The Essential Galileo (32 page)

[§7.2 Heat, Atoms, and Primary vs. Secondary Qualities]
¹⁰

[347] In accordance with my earlier promise to Your Most Illustrious Lordship, there now remains for me to say what I think about the proposition “motion is the cause of heat,” indicating in what sense I think it may be true. But first I must make a comment about what we call
heat
. I very much suspect that the conception which people generally form of it is very far from the truth inasmuch as it is believed to be a real attribute, property, and quality that truly inheres in the material by which we feel warmed.

Accordingly, I say that as soon as I conceive of a corporeal substance or material, I feel indeed drawn by the necessity of also conceiving that it is bounded and has this or that shape; that it is large or small in relation to other things; that it is in this or that location and exists at this or that time; that it moves or stands still; that it touches or does not touch another body; and that it is one, a few, or many. Nor can I, by any stretch of the imagination, separate it from these conditions. However, my mind does not feel forced to regard it as necessarily accompanied by such conditions as the following: that it is white or red, bitter or sweet, noisy or quiet, [348] and pleasantly or unpleasantly smelling; on the contrary, if we did not have the assistance of our senses, perhaps the intellect and the imagination by themselves would never conceive of them. Thus, from the point of view of the subject in which they seem to inhere, these tastes, odors, colors, etc., are nothing but empty names; rather they inhere only in the sensitive body, such that if one removes the animal, then all these qualities are taken away and annihilated. However, since we have given them particular names different from those of the primary and real attributes, we have a tendency to believe that these qualities are truly and really different from the primary ones.

I think I can explain my meaning more clearly with some examples. Suppose I move my hand first over a marble statue and then over a living man. Regarding the action coming from my hand, from the point of view of the hand the action over one subject is the same as that over the other; it consists of primary attributes, namely, motion and touch, and we do not use any other names. But the animate body that receives such an action feels various sensations depending on where it is touched. For example, if it is touched on the soles of the feet, on the knees, or on the armpits, besides touch it feels another sensation to which we have given a particular name, calling it
tickling
. This sensation is entirely ours and not at all in the hand; I think it would be a great error to want to say that, besides motion and touching, the hand has within itself another property different from these, namely, the power to tickle, such that tickling is an attribute inherent in it. Similarly, a piece of paper or a feather lightly brushed over any part of our body performs exactly the same operation with regard to itself, namely, moving and touching. But with regard to us, by touching between the eyes, or on the nose, or under the nostrils, it produces an almost intolerable titillation, whereas in other parts it is hardly felt. That titillation is entirely in us and not in the feather, and if the animate and sensitive body is removed, it is nothing but an empty name. Now, I believe that many qualities that are attributed to natural bodies (such as tastes, odors, colors, and others) may have a similar and not greater reality.

A solid and, so to speak, highly material body, when moved and applied to any part of my person, produces in me [349] a sensation which we call
touch
. Although this sensation covers the whole body, nevertheless it seems to reside chiefly in the palms of the hands, and especially in the fingertips, with which we feel extremely small differences of roughness, smoothness, softness, and hardness, whereas with other parts of the body we do not distinguish them as well. Some of these sensations are more pleasant, others less so, depending on the shapes of tangible bodies, whether they are smooth or rough, acute or obtuse, hard or soft. This sense, being more material than the others and deriving from the solidity of matter, seems to correspond to the element earth.

Now, some of these bodies are constantly being subdivided into tiny particles,
¹¹
of which some are heavier than air and fall downwards and others are lighter and rise upwards. And perhaps here is how two other senses are generated, when those particles go and strike two parts of our body that are much more sensitive than our skin, which does not feel the effect of materials that are so fine, delicate, and soft. The particles that go down are received by the upper part of the tongue, becoming mixed with its humidity and penetrating its substance; thus they produce taste, likable or disagreeable, depending on the kind of contact with the various shapes of the particles, on the greater or smaller number of particles, and on their velocity. The other particles, which go up, enter through the nostrils and strike some small nodules that are the instrument of our sense of smell; here likewise their touch and movements are recorded with pleasure or annoyance, depending on whether their shapes are these or those, their movements are slow or fast, and their number is small or large. And indeed we see that, with regard to their location, the tongue and the nasal passages are wisely arranged: the former is extended underneath in order to receive the descending signals; the latter are set up for the ascending ones. And perhaps there is an analogy between the production of tastes and the descent of fluids through air and between the production of odors and the ascent of fires.

There remains the question of the correspondence between the element air and sounds. These come to us equally from all parts (lower, higher, and lateral) since we are located in air, whose motion in its own region is propagated equally in all directions. And the placement of the ear is arranged as much as possible to respond to all positions. Sounds are produced in us and heard when, without any sonorous or sound-like qualities, a rapid vibration of the air in the form of extremely minute waves moves some cartilage in the tympanum that is [350] in our ear. The external means capable of producing this rippling in the air are extremely numerous, but perhaps they reduce mostly to the vibration of bodies that strike the air and thereby ripple it; the waves propagate through it at great velocity, with higher frequencies generating sharper sounds and lower frequencies deeper tones.

However, I do not believe that in order to stimulate in us tastes, odors, and sounds, external bodies require anything other than sizes, shapes, quantity, and slow or fast motions. I think that if one takes away ears, tongues, and noses, there indeed remain the shapes, numbers, and motions, but not the odors, tastes, or sounds; outside the living animal these are nothing but names, just as tickling and titillation are nothing but names if we remove the armpits and the skin around the nose. And just as the four elements correspond to the four senses considered so far, I believe that light corresponds to vision, the sense that is the most eminent of all; indeed its excellence is such that the comparison is like that of finite to infinite, time consuming to instantaneous, divisible to indivisible, and dark to light. I understand very little about this sense and related matters, and to explain the little I do understand, or better to adumbrate it on paper, I would need a long time; so I pass it over in silence.

But let us return to my primary purpose here. We have already seen that many properties, which are considered to be qualities inherent in external objects, do not really have any other existence except in us, and that outside of us they are nothing but names. Now I say that I am inclined to believe that heat is of this kind. The materials which produce heat in us and make us feel it, and which we call by the general name
fire
, are large collections of tiny corpuscles shaped in such and such a manner and moving with such and such a speed; when they meet our body they penetrate it because of their extremely small size. Their contact, which they make as they pass through our bodily substance and which we feel, is the property we call
heat
, which is pleasing or hurtful depending on the lesser or greater number and speed of the particles that are pricking and penetrating us. Such penetration is pleasing when it facilitates our unfelt but necessary perspiration, and hurtful when it causes too much division and separation of our bodily substance. In short, the action of [351] fire works exactly this way: because of its extreme flexibility, by moving it penetrates all bodies and so dissolves them sooner or later depending on the number and velocity of the igneous particles in it and on the density or rarity of the matter in those bodies; as they are being destroyed, the greater part of many bodies turns into tiny igneous particles, and the decomposition continues as long as there remains decomposable material.

However, I do not believe in the least that besides shape, quantity, motion, penetration, and touch, there is in fire another quality, and that this quality is heat. Rather, I think that heat is in us, so much so that if we remove the animate and sensitive body, heat remains nothing but a simple word. Furthermore, since this property is produced in us by the touch of the tiny igneous particles and their passing through our bodily substance, it is clear that if they were to stand still then their operation would remain null. Thus we see that the considerable amount of fire contained in the pores and cavities of a piece of quicklime does not warm us when we hold it in our hand, because the fire is standing still. But let us place the quicklime in water, where the fire has a greater propensity to move than it had in air because of the weight of the water, and where the cavities are opened more by the water as compared to the situation in air; then the tiny igneous particles escape and meet and penetrate our hand, and we feel the heat.

Since, then, the presence of the igneous particles is not sufficient to stimulate heat, but their motion is also needed, therefore it seems to me very reasonable to say that motion is the cause of heat.

This is the motion that burns arrows and the wood of catapults and liquefies the lead of gunshots and other metals:
¹²
moving at high speed, whether by their own power or by the strong blast of a bellows if that is insufficient, the tiny particles of fire penetrate all bodies; some of these are decomposed into flying igneous particles, others are decomposed into extremely minute dust, and still others are liquefied into fluids like water. But if this proposition is taken in its ordinary meaning (i.e., that moving a rock or a piece of iron or of wood heats them up), then I regard it as a solemn falsehood. Now, the friction and rubbing of two hard bodies does reduce them to motion, in the sense that either parts of them are decomposed into extremely fine flying particles, or the igneous particles contained in them are allowed to escape; as these moving particles meet our bodies, penetrate them, and pass through them, the sensitive soul feels their motion and touch and experiences the pleasing or hurtful sensation which we have [352] named
heat, burning
, or
scorching
. Perhaps while the rubbing and grinding are limited to producing particles that are tiny but still finite, their motion is temporal and their operation merely calorific. But then if one arrives at the ultimate and highest decomposition into really indivisible atoms, one creates light, whose motion (or rather, expansion or propagation) is instantaneous; and it is capable of filling immense spaces on account of its subtlety, rarefaction, and immateriality, although I do not know whether these words are correct or whether we should speak of some other property as yet unnamed and different from all these.

Your Most Illustrious Lordship, I do not want inadvertently to engulf myself in an infinite ocean such that I cannot get back to port. Nor do I want, while removing one doubt, to give rise to a hundred others, as I fear it may have happened as a result of my little departure from the shore. So I want to postpone further discussion to some other more appropriate occasion.

1.
For the historical background, see the Introduction, especially §0.8.

2.
Galilei 1890–1909, 6: 228.31-233.23; newly translated by Finocchiaro.

3.
Recall (from the Introduction) that
The Assayer
is written in the form of a letter to Virginio Cesarini; that it is structured as a series of criticisms of quotations from Orazio Grassi's
Astronomical and Philosophical Balance
, published under the pseudonym of Lotario Sarsi; that the Balance was a critique of Mario Guiducci's
Discourse on the Comets
; and that the
Discourse was
in turn a critique of Grassi's
Astronomical Disputation on the Three Comets of the Year 1618.

4.
These passages were quoted in the
Balance
from the
Discourse
; the page numbers refer to the original edition.

5.
The following criticism of Tycho is unfair, since he was aware of the approximations involved. For some details, see Pagnini 1964, 1: 144–46 n. 1; Drake and O'Malley 1960, 339–40, 369 n. 8.

6.
Thaddaeus Hagecius (1525-1600), professor of mathematics at the University of Prague and personal physician to several Holy Roman Emperors.

7.
Scipione Chiaramonti (1565-1652) was at the time (1623) professor of mathematics at the University of Perugia and later (1627-36) professor of philosophy at the University of Pisa. His
Anti-Tycho
had been published in Venice in 1621. He is mentioned favorably in The
Assayer
in regard to the nature of comets, but he is frequently and sharply criticized in the
Dialogue
for his anti-Copernican views.

8.
Bartolo da Sassoferrato (1313-55), a jurist who wrote a famous commentary on ancient Roman law.

9.
Orlando Furioso
, by Ludovico Ariosto (1474-1533), is one of the greatest epic poems of the Italian language and was especially liked by Galileo.