The speaker remarked, that his object was to bring before his audience the principles and mode of action of a method employed for the measurement of the chemical action of light.*

As an illustration of the chemical action of light, attention was directed to the fact, that when a perfectly pure mixture of exactly equal volumes of chlorine and hydrogen gases is exposed to light, the gases combine, producing an equal volume of hydrochloric acid gas, whilst no such combination occurs in the dark. This combination may occur gradually, or with great rapidity. If the chemical activity of the light be great, the union takes place quickly, great heat is evolved, a sudden expansion takes place, and the vessel containing the mixture of chlorine and hydrogen is shattered by the explosion. The gradual or slow combination may be rendered evident by allowing the hydrochloric acid thus formed to be absorbed by water; the consequent diminution of bulk of the gas accurately representing the chemical action effected.

This mixture of equal volumes of chlorine and hydrogen is used as the sensitive substance for measuring the chemical action of light. It is evolved in the perfectly pure state by the electrolytic decomposition of strong aqueous hydrochloric acid; and it is by this method only, that it can be prepared. The gases thus evolved are in the exact proportion in which they exist in hydrochloric acid; so that, if by any means, we re-combine these gases, no trace of either substance will remain behind, the whole uniting to form hydrochloric acid.

For the purpose of measuring this chemical action, effected, not only by solar light, but also by light from many artificial sources, we require some instrument, which is to the chemical action of light what the thermometer is to the heat actions; an instrument which will show objectively the amount of chemically active light. We must be sure, in the first place, that our mode of measurement is a reliable one. That, as in the case of the thermometer, equal increments of volume, correspond to equal increments of heat, so, in the new instrument, the indications, however obtained, shall be proportional to, and represent the amount of chemical rays emanating from, any source.

This has been accomplished in the chemical photometer; by the help of which an accurate measurement of the chemical action of light is effected.

The facts upon which this mode of measurement is based, may be summed up as follows:

1. Exactly equal volumes of chlorine and hydrogen gases, when mixed, combine together on exposure to light, forming hydrochloric acid gas.

2. This combination does not occur in the dark.

* For a detailed description of apparatus, &c. see “ Photochemical Researches," Part 1. "Measurement of the Chemical Action of Light," by R. Bunsen and H. E. Roscoe.-Phil. Trans. 1857, p. 355.

3. The quantity of hydrochloric acid thus formed is directly pro-
portional to the intensity of the incident light, and serves,
therefore, as a measure of the chemical action produced.
4. The chemical photometer is an instrument, by help of which
the quantity of hydrochloric acid thus formed, can be
accurately measured.

The chemical photometer consists essentially of three parts; namely, first, the apparatus in which the sensitive gas is generated; secondly, the apparatus in which the gas is exposed to the light; and thirdly, the apparatus in which the volume of hydrochloric acid produced in a given time is read off.

When very numerous precautions in the management of the photometer are taken, it proves a most sensitive and reliable instrument. Having thus obtained an instrument by which the chemical action of light can be accurately measured, it only remains to graduate it. For this purpose we require a standard of light, from which the determination is to proceed. For this comparative measurement, the possession of a constant source of light is the first essential. This is obtained as follows:

:

1. A flame of pure carbonic oxide gas, burning in the air and issuing from an opening of given size at a given rate, is employed as the standard flame.

2. The unit amount of chemical action, is that effected by such a flame upon the sensitive mixture of chlorine and hydrogen during one minute, at the distance of one metre.

3. The quantity of chemically active light producing this action is called one chemical unit of light; and ten thousand of such units one chemical degree of light.

4. The chemical photometer is graduated by observing how many of these chemical units of light correspond to one division on the scale of the instrument.

As an illustration of the mode in which this measurement of the chemical action of light is employed, the speaker described the method by which the chemical action produced by the direct solar rays has been determined.* For this purpose, it was necessary to admit a very small, but a known, portion of direct sun-light into the dark room in which the instrument was placed, and to allow the insolation vessel to be bathed in the pencil of rays thus admitted. By help of Silbermann's heliostate, the sun's image was reflected during the whole day upon one spot, a small opening of known size, in the window shutter of a dark room. The fraction of the total sun's rays thus admitted and allowed to fall upon the chemical photometer can be calculated, and the action thus effected, observed; hence the amount

*The full memoir on this subject is to be found in Poggendorff's Annal. Bd. CVIII. p. 193. In Abstract, Proceedings Royal Society, Vol. x. p. 39, 1859. Photochemical Researches, Part 4, by R. Bunsen and H. E. Roscoe.

of action can be found which the sun would have produced if directly shining upon the instrument; a condition, impossible of course to fulfil, as the action would become too rapid and the whole apparatus would be shattered by explosion.

The day chosen for observation of the sun's action must obviously be cloudless, if we wish to obtain an idea of the relation existing between the chemical action and the height of the sun. Beginning the observations as near sunrise as possible, we find, for instance, on September 15th, 1858, one of the days on which such a series of experiments was made, that at 7b. 9m. a.m., when the sun's zenith distance was 76o 30', the observed action amounted to 1.52. That is, in one minute the column of water moved through 1 52 division; or the quantity of hydrochloric acid formed, when the sun stood at the height mentioned, was represented by 1.52 division on the scale.

Gradually, as the day wore on, the observed action for each minute became larger; until at 9h 14m. a.m., the latest observation possible on the day in question, owing to the formation of clouds, the action reached 18 5 divisions, or was thirteen times as large as at 7 9. In the last column of the accompanying table is found the action, expressed in degrees of light, which would have been observed at the foregoing times, if the whole sunlight had been allowed to fall on the instrument.

This great increase in the chemical action with the rise of the sun in the heavens simply results from the fact that the solar rays, in passing through the air, are extinguished or absorbed, lost in fact as light; and that as the sun rises higher above the horizon, the column of air through which the rays pass is constantly being lessened; consequently more of the direct rays reach the earth.

Now, the law according to which the direct rays of the sun are thus absorbed in the air can be obtained from the experiments, of which the foregoing is only an example; hence, if the action which the sun produces, when at a given height, is known, it is possible to calculate the action which it would produce at any other height.

VOL. III. (No. 31.)

Q

That these calculated results agree very closely with the experimental data, with the observed action,-is seen by comparing the numbers in Table, No. II., expressing the observed and calculated action.

TABLE II.

The amount of Chemical Action effected at a point upon the Earth's Surface on any cloudless day, by the direct Solar Rays, depends alone upon the Sun's zenith distance; or upon the height of the column of air through which the Rays have to pass.

Knowing the law which regulates the absorption of the chemical rays, we can calculate what the action would be if there were no atmosphere to diminish the power of the rays. It is thus found that if the sun's rays were not thus weakened, by passage through the atmosphere, they would produce an illumination represented by 318 degrees of light or they would effect a combination in one minute, upon an unlimited atmosphere of chlorine and hydrogen on which they fell perpendicularly, of a column of hydrochloric acid, 35 3 metres in height. The sun's rays having passed perpendicularly through our atmosphere to the sea's level, effect an action of only 14.4 light metres; or nearly two-thirds of their chemical activity has been lost by extinction and dispersion in the atmosphere.

A large number of most interesting conclusions may be drawn from the facts already noticed. Thus, for instance, we may determine the chemical action which the solar rays will produce on the various planets; for we know that the intensity of the chemical illumination varies inversely as the square of the distance of the planet from the sun. The numbers in Table III., express this chemical action in degrees of light, and in heights of columns of hydrochloric acid called

light metres. Hence, we see how much the sun's chemical action varies on the different planets; the superior planets receiving so small a portion as to render it impossible that the kind of animal and vegetable life which we here enjoy can there exist.

TABLE III.-Chemical Action produced by Direct Sunlight on

each Planet.

Interesting conclusions can be drawn from these facts, concerning the distribution of the chemical rays on the surface of our earth in different latitudes, and at different elevations above the sea's level. The farther removed a situation is from the level of the sea, the higher up in the atmosphere it is placed, the greater amount of chemical action it will receive. Thus, in the highlands of Thibet, where corn and grain flourish at a height of from 12,000 to 14,000 feet, the chemical action of the direct sunlight is 1 times as great as in the neighbouring lowland plains of Hindostan. In the same way we can calculate for any point of the earth's surface whose latitude is known, the amount of chemical action which the direct sunlight effects at any given time of day or year. In Table IV. the numbers represent the chemical

TABLE IV.- Chemical Action effected by Direct Sunlight in One Minute on the Vernal Equinox at

0.67

6.62 10.74 18.71 23.99 35.88 1.86 13.27 20.26 32.91 40.94 58.46 3.02 18.60 27.55 43.34 53.19 74.37 3.51 20.60 30.26 47.15 57.62 80.07 105.3

50.01

78.61

98.33