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at is colourless includes （a） what is transparent and （b） what is invisible or scarcely visible； i。e。 what is 'dark'。 The latter （b） is the same as what is transparent； when it is potentially； not of course when it is actually transparent； it is the same substance which is now darkness； now light。 Not everything that is visible depends upon light for its visibility。 This is only true of the 'proper' colour of things。 Some objects of sight which in light are invisible； in darkness stimulate the sense； that is； things that appear fiery or shining。 This class of objects has no simple common name； but instances of it are fungi； flesh； heads； scales； and eyes of fish。 In none of these is what is seen their own proper' colour。 Why we see these at all is another question。 At present what is obvious is that what is seen in light is always colour。 That is why without the help of light colour remains invisible。 Its being colour at all means precisely its having in it the power to set in movement what is already actually transparent； and； as we have seen； the actuality of what is transparent is just light。 The following experiment makes the necessity of a medium clear。 If what has colour is placed in immediate contact with the eye； it cannot be seen。 Colour sets in movement not the sense organ but what is transparent； e。g。 the air； and that； extending continuously from the object to the organ； sets the latter in movement。 Democritus misrepresents the facts when he expresses the opinion that if the interspace were empty one could distinctly see an ant on the vault of the sky； that is an impossibility。 Seeing is due to an affection or change of what has the perceptive faculty； and it cannot be affected by the seen colour itself； it remains that it must be affected by what comes between。 Hence it is indispensable that there be something in between…if there were nothing； so far from seeing with greater distinctness； we should see nothing at all。 We have now explained the cause why colour cannot be seen otherwise than in light。 Fire on the other hand is seen both in darkness and in light； this double possibility follows necessarily from our theory； for it is just fire that makes what is potentially transparent actually transparent。 The same account holds also of sound and smell； if the object of either of these senses is in immediate contact with the organ no sensation is produced。 In both cases the object sets in movement only what lies between； and this in turn sets the organ in movement： if what sounds or smells is brought into immediate contact with the organ； no sensation will be produced。 The same； in spite of all appearances； applies also to touch and taste； why there is this apparent difference will be clear later。 What comes between in the case of sounds is air； the corresponding medium in the case of smell has no name。 But； corresponding to what is transparent in the case of colour； there is a quality found both in air and water； which serves as a medium for what has smell…I say 'in water' because animals that live in water as well as those that live on land seem to possess the sense of smell； and 'in air' because man and all other land animals that breathe； perceive smells only when they breathe air in。 The explanation of this too will be given later。
8
Now let us； to begin with； make certain distinctions about sound and hearing。 Sound may mean either of two things （a） actual； and （b） potential； sound。 There are certain things which； as we say； 'have no sound'； e。g。 sponges or wool； others which have； e。g。 bronze and in general all things which are smooth and solid…the latter are said to have a sound because they can make a sound； i。e。 can generate actual sound between themselves and the organ of hearing。 Actual sound requires for its occurrence （i； ii） two such bodies and （iii） a space between them； for it is generated by an impact。 Hence it is impossible for one body only to generate a sound…there must be a body impinging and a body impinged upon； what sounds does so by striking against something else； and this is impossible without a movement from place to place。 As we have said； not all bodies can by impact on one another produce sound； impact on wool makes no sound； while the impact on bronze or any body which is smooth and hollow does。 Bronze gives out a sound when struck because it is smooth； bodies which are hollow owing to reflection repeat the original impact over and over again； the body originally set in movement being unable to escape from the concavity。 Further； we must remark that sound is heard both in air and in water； though less distinctly in the latter。 Yet neither air nor water is the principal cause of sound。 What is required for the production of sound is an impact of two solids against one another and against the air。 The latter condition is satisfied when the air impinged upon does not retreat before the blow； i。e。 is not dissipated by it。 That is why it must be struck with a sudden sharp blow； if it is to sound…the movement of the whip must outrun the dispersion of the air； just as one might get in a stroke at a heap or whirl of sand as it was traveling rapidly past。 An echo occurs； when； a mass of air having been unified； bounded； and prevented from dissipation by the containing walls of a vessel； the air originally struck by the impinging body and set in movement by it rebounds from this mass of air like a ball from a wall。 It is probable that in all generation of sound echo takes place； though it is frequently only indistinctly heard。 What happens here must be analogous to what happens in the case of light； light is always reflected…otherwise it would not be diffused and outside what was directly illuminated by the sun there would be blank darkness； but this reflected light is not always strong enough； as it is when it is reflected from water； bronze； and other smooth bodies； to cast a shadow； which is the distinguishing mark by which we recognize light。 It is rightly said that an empty space plays the chief part in the production of hearing； for what people mean by 'the vacuum' is the air； which is what causes hearing； when that air is set in movement as one continuous mass； but owing to its friability it emits no sound； being dissipated by impinging upon any surface which is not smooth。 When the surface on which it impinges is quite smooth； what is produced by the original impact is a united mass； a result due to the smoothness of the surface with which the air is in contact at the other end。 What has the power of producing sound is what has the power of setting in movement a single mass of air which is continuous from the impinging body up to the organ of hearing。 The organ of hearing is physically united with air； and because it is in air； the air inside is moved concurrently with the air outside。 Hence animals do not hear with all parts of their bodies； nor do all parts admit of the entrance of air； for even the part which can be moved and can sound has not air everywhere in it。 Air in itself is； owing to its friability； quite soundless； only when its dissipation is prevented is its movement sound。 The air in the ear is built into a chamber just to prevent this dissipating movement； in order that the animal may accurately apprehend all varieties of the movements of the air outside。 That is why we hear also in water； viz。 because the water cannot get into the air chamber or even； owing to the spirals； into the outer ear。 If this does happen； hearing ceases； as it also does if the tympanic membrane is damaged； just as sight ceases if the membrane covering the pupil is damaged。 It is also a test of deafness whether the ear does or does not reverberate like a horn； the air inside the ear has always a movement of its own； but the sound we hear is always the sounding of something else； not of the organ itself。 That is why we say that we hear with what is empty and echoes； viz。 because what we hear with is a chamber which contains a bounded mass of air。 Which is it that 'sounds'； the striking body or the struck？ Is not the answer 'it is both； but each in a different way'？ Sound is a movement of what can rebound from a smooth surface when struck ag