Contents Index

extracts from A Syllabus of a Course of Lectures on Chemistry, delivered at the Royal Institution of Great Britain

in The Collected Works of Sir Humphry Davy, ed. John Davy (London: Smith, Elder and Co., 1839), II, 386-409.





1. Introduction.
2. Of the Effects of Heat
  1. Heat was considered as the general power of expansion in Part I. Div. I. Its particular agencies, and {387} the laws by which it is governed, are worthy of a minute examination.

  2. The effects of those actions of heat upon living organs, by which the peculiar sensations of heat and cold are produced, are well known. They are relative, and influenced by the state of the organ.

  3. Bodies increase in volume when heat is added to them, and diminish in volume when it is subtracted from them. The exceptions to this law are very few. Different bodies, and even the same bodies, when differently heated, expand in different ratios, by the additions of equal quantities of heat. In general, gases are more expansible by heat than fluids, and fluids than solids.

  4. The more powerful agencies of heat upon bodies, are often connected with changes in their forms of aggregation. Solids by a certain increase of heat are converted into fluids, and fluids into gases. Also by a subtraction of heat, gases become fluids, and fluids solids.

  5. It was formerly supposed that the absolute weights of bodies were diminished by heat, but some delicate experiments lately made have proved that this opinion is erroneous.

  6. Heat is possessed of most extensive powers in producing chemical combinations, and decompositions. For as it expands different bodies in different ratios, so it likewise diminishes in different ratios the attractions of their particles for each other. See Part I. Div. I. and VI.

3. Of Radiant Heat.
  1. Heat, when existing in the radiant state, moves {388} through space with a velocity almost inconceivable. Like light, it is capable of being reflected, and refracted. And its rays, as would appear from the valuable discoveries of Dr. Herschel, are of different degrees of refragibility, being for the most part less refrangible than those of light.

  2. When radiant heat falls upon the surface of an opaque body, it is never wholly reflected. Portions of it are absorbed by the body, which become common heat of temperature or free caloric. The powers of absorbing radiant heat, appear to be different in different bodies, and they are, in some measure, connected with their peculiar colours.

  3. The radiant heat emitted from the sun is uniformly combined, or mixed with light; but these agents are capable of being separated to a certain extent by the prism. And, as the experiments of Professor Pictet have proved, the radiation of heat from bodies, on the surface of the earth, is apparently perfectly independent of the presence of light.

4. Of Temperature, Capacity for Heat, &c.
  1. The temperatures of bodies are said to be high, or low, in proportion as they are capable of communicating, or receiving heat.

  2. And their capacities for heat, are considered as great, or small, in proportion as their temperatures are less or more raised by the addition, or diminished by the subtraction of equal quantities of heat.

  3. Bodies, in changing their states of existence, have their capacities uniformly changed. And in this case, they absorb or give out that heat, in consequence {389} of its peculiar relation to the heat of temperature, is called latent heat, or caloric of combination.

5. Of the Communication of Heat.
  1. When many bodies of different temperatures, and in different forms of aggregation, are brought into contact, they all, after a certain time, whatever change they may have undergone, acquire a common temperature; though their heat is communicated, or received in different manners, and with different degrees of celerity.

  2. Solids are the only perfect conductors of heat, for they are the only substances through which heat is communicated, from particle to particle, till the mass or system of bodies becomes of the same temperature.

  3. Fluid and aëriform substances, strictly speaking, are either perfect or nearly perfect nonconductors of heat. For, according to the important discoveries of Count Rumford, though their particles are capable of receiving heat from other bodies, or of communicating it to them, yet, amongst those particles themselves, all communication of heat appears impossible. And masses of fluids are heated, or cooled, only in consequence of changes in the specific gravities of their particles, by which they are carried in succession to the heating, or cooling body.

6. Of the Artificial Production of Heat, and of Cold.
  1. Whenever, during chemical combination, the capacities of bodies are diminished, their temperatures are uniformly increased; and vice versa, when their {390} capacities are increased, their temperatures are diminished. See Part I. Div. I. Sect. 5.

  2. The chemical changes which are connected with the production of the greatest quantities of radiant heat, and of heat of temperature are, 1. The absorption of oxygen gas by phosphorous, charcoal, hydrogen, iron, &c. 2. The deflagration of combustible bodies by means of nitrate, or oxygenated muriate, of potash. 3. The combination of the alkalies with the acids. 4. The combination of sulphur with certain of the metals.

  3. One of the greatest dimunitions of temperature, producible by chemical change, is that which takes place from the action of muriate of lime, or of nitrate of ammonia, or of potash, upon snow.

  4. Heat is capable of being excited by mechanical means. Whenever hard solid bodies are violently struck together, their temperatures are uniformly raised. Likewise an increase of temperature is produced by the friction of solids; and that, as it would appear from the valuable experiments of Count Rumford, under circumstances, when these substances are incapable of undergoing either a chemical change, or a change of capacity.

7. Theories concerning the Nature of Heat.
  1. Two theories concerning the nature of heat have been most prevalent amongst philosophers. 1. It has been supposed to be a peculiar ethereal fluid. 2. It has been conjectured to be a property of common matter; a specific motion of the particles of bodies.

  2. The arguments in favour of the first of these theories, have been chiefly deduced from the phænomena {391} of latent heat, of radiant heat, and of change of capacity; whilst the last of them has been supported by experiments on the excitation of heat by friction, in cases in which there existed no perceptible source, from which, considered as a substance, it could possible be derived.

  3. The late experiments of Dr. Herschel have demonstrated, that radiant heat must be constituted by the motions of a peculiar substance. And these motions may be conceived to be either rectilinear projections, or undulations.

  4. It has been lately supposed that they are undulations. And on this theory it has been assumed, 1. That an elastic ethereal medium exists in space. 2. That this medium is diffused through the pores of different ponderable substances, in different states of density. 3. That radiant heat is constituted by particular undulations of it, when in a free state. 4. The sensible heat is occasioned by particular undulations of it, in its states of diffusion through the pores of ponderable substances. 5. That certain peculiar vibratory motions of the particles of ponderable substances are capable of producing the undulations in the ethereal medium which constitute heat. 6. And reciprocally that those undulatory motions of the ethereal medium are capable of producing peculiar vibrations of the particles of ponderable substances.

  5. These propositions are evidently countenanced by the experiments of Count Rumford and Professor Pictet, on the heat produced by friction. They are rendered more conclusive by the analogy between the laws of the motions of radiant heat, and those of sound. And they, in some measure, reconcile the two different theories.


8. Of the Instruments used in Experiments upon Heat.
  1. Thermometers and pyrometers are instruments for measuring the temperatures of bodies.

  2. Thermometers are used for ascertaining degrees of temperature, which are generally not much higher or lower than those of the atmosphere. They are formed on the principle of the expansion of bodies by heat; and such substances are employed in them, as expand most uniformly by successive equal increments of heat. The common thermometers with mercury and alcohol are well known. As thermometers in all cases act only as substances giving, or receiving heat, the quantity of fluid they contain should in all cases be very small: for, in proportion as it is small, so in proportion will the thermometrical expansions or contractions more accurately denote the real temperature of the body experimented upon.

  3. Pyrometers are employed for measuring very high temperatures. They are usually formed of baked clay, a substance which, differing from almost all other bodies, contracts in volume by heat. The contractions of the pyrometer are considered as the measure of its temperature; and they are apparently great, in proportion as the temperatures to which it is exposed are high.

  4. The calorimeter is an instrument that has been used for the purpose of ascertaining the relative capacity of bodies for heat. It is founded upon the principle, that ice, during its conversion into water, absorbs uniformly the same quantities of heat: and it is composed of tin vessel filled with ice, and surrounded by ice. In this vessel, the substance experimented upon is placed; and its capacity is supposed to be directly as the {393} quantity of ice, which, when of a given temperature, it is capable of converting into water.

9. Agency of Heat in Nature.



1. Effects of Light.
2. Laws of Light.
3. Of the artificial Production of Light.
  1. Light is produced during a number of chemical operations. 1. By the union of oxygen gas, and of oxygen in particular states of combination, with certain combustible bodies. 2. By the action of the mineral acids on the fixed alkaline substances. 3. By the combination of sulphur with the metals. 4. By the action of sulphuric acid upon the moistened oxygenated muriates.

  2. All solid, and fluid substances become luminous, when heated to a temperature corresponding to about 850°.

  3. Certain bodies, called solar phosphori, after having been exposed to light, exhibited a luminous appearance in the dark; and this appearance is rendered more vivid by increase of their temperature. By {395} a high degree of heat indeed, it is at length destroyed; but it is capable of being restored by a second exposure to light.

  4. Light is produced during the collision of different bodies, but this phænomenon is probably in general either dependent on combustion, or on the excitation of electricity.

4. Opinions concerning the Nature of Light.
  1. Two opinions have been formed concerning the nature of light, in its visible state. 1. I has been supposed to be produced by the rectilinear rapid motions of the minute particles of a peculiar substance. 2. It has been considered as the undulatory motion of an elastic ethereal medium extended through space.

  2. The first of these opinions has been for a long while most prevalent; and, indeed, even now, it is generally adopted. The last of them, however, has been lately supported by some important arguments deduced by Dr. Young, from the analogy between the laws of known undulatory motions, and those of light.

  3. Though the ph&alig;enomena of the reflection and refraction of light are very analogous to those of radiant heat; and though these bodies are usually present at the same time, yet the distinctness between their physical, as well as their chemical powers of action is sufficient to induce us to believe that they are perfectly different agents. The heat and light in the solar spectrum produce perfectly different effects: for if muriate of silver be exposed to the differently refracted rays, it is found that the invisible heat-making rays produce no effect upon it; its colour is altered by the {396} violet rays in about 1/80 part of the time in which it is changed by the red; and, what is very curious, it is likewise acted upon in the space beyond the violet rays. This circumstance has been noticed by Messrs. Ritter and Böckman, and by Dr. Wollaston. It would appear from it, that invisible rays exist, which, though possessed of chemical agencies, and of the highest degree of refrangibility, are, nevertheless, incapable of producing heat.

5. Of the Photometer.
6. Agencies of Light in Nature.
  1. The action of solar light upon living vegetables, is in most cases connected with the production of {397} oxygen gas; a circumstance, probably intimately related to the uniformity of the constitution of our atmosphere; from which oxygen is constantly absorbed, by the respiration of animals, combustion, and other causes.

  2. Light is possessed of a decided influence in modifying many natural phænomena. The crystallization of salts is materially affected by its agency. It is capable of altering most of the colours of bodies. And it is perpetually producing numerous decompositions, and new combinations upon every part of the surface of the globe.

  3. Light is possessed of great powers of action upon organized beings. The colours of the leaves of vegetables, and of their flowers, are almost wholly dependent upon it: and vegetables are incapable of existing for any great length of time, when deprived of its benign influence.



1. Effects of Electricity.
  1. Electricity in its different states of accumulation produces different effects. In general, its presence is denoted in substances, by their powers of attracting, or repelling, under different circumstances, light bodies. Electricity often appears in the form of light; and, in this state, it is capable of igniting bodies, and of inflaming such of them as are combustible. It occasions vivid sensations, called electrical shocks, in passing {398} through living bodies. It is sensible to the organs of taste and smell.

  2. Electricity is possessed of very powerful chemical agencies. It is capable of producing a number of combinations, and of decompositions; and some of its effects are analogous to those produced by very intense degrees of heat.

  3. Nitric acid is formed from oxygen gas and nitrogen gas, by means of electricity. Water is converted by it into oxygen and hydrogen. And ammonia is decomposed into nitrogen and hydrogen.

2. Of the Conductors and Nonconductors of Electricity, and of their different States with regard to Electricity.
3. Of the Excitation of Electricity.
  1. One of the most simple modes of exciting electricity, is by the friction of two nonconductors, or of a nonconductor and a conductor.

  2. When a glass cylinder, or a circular plate of glass, mounted upon a nonconducting stand, is made to revolve upon its axis, so as to rub against cushions of leather covered with an amalgam of mercury and zinc, the electrical equilibrium is destroyed. The cushions, and the conductors in contact with them, become negatively electrified; and a conductor placed near any part of the glass, which is not in contact with the cushion, becomes positively electrified.

  3. When globes, or cakes of resin, are used in the same manner, electricity is likewise excited; but in this case, the cushions and their conductors become positively, and the other conductors or the prime conductors negatively, electrified.

  4. Electricity is excited in certain nonconducting bodies, merely by changes in their states of temperature. The topaz of Brazil, and the tourmaline, become electrical whenever they are gently heated; and sulphur and sealing wax when cooled after having been melted, are found strongly electrified.

  5. Indeed, whenever bodies change their forms of aggregation, whether from the agency of heat, or other causes, there is every reason to believe, that their states, with regard to electricity, are changed. Water, during its conversion into vapour, appears to absorb much electricity from the bodies in contact with it, so that they become negatively electrified. And during the condensation of aqueous vapour, electricity is evolved.

  6. Electricity is capable of being excited by the {400} action of different conducting substances on each other; but the modes of this excitation, and its general connection with chemical changes, constitute a science which has been hitherto distinguished from common electricity, by the name of galvanism, and which will be immediately considered.

4. Theories concerning Electricity.
5. Of Electrical Instruments.
  1. The Leyden phial is a glass vessel, partially coated on its internal and external surfaces with tinfoil, or other conducting substances. It is charged by electrifying one of its surfaces positively, at the same time that the other surface is connected with the ground, by which it is enabled to become negatively electrified. It is discharged by connecting, by means of a conductor, its positively and negatively electrified surfaces; when the equilibrium is restored.

  2. The electrophorous is composed of a nonconductor plate, which is generally of resin, attached to the upper surface of a conductor, and of a conducting plate having a nonconducting handle. After the nonconducting plate has been once excited, its under surface being connected with the ground; the conducting plate, as often as it is laid upon it and brought in contact for a moment with another conductor, will so often be capable of furnishing a spark, after having been removed from it, by the nonconducting handle.

  3. Bennet's electrometer is composed of two gold leaves, attached to a conducting plate, and enclosed in a tube of glass. It is used for ascertaining the presence of small quantities of electricity in bodies; which are denoted by the separation of the leaves.

  4. The condenser, the doubler, and the multiplier, are instruments generally employed for the purpose of rendering sensible, by means of the electrometer, quantities of electricity immediately imperceptible.

6. Agency of Electricity in Nature.



1. Of the Nature of Galvanism.
2. Of the Construction of the least complicated Galvanic Arrangements, i.e. Simple Circles.
  1. The conductors of electricity, which by their mutual agencies are capable of producing galvanic appearances may be divided into two classes. 1. Perfect conductors, which are either metals or charcoal. 2. Imperfect conductors, which are either oxydated fluids, or substances containing such fluids.

  2. For the composition of a simple galvanic circle, at least two conductors of one class, and one of another class are required. And they must be so disposed that the conductors of the one class may be in contact with each other in one or more points, at the same time that they are connected in other distinct points with the conductor of the other class. Thus, if plates of zinc and gold be made to touch each other in one point, and be connected together in other points, by a portion of common water, or diluted muriatic acid, a simple galvanic circle is formed. A simple galvanic circle is likewise formed when separate portions of water, and solution of hydrogenated sulphuret of potash are connected together in one point, and brought in contact in other points with a piece of silver.

  3. Every arrangement, however, of two conductors of one class with one of the other, is not an acting galvanic circle. For the production of galvanic effects, it is necessary, that the fluid part of parts of the circle should be capable of acting chemically upon the solid part or parts of the circle. The most powerful circles are those in which two different chemical actions are exerted in different parts of the arrangement; and in every circle there is at least one point, in which oxydation, or some other chemical change is taking place.

    The following tables, in which some different simple circles are arranged in the order of their powers, will show how intimately primary chemical changes are connected with the production of galvanism.




3. Of the Agencies of Single Galvanic Circles.
    The galvanic influence is manifested in single circles, {405} either by its power of acting upon living animal organs, or by its chemical agencies.

  1. When the tongue is made part of a simple galvanic circle, and acid taste is perceived, and when the eye is connected with it, a flash of light is produced.

  2. When the galvanic agency is made to act on a muscle, and the nerve attached to it, which have just been separated from the body of a living animal, muscular contractions are uniformly produced.

  3. In all the single galvanic circles, the primary chemical actions are increased, and to a certain extent modified. Thus, when zinc, which oxydates slowly when exposed to the atmosphere in contact with water, is made to form a galvanic circle with that fluid and silver, it oxydates rapidly; and an alkali appears to be formed in the water at its points of contact with the silver. Likewise, when iron and silver form a circle with diluted muriatic acid, the action of the acid upon the iron is increased; and hydrogen gas is not only evolved from the water in contact with the iron, but likewise from that contact with the silver.

4. Of Compound Galvanic Circles or Galvanic Batteries.
  1. Galvanic batteries are composed by series of the substances which form simple circles; and these substances are arranged in such a manner, that the conductors of the same class in every series are in contact with each other in one point or more, at the same time that they are respectively connected with different conductors of the other class, so that a regular alternation is observed; the order being, conductor of the one class, conductor of the one class, conductor of the other class, and so on. Thus, if plates of zinc and of silver, and {406} pieces of cloth of the size of the plates, moistened in water, be arranged in the order of zinc, silver, cloth; zinc, silver, cloth, and so on, till twenty series are connected together, the galvanic battery of Volta will be formed. Likewise a galvanic battery will be formed if pieces of tin, iron, or charcoal, be introduced into glasses filled some with water, and some with nitric acid, and connected in pairs by siphons, in the order of metal or charcoal, acid, water; metal or charcoal, acid, water, and so on, till twenty series are constructed.

  2. The substances most active in the simple circles are likewise most active in the compound circles. And in all cases, the relative quantities of galvanic power exhibited by equal numbers of different series, are in some measure proportional to the intensity of the peculiar primary chemical agencies, exerted by the different conductors composing them, on each other. The tables of the simple circles will indicate, with the necessary changes of arrangement, the powers of such compound circles as are composed of similar parts.

5. Of the Agencies of Galvanic Batteries.
  1. The galvanic influence in its accumulated state in compound circle, produces very singular physical and chemical effects, and exhibits many of the appearances of common electricity.

  2. It produces shocks, when made to act upon the human body, which are very analogous to those occasioned by the electrical battery.

  3. It passes through air and certain other nonconductors in the form of sparks; and in this state it is capable of burning charcoal, and metallic bodies, when they are in contact with the atmosphere; and {407} likewise of inflaming mixtures of oxygen gas and hydrogen gas.

  4. It affects the electrometer; and is capable of communicating weak charges to the condenser, and Leyden phial.

  5. In passing through common water from perfect conductors, it effects chemical changes very analogous to those taking place in the different primary series, in which it was excited; producing in its positive state, oxygen and an acid; and in its negative state, hydrogen and an alkali.

6. General Observations on Galvanic Circles.
  1. Provided those points of contact, in both the simple and compound galvanic circles, in which the chemical agencies of the different conductors on each other are more particularly exerted, remain permanent, the parts of the series, which do not immediately act chemically on each other, may be connected together by means of conductors of their own class, without any other change in the agencies of the circle than a diminution of their intensity. This diminution, with regard to perfect conductors, is barely perceptible; though in the case of imperfect conductors it is evidently, in some measure, correspondent to the increase of the length, or what is apparently equivalent, the diminution of the surface of the chain that they compose.

  2. The agency of the galvanic influence which occasions chemical changes in water, and communicates shocks to the living body, is probably, in some measure, distinct from that agency which produces sparks, and the combustion of bodies. The one appears, all other circumstances being similar, to have {408} little relation to surface in compound circles; but to be great in some unknown proportion as the series are numerous. The intensity of the other seems to be as much connected with the extension of the surfaces of the series, as with their number.

  3. A measure of the intensity of that agency in galvanic batteries, which produces chemical changes in water, may be derived from the quantity of gas it is capable of evolving in a given time; or from the length of the fluid chain through which it can be transmitted. The comparative forces of different batteries may be determined by connecting them in an order different from that of the regular alternation, so as to produce a certain annihilation of power; for in all instances when the most oxydable part of one series is connected, by means of a fluid, with the analogous part of another equal series, the galvanic powers of both are destroyed.

7. Theories concerning Galvanism.
  1. The obscurity of the galvanic facts which were first noticed, and their apparent insulation, gave rise at an early period of the science, to many different theories concerning galvanism. It was supposed, 1. That it was a peculiar ethereal fluid. 2. That it depended upon hydrogen, disengaged during the decomposition of water by metals. 3. That it was the electrical fluid existing in a peculiar state.

  2. Since the discovery of the agencies of the accumulated galvanic influence, the last of these suppositions has been adopted by the greater number of those philosophers who have studied the subject with accuracy. Galvanism is now generally believed to be electricity; and the chief difference that prevails in theory, is with {409} respect to the manner in which this electricity is excited.

  3. M. Volta has supposed, that an electrical current is always produced by the mere contact of certain different conductors of electricity. But many of the British philosophers have denied this position; accounting for galvanism from the destruction of the equilibrium of electricity in galvanic circles, in consequence of the chemical agencies of the different bodies composing them.

8. Of the Appearance of Galvanism in Animal Organs.
  1. The well known facts relating to the torpedo, electrical eel, &c. prove that galvanic electricity is capable of being excited by the agencies of living organs. These facts, compared with the phænomena of the production of muscular contractions by galvanism, lead to interesting inquiries concerning by galvanism, lead to interesting inquiries concerning the relation of this influence to living action. The general connection of electricity with physiology and with chemistry, which is at present involved in obscurity, is probably capable of experimental elucidation; and the knowledge of it would evidently lead to novel views of the philosophy of the imponderable substances.