Imágenes de páginas

may be observed in summer. Indeed we should make a more diligent inquiry into the nature of confined air. For, in the first place, the qualities of air in its own nature with regard to heat and cold, may reasonably be the subject of doubt. For air evidently derives its heat from the effects of celestial bodies, and possibly its cold from the cxhalation of the earth, and in the mid region of air (as it is termed) from cold vapours and snow, so that no judgment can be formed of the nature of air by that which is out of doors and exposed, but a more correct one might be derived from confined air. It is necessary, however, that the air should be enclosed in a vessel of such materials as would not imbue it with heat or cold of theinselves, nor easily admit the influence of the exterternal atmosphere. The experiment should be made therefore with an earthen jar, covered with folds of leather to protect it from the external air, and the air should be kept three or four days in this vessel well closed. On opening the jar, the degree of heat may be ascertained either by the hand or a graduated glass tube.

Nineteenth negative to the thirteenth affirmative.


There is a similar doubt as to whether the warmth of wool, skins, feathers, and the like, is derived from a slight inherent heat, since they are animal excretions, or from their being of a certain fat and oily nature that accords with heat, or merely from the confinement and separation of air which we spoke of in the preceding paragraph. For all air appears to possess a certain degree of warmth when separated from the external atmosphere. Let an experiment be made, therefore, with fibrous substances of linen, and not of wol, feathers, or silk, which are animal excretions. For it is to be observed that all powders (where air is manifestly enclosed) are less cold than the substances when whole, just as we imagine froth (which contains air) to be less cold than the liquid itself.

Twentieth negative to the fourteenth affirmative.

We have here no exactly negative instance, for we are not acquainted with any body tangible or spirituous which does not admit of heat when exposed to the fire. There is, however, this difference, that some admit it more rapidly, as air, oil, and water, others more slowly, as stone and metals. This, however, belongs to the table of degrees.

Twenty-first negative to the fifteenth affirmative. No negative is here subjoined, except the re

• This last is found to be the real air not being a good conductor, and therefore not allowing the escape of heat. The confined air is disengaged when these substances are placed

under an exhausted receiver.

This is erroneous. Air, in fact, is one of the worst. and

metals are the best conductors of heat.

mark that sparks are not kindled by flint and steel, or any other hard substance, unless some small particles of the stone or metal are struck off, and that the air never forms them by friction, as is commonly supposed; besides, the sparks from the weight of the ignited substance, have a tendency to descend rather than to rise, and when extinguished become a sort of dark ash.

Twenty-second negative to the sixteenth affirmative. We are of opinion that here again there is no negative. For we are not acquainted with any tangible body which does not become decidedly warm by friction, so that the ancients feigned that the gods had no other means or power of creating heat than the friction of air, by rapid and violent rotation. On this point, however, further inquiry must be made, whether bodies projected by machines (as balls from cannon) do not derive some degree of heat from meeting the air, which renders them somewhat warm when they fall. The air in motion rather cools than heats, as in the winds, the bellows, or breath when the month is contracted. The motion, however, in such instances is not sufficiently rapid to excite heat, and is applied to a body of air and not to its component parts, so that it is not surprising that heat should not be generated.

Twenty-third negative to the seventeenth affirmative. We must make a more diligent inquiry into this instance. For herbs, and green and moist vegetables appear to possess a latent heat, so small, however, as not to be perceived by the touch in single specimens, but when they are united and confined, so that their spirit cannot exhale into the air, and they rather warm each other, their heat is at once manifested, and even flame occasionally in suitable substances.

Twenty-fourth negative to the eighteenth affirmative.

Here, too, we must make a more diligent inquiry. For quicklime, when sprinkled with water, appears to conceive heat, either from its being collected into one point, (as we observed of herbs when confined,) or from the irritation and exasperation of the fiery spirit by water, which occasions a conflict and struggle. The true reason will more readily be shown if oil be used instead of water, for oil will equally tend to collect the confined spirit, but not to irritate. The experi ment may be made more general, both by using the ashes and calcined products of different bodies, and by pouring different liquids upon them.

Twenty-fifth negative to the nineteenth affirmative. A negative instance may be subjoined of other metals which are more soft and soluble. For leaf gold dissolved by aqua regia, or lead by aqua fortis, are not warm to the touch whilst dissolving, no more is quicksilver, (as far as I remember,) but

silver excites a slight heat, and so does copper, | palate, and still more so when taken internally. and tin yet more plainly, and most of all, iron and steel, which excite not only a powerful heat, but a violent bubbling. The heat, therefore, appears to be occasioned by the struggle which takes place when these strong dissolvents penetrate, dig into, and tear asunder the parts of those substances, whilst the substances themselves resist. When, however, the substances yield more easily, scarcely any heat is excited.

Twenty-sixth negative to the twentieth affirmative. There is no negative instances with regard to the heat of animals, except in insects, (as has been observed,) owing to their small size. For, in fishes, as compared with land animals, a lower degree rather than a deprivation of heat is observable. In plants and vegetables, both as to their exudations and pith when freshly exposed, there is no sensible degree of heat. But in animals there is a great difference in the degree, both in particular parts, (for the heat varies near the heart, the brain, and the extremities,) and in the circumstances in which they are placed, such as violent exercise and fevers.

Twenty-seventh negative to twenty-first affirmative.

Here again there is scarcely a negative instance. I might add that the excrements of animals, even when they are no longer fresh, possess evidently some effective heat, as is shown by their enriching the soil.

Twenty-eighth negative to the twenty-second and twentythird affirmative.

Such liquids (whether oily or watery) as are intensely acrid, exhibit the effects of heat, by the separation and burning of bodies after some little action upon them, yet they are not at first warm to the touch. But they act according to their affinity and the pores of the substances to which they are applied. For aqua regia dissolves gold, but not silver, on the contrary, aqua fortis dissolves silver, but not gold; neither of them dissolves glass, and so of the rest.

Twenty-ninth negative to twenty-fourth affirmative. Let spirits of wine be tried on wood, or butter, wax, or pitch, to see if this will melt them at all by their heat. For the 24th instance shows that they possess properties resembling those of heat in causing incrustation. Let an experiment also be made with a graduated glass or calendar,* concave at the top, by pouring well rectified spirits of wine into the cavity, and covering it up in order that they may the better retain their heat, then observe whether their heat make the water descend.

Thirtieth negative to twenty-fifth affirmative.

One should see, therefore, on what other substances they exhibit the effects of heat. Now, sailors tell us that when large quantities of spices are suddenly opened, after having been shut up for some time, there is some danger of fever and inflammation to those who stir them or take them out. An experiment might therefore be made whether such spices and herbs when produced will, like smoke, dry fish and meat hung up over them.

Thirty-first negative to twenty-sixth affirmative. There is an acrid effect, and a degree of penetration in cold liquids, such as vinegar and oil of vitriol, as well as in warm, such as oil of marjoram and the like. They have, therefore, an equal effect in causing animated substances to smart, and separating and consuming inanimate parts. There is not any negative instance as to this, nor does there exist any animal pain unaccompanied by the sensation of heat.

Thirty-second negative to twenty-seventh affirmative.

There are many effects common to cold and heat, however different in their process. For, snow balls appear to burn boys' hands after a little time, and cold no less than fire preserves bodies from putrefaction, besides, both heat and cold contract bodies. But it is better to refer these instances and the like to the investigation of cold.

13. In the third place, we must exhibit to the understanding the instances in which that nature, which is the object of our inquiries, is present in a greater or less degree, either by comparing its increase and decrease in the same object, or its degree in different objects. For, since the form of a thing is its very essence, and the thing only differs from its form as the apparent from the actual object, or the exterior from the interior, or that which is considered with relation to man from that which is considered with relation to the universe; it necessarily follows that no nature can be considered a real form, which does not uniformly diminish and increase with the given nature. We are wont to call this our table of degrees or comparative instances.

Table of the Degrees or Comparative Instances of


We will first speak of those bodies which exhibit no degree of heat sensible to the touch, but appear rather to possess a potential heat, or disposition and preparation for it. We will then go on to others, which are actually warm to the touch, and observe the strength and degree of it. 1. There is no known solid or tangible body

Spices and acrid herbs are sensibly warm to the which is by its own nature originally warm

*See No 38, in the table of the degrees of heat.

For neither stone, metal, sulphur, fossils, wood. water, nor dead animal carcasses, are found warm.

The warm springs in baths appear to be heated accidentally, by flame, subterraneous fire, (such as is thrown up by Etna and many other mountains,) or by the contact of certain bodies, as heat is exhibited in the dissolution of iron and tin. The degree of heat, therefore, in inanimate objects is not sensible to our touch, but they differ in their degrees of cold, for wood and metal are not equally cold. This, however, belongs to the table of degrees of cold.

2. But with regard to potential heat and predisposition to flame, we find many inanimate substances wonderfully adapted to it; as sulphur, naphtha, and saltpetre.

3. Bodies which have previously acquired heat, as horse-dung from the animal, or lime, and perhaps ashes or soot from fire, retain some latent portion of it. Hence distillations and separations of substances are effected by burying them in horse-dung, and heat is excited in lime by sprinkling it with water, (as has been before observed.) 4. In the vegetable world we know of no plant, nor part of any plant, (as the exudations or pith) that is warm to man's touch. Yet, as we have before observed, green weeds grow warm when confined, and some vegetables are warm and others cold to our internal touch, i. e. the palate and stomach, or even, after a while, to our external skin, (as is shown in plasters and ointments.)

and weather, and is not so acute as to be insufferable to the hand. It is said, however, of Constantius, and some others of a very dry constitution and habit of body, that when attacked with violent fevers, they became so warm as to appear almost to burn the hand applied to them.

9. Animals become more warm by motion and exercise, wine and feasting, venery, burning fevers, and grief.

10. In the paroxysm of intermittent fevers the patients are at first seized with cold and shivering, but soon afterwards become more heated than at first; in burning and pestilential fevers they are hot from the beginning.

11. Let further inquiry be made into the comparative heat of different animals, as fishes, quadrupeds, serpents, birds: and also of the different species, as the lion, the kite, or man. For, according to the vulgar opinion, fishes are the least warm internally, and birds the most; particularly doves, hawks, and ostriches.

12. Let further inquiry be made as to the comparative heat in different parts and limbs of the same animal. For milk, blood, seed, and eggs are moderately warm, and less hot than the outward flesh of the animal when in motion or agitated. The degree of heat of the brain, stomach, heart, and the rest, has not yet been equally well investigated.

13. All animals are externally cold in winter and cold weather, but are thought to be internally warmer.

5. We know of nothing in the various parts of animals, when dead or detached from the rest, that is warm to the touch. For horse-dung itself does not retain its heat, unless it be confined and 14. The heat of the heavenly bodies, even in buried. All dung, however, appears to possess the warmest climates and seasons, never reaches a potential heat, as in manuring fields. So, also, such a pitch as to light or burn the dryest wood dead bodies are endued with this latent and po- or straw, or even tinder without the aid of burningtential heat, to such a degree that, in cemete-glasses. It can, however, raise vapour from ries where people are interred daily, the earth acquires a secret heat which consumes any recently deposited body much sooner than pure earth and they tell you that the people of the East are acquainted with a fine soft cloth, made of the down of birds, which can melt butter wrapt gently up in it by its own warmth.

6. Manures, such as every kind of dung, chalk, sea-sand, salt, and the like, have some disposition towards heat.

7. All putrefaction exhibits some slight degree of heat, though not enough to be perceptible by the touch. For, neither the substances, which by putrefaction are converted into animalculæ, as flesh and cheese, nor rotten wood, which shines in the dark, are warm to the touch. The heat, however, of putrid substances displays itself occasionally in a disgusting and strong scent.

8. The first degree of heat, therefore, in substances which are warm to the human touch, appears to be that of animals, and this admits of a great variety of degrees, for the lowest (as in insects) is scarcely perceptible, the highest scarcely equals that of the sun's rays in warm climates

moist substances.

15. Astronomers tell us that some stars are hotter than others. Mars is considered the warmest after the sun, then Jupiter, then Venus. The moon and, above all, Saturn are considered to be cold. Among the fixed stars, Sirius is thought the warmest, then Cor Leonis, or Regu lus, then the lesser dog-star.

The sun gives out more heat as it approaches towards the perpendicular or zenith, which may be supposed to be the case with the other planets according to their degree of heat; for instance, that Jupiter gives out more heat when situated beneath Cancer or Leo, than when he is beneath Capricorn and Aquarius.

17. It is to be supposed that the sun and other planets give more heat in perigee, from their approximation to the earth, than when in apogee. But if in any country the sun should be both in its perigee and nearer to the perpendicular at the same time, it must necessarily give out more heat than in a country where it is also in perigee, but situated more obliquely. So that the comparative altitude of the planets should be observed, and

their approach to or declination from the perpen- from some imperfect metals is very strong and dicular in different countries. active: but on all these points further inquiry should be made.

18. The sun and other planets are thought also to give ont more heat in proportion as they are nearer to the larger fixed stars; as when the sun is in Leo he is nearer Cor Leonis, Cauda Leonis, Spica Virginis, Sirius, and the lesser dog-star, than when he is in Cancer, where, however, he approaches nearer to the perpendicular. It is probable also that the quarters of the heavens produce a greater heat (though not perceptibly) in proportion as they are adorned with a greater number of stars, particularly those of the first magnitude.

19. On the whole, the heat of the heavenly bodies is augmented in three ways: 1. The approach to the perpendicular; 2. Proximity or their perigee; 3. The conjunction or union of stars.

20. There is a very considerable difference between the degree of heat in animals, and even in the rays of the heavenly bodies, (as they reach us,) and the heat of the most gentle flame, and even of all ignited substances, nay, liquids, or the air itself, when unusually heated by fire. For the flame of spirit of wine, though diffused and uncollected, is yet able to set straw, linen, or paper on fire, which animal heat, or that of the sun, will never accomplish without a burning-glass.

21. There are, however, many degrees of strength and weakness in flame and ignited bodies: but no diligent inquiry has been made in this respect, and we must, therefore, pass it hastily over. Of all flames, that of spirits of wine appears to be the most gentle, except, perhaps, the ignis fatuus, or the flashes from the perspiration of animals. After this we should be inclined to place the flame of light and porous vegetables, such as straw, reeds, and dried leaves; from which the flame of hair or feathers differs but little. Then, perhaps, comes the flame of wood, particularly that which contains but little rosin or pitch, that of small wood, however, (such as is usually tied up in fagots,) is milder than that of the trunks | or roots of trees. This can be easily tried in iron furnaces, where a fire of fagots or branches of trees is of little service. Next follows the flame of oil, tallow, wax, and the like oily and fat substances, which are not very violent. But a most powerful heat is found in pitch and rosin, and a still greater in sulphur, camphire, naphtha, saltpetre, and salts, (after they have discharged their crude matter,) and in their compounds; as in gunpowder, Greek fire, (vulgarly called wild file,) and its varieties, which possess such a stubborn heat as scarcely to be extinguished by

23. The flame of vivid lightning appears to exceed all the above, so as sometimes to have melted even wrought iron into drops, which the other flames cannot accomplish.

21. In ignited bodies there are different degrees of heat, concerning which also a diligent inquiry has not been made. We consider the faintest heat to be that of tinder, touchwood, and dry rope match, such as is used for discharging cannon. Next follows that of ignited charcoal, or cinders, and even bricks, and the like; but the most violent is that of ignited metals, as iron, copper, and the like. Further inquiry, however, must be made into this also.

25. Some ignited bodies are found to be much warmer than some flames; for instance, red-hot iron is much warmer, and burns more than the flame of spirits of wine.

26. Some bodies even not ignited, but only heated by the fire, as boiling water, and the air confined in reverberatories, surpass in heat many flames and ignited substances.

27. Motion increases heat, as is shown in the bellows and the blow-pipe, for the harder metals are not dissolved or melted by steady, quiet fire. without the aid of the blow-pipe.

28. Let an experiment be made with burningglasses; in which respect I have observed, that if a glass be placed at the distance of ten inches, for instance, from the combustible object, it does not kindle or burn it so readily as if the glass be placed at the distance of five inches, (for instance,) and be then gradually and slowly withdrawn to the distance of ten inches. The cone and focus of the rays, however, are the same, but the mere motion increases the effect of the heat.

29. Conflagrations, which take place with a high wind, are thought to make greater way against than with the wind, because, when the wind slackens, the flame recoils more rapidly than it advances, when the wind is favourable.

30. Flame does not burst out or arise unless it have some hollow space to move and exert itself in, except in the exploding flame of gunpowder and the like, where the compression and confine. ment of the flame increases its fury.

31. The anvil becomes so hot by the hammer, that if it were a thin plate, it might probably grow red, like ignited iron, by repeated strokes. Let the experiment be tried.

32. But in ignited bodies that are porous, so as to leave room for the fire to move itself, if its motion be prevented by strong compression, the 22. We consider that the flame which results fire is immediately extinguished; thus it is with


*This notion is erroneous, but the sun is in Leo about August, when the earth has become heated by the accumula tion of heat after the solstice. The maximum of heat in the day is not at noon, but about two o'clock, for the same reason.

tinder, or the burning snuff of a candle or lamp. or even hot charcoal cinders, for when they are squeezed by snuffers, or the foot, and the like the effect of the fire instantly ceases.

33. The approach towards a hot body, increases heat in proportion to the approximation; a similar effect to that of light, for the nearer any object is placed towards the light, the more visible it


34. The union of different heats increases heat, unless the substances be mixed. For a large and small fire in the same spot, tend mutually to increase each other's heat, but lukewarm water poured into boiling water cools it.

we shall presently speak of, and which is very gentle and delicate, should be impeded.

Before the first glass be inserted in the other, its upper part (the belly) should be warmed at the fire. Then upon placing it as we have described, the air, (which was dilated by the heat,) after a sufficient time has been allowed for it to lose the additional temperature, will restore and contract itself to the same dimensions as that of the external or common atmosphere at the moment of immersion, and the water will be attracted upwards in the tube to a proportionate extent. A long, nardivided into as many degrees as you please. You will then perceive, as the weather grows warmer or colder, that the air contracts itself into a narrower space in cold weather, and dilates in the warm, which will be exhibited by the rising of the water as the air contracts itself, and its depression as the air dilates. The sensibility of the air with regard to heat or cold is so delicate and exquisite, that it far exceeds the human touch, so that a ray of sunshine, the heat of the breath, and, much more, that of the hand placed on the top of the tube, immediately causes an evident depression of the water. We think, however, that the spirit of animals possesses a much more delicate susceptibility of heat and cold, only that it is impeded and blunted by the grossness of their bodies.

35. The continued neighbourhood of a warm body increases heat. For the heat, which perpetually passes and emanates from it, being mix-row slip of paper should be attached to the tube, ed with that which preceded it, multiplies the whole. A fire, for instance, does not warm a room in half an hour as much as the same fire would in an hour. This does not apply to light, for a lamp or candle placed in any spot, gives no more light by remaining there, than it did at first. 36. The irritation of surrounding cold increases heat, as may be seen in fires during a sharp frost. We think that this is owing not merely to the confinement and compression of the heat, (which forms a sort of union;) but also by the exasperation of it, as when the air or a stick are violently compressed or bent, they recoil, not only to the point they first occupied, but still further back. Let an accurate experiment, therefore, be made with a stick, or something of the kind, put into the flame, in order to see whether it be not sooner burnt at the sides than in the middle of it.†

37. There are many degrees in the susceptibility of heat. And, first, it must be observed how much a low, gentle heat changes and partially warms even the bodies least susceptible of it. For even the heat of the hand imparts a little warmth to a ball of lead or other metal held a short time in it. So easily is heat transmitted and excited, without any apparent change in the body.

38. Of all bodies that we are acquainted with, air admits and loses heat the most readily, which is admirably seen in weather-glasses, whose construction is as follows. Take a glass with a hollow belly, and a thin and long neck; turn it upside down, and place it with its mouth downwards into another glass vessel containing water; the end of the tube touching the bottom of the vessel, and the tube itself leaning a little on the edge, so as to be fixed upright. In order to do this more readily, let a little wax be applied to the edge, not, however, so as to block up the orifice, lest by preventing the air from escaping, the motion, which

The fires supply fresh heat, the water has only a certain quantity of heat, which being diffused over a fresh supply of cooler water, must be, on the whole, lowered.

If condensation were the cause of the greater heat, Bacun concludes the centre of the flame would be the hotter part, and vice versa. The fact is, neither of the causes assigned by Bacon is the true one; for the fire burns more quickly only because the draught of air is more rapid, the cold, dense air pressing rapidly into the heated room and towards the chin.ney.

39. After air we consider those bodies to be most sensible of heat, which have been recently changed and contracted by cold, as snow and ice; for they begin to be dissolved and melt with the first mild weather. Next, perhaps, follows quicksilver; then greasy substances, as oil, butter, and the like; then wood; then water; lastly, stones and metals, which do not easily grow hot, particularly towards their centre. When heated, however, they retain their temperature for a very long time; so that a brick or stone, or hot iron plunged in a basin of cold water, and kept there for a quarter of an hour or thereabouts, retains such a heat as not to admit of being touched.

40. The less massive the body is, the more readily it grows warm at the approach of a heated body, which shows that heat with us is somewhat averse to a tangible mass.†

41. Heat, with regard to the human senses and touch, is various and relative, so that lukewarm

Bacon appears to have confounded combustibility and fusibility with susceptibility of heat; for, though the metals will certainly neither dissolve as soon as ice or butter, nor be consumed as soon as wood, that only shows that different degrees of heat are required to produce similar effects on dif ferent bodies; but metals much more readily acquire and transmit the same degree of heat than any of the above substances. The rapid transmission renders them generally cold to the touch. The convenience of fixing wooden handles to vessels containing hot water illustrates these observations.

Another singular error, the truth being that solid bodies are the best conductors; but of course where heat is diffused over a large mass, it is less in each part, than if that part also alone received the whole quantum of heat.

« AnteriorContinuar »