground very elaborately with Sand on a Copper Plate,
'till it be well polish'd, makes the Sand, together with what is worn
off from the Glass and Copper, become very black: why black Substances
do soonest of all others become hot in the Sun's Light and burn, (which
Effect may proceed partly from the multitude of Refractions in a little
room, and partly from the easy Commotion of so very small Corpuscles;)
and why blacks are usually a little inclined to a bluish Colour. For
that they are so may be seen by illuminating white Paper by Light
reflected from black Substances. For the Paper will usually appear of a
bluish white; and the reason is, that black borders in the obscure blue
of the order described in the 18th Observation, and therefore reflects
more Rays of that Colour than of any other.
In these Descriptions I have been the more particular, because it is not
impossible but that Microscopes may at length be improved to the
discovery of the Particles of Bodies on which their Colours depend, if
they are not already in some measure arrived to that degree of
perfection. For if those Instruments are or can be so far improved as
with sufficient distinctness to represent Objects five or six hundred
times bigger than at a Foot distance they appear to our naked Eyes, I
should hope that we might be able to discover some of the greatest of
those Corpuscles. And by one that would magnify three or four thousand
times perhaps they might all be discover'd, but those which produce
blackness. In the mean while I see nothing material in this Discourse
that may rationally be doubted of, excepting this Position: That
transparent Corpuscles of the same thickness and density with a Plate,
do exhibit the same Colour. And this I would have understood not without
some Latitude, as well because those Corpuscles may be of irregular
Figures, and many Rays must be obliquely incident on them, and so have
a shorter way through them than the length of their Diameters, as
because the straitness of the Medium put in on all sides within such
Corpuscles may a little alter its Motions or other qualities on which
the Reflexion depends. But yet I cannot much suspect the last, because I
have observed of some small Plates of Muscovy Glass which were of an
even thickness, that through a Microscope they have appeared of the same
Colour at their edges and corners where the included Medium was
terminated, which they appeared of in other places. However it will add
much to our Satisfaction, if those Corpuscles can be discover'd with
Microscopes; which if we shall at length attain to, I fear it will be
the utmost improvement of this Sense. For it seems impossible to see the
more secret and noble Works of Nature within the Corpuscles by reason of
their transparency.