I have heated several of my specimens of itacolumite to ascertain whether any petroleum odor was evolved, but with negative results. If the diamond proceeded from a slow and gradual oxydation of the hydrocarbon perhaps we should not expect to find any petroleum left.

In this connection the small and rarely occurring black specks, seen with the microscope, are to be noted; are they minute black diamonds, and have they any relation to the experiment where the agate mortar was so deeply scratched?

Bethlehem, Pennsylvania, April 6.

ART. VIII. On the Laws which govern the general distribution of Heat over the Earth;* and on Brewster's Neutral Point; by PLINY EARLE CHASE.

1. The laws which govern the distribution of heat.

THE principal elements of general thermometric variation are: 1, the heat imparted by the sun; 2, terrestrial absorption and radiation; 3, atmospheric currents. Of these three agencies, the first is, in one sense at least, the chief, since it is the one on which the others depend; the second is mainly instrumental in modifying the other two, and especially in retarding the daily and yearly changes; the third is a subject of hourly experience, and its meteorological importance is now generally recognized.

The amount of heat which is received directly from the sun, evidently varies as the cosine of the zenith distance, or the sine of the sun's altitude. In the daily distribution of temperature this is the most important element, as is evident from the tabular comparisons in my communication of Sept. 21, 1866.† Absorption and radiation proceed at nearly uniform rates, therefore it may be assumed that their effects are approximately proportional to the time during which they operate. The average gen. eral variation which is due to the influence of the winds is a difficult point to determine, but the present investigation has led me to believe that it may be measured by the difference of arc (instead of the sine-difference) of the sun's meridian altitude. My reasons for this inference are the following: 1, the general average temperature of the year often appears to vary very nearly as the arc in question; 2, it seems unreasonable to suppose that a variation of this character ean be attributable either to the heat communicated by the sun or to terrestrial absorption and radiation; 3, the tendency of the air, so far as it is deter

Abridged from the Proceedings of the Am. Philosophical Society, Feb. 1, 1867. Proc. Amer. Phil. Soc., vol. x, pp. 261-269. See, especially, the observations at St. Bernard, and the general average of Table I, p. 267.

mined by the direct heat of the sun, is at all times toward that point of the earth's surface at which the sun is vertical, and we may readily believe that that tendency should be proportional to the distance, measured on a great circle of the earth, through which the air would be obliged to move in order to reach the sub-solar point. This distance evidently varies as the arc of the sun's zenith-distance.

We have, then, three natural standards for admeasurement, by means of which, if we rightly eliminate special and limited perturbations, we may perhaps be able to determine the predominating influence, in many cases both of local and of general thermal disturbance. In order to institute as broad a comparison as possible, I have adopted a method of elimination which may be illustrated by a single example.

The average monthly temperatures of the United States, as deduced from Prof. Coffin's reductions, appear to be as follows: Jan. 28.352 Feb. 30.873 Mar. 39-049 Apr. 49-744 May 60-902 June 69-780 July 75-640 Aug. 71-754 Sept. 65:643 Oct. 53-922 Nov. 42-350 Dec. 32-132 Averaging the temperature at equal intervals from January (taking the mean temperature of Dec. and Feb., of Nov. and March, &c.), we get the following results.

The second of the above series of ratios (that of the differences in the arcs of the sun's zenith-distance) is based upon the following estimate of the average monthly increase of solar altitude at all places in the temperate zones.

If we allow about 24 days for the cumulative effects of increasing heat and cold, these ratios become properly comparable with the monthly ratios of temperature-variation, as in the following table, which is compiled from the works of Dove and Guyot.

An extensive series of comparisons* seems to warrant the following inferences, all of which are confirmed by other considerations.

1. Taking into view the entire land surface of the globe and the entire range of the year, the direct heat of the sun and the induced aerial currents appear to be about equally instrumental in determining fluctuations of temperature.

2. The influence of the winds is most marked in the Northern and Western hemispheres; that of solar obliquity, in the Southern and Eastern hemispheres.

3. Where the sun's rays are least intense (as in the Polar Regions) and where the winds are most variable, the ratios exhibit the nearest parallelism to the increments of arc; but where the winds are most uniform (in and near the region of monsoons), they correspond more closely with the sinal increments.

4. The general changes of temperature at midwinter, and at the equinoctial seasons (when the sun's declination is changing most rapidly), are most dependent upon the local solar heat; the midsummer changes are more subject to the influence of the winds.

5. The greatest conflict of opposing forces occurs during the sun's passage between the comparatively wind-governed Northern hemisphere and the sun-governed Southern hemisphere. This conflict is manifested in the spring and autumn rains.

6. The closest and most general approximation of ratios is shown in the monthly temperature change at midsummer, which corresponds almost precisely with the change of arc.

2. On Brewster's Neutral Point.

In the April number of the Philosophical Magazine, Sir David Brewster says: "Dr. Rubenson has never been able to see, even under the fine sky of Italy, the neutral point which I discovered under the sun, and which, I believe, has never been seen by any other observer than Mr. Babinet."

The point is question can be easily seen in Philadelphia on any clear day, when the sun is more than 20° above the horizon, and I have reason to believe that it can be found with equal ease at many other places in the United States, although I have not been able to find any published observations except my own.†

As all the phenomena of skylight polarization are very interesting, and as some of its laws are still imperfectly understood, others may, perhaps, be induced to turn their attention in this direction, so as to determine whether the difficulty experienced by European observers is owing to a higher latitude, to a moister atmosphere, or to some other cause.

* See Proceedings, &c., loc. cit.

† Proc. Amer. Phil. Soc., vol. x; this Journal, vol. xlii, pp. 111, 112.

A simple Savart polariscope is sufficient for making the observations. In positing Brewster's neutral point, I have usually raised the lower sash of an attic window so that the bottom of the sash will screen the sun from the polariscope. I have thus been able, in every instance when the atmospheric conditions seemed favorable, to see very distinctly the neutral point, and the oppositely polarized bands above and below.

ART. IX.—Contributions toward a Theory of Photo-chemistry; by M. CAREY LEA.

IN a somewhat extended series of experiments published at various times,* I endeavored to fix, as far as I was able, some of the facts of photo-chemistry, and more especially the nature of the action of light upon iodid of silver, at once the most important and the most difficult of explanation of all the facts of photo-chemistry which fall under our notice. The phenomena exhibited by iodid of silver, in the point of view which they assume to me, are the key to the whole matter, and based upon them, I propose to offer some theoretical views upon the general subject.

The study of light has always been largely aided by analogical reasoning from another source-that of sound, whose phenomena probably afforded the first conception of the undulatory theory, and in turn, discoveries made in light have aided our knowledge of the phenomena of heat, many of which would perhaps have been still unknown but for the aid so obtained. It is therefore perfectly allowable to reason analogically back from heat to light.

The tendency of heat is always to equalize itself, by radiation and conduction. The loss of heat in this way where the body affected is much above the temperature of those that surround it is enormously rapid, and this loss continues with diminishing rapidity till an equilibrium is attained.

The same is the case with light, though the loss is there usually so much more rapid as to be almost simultaneous with the reception, to our senses, and in the ordinary conditions of observation it is quite so. But the exceptions are perfectly well marked. The phenomena of phosphorescence show that a body may retain the impression of light for a considerable time.. And the phenomena of phosphorescence received an immense extension from the ingenious and beautiful experiments of Becquerel,

* A brief résumé of many of these experiments was published in this Journal in the year 1866.

who showed that a very large number of bodies continued to emit light for an appreciable time after the direct influence of light ceased to operate upon them. Although the time might be but a very small fraction of a second, still it was rendered brilliantly evident to the sense, and the exact period could be measured. And when we consider the enormous rapidity with which the phenomena of light take place, even the fraction of a second is a long time, and it would be exceedingly rash to attempt to limit such phenomena to our powers of observation.

Just as with heat there exists in all probability an absolute zero at which heat-vibrations cease, so probably there is a light zero at which the body ceases to vibrate luminously. Most bodies (to our perception) reach this zero immediately when carried into darkness. Phosphorescent bodies, form, however, a striking exception.

Let us suppose that a body be surrounded by other bodies equally illuminated, and that temporarily an additional quantity of light falls upon it. On the cessation of this illumination, the body will recover its condition of equilibrium with surrounding bodies, by losing its excess of light in the following manner: 1. By reflection. 2. By transmission. 3. By conversion into heat. 4. By chemical action. 5. By radiation.

That is, the body, if it be transparent, or have reflecting surfaces, will part with a certain quantity of its light in those ways. If it is susceptible of chemical decomposition, a certain portion of light will be consumed in effecting that decomposition. And what farther loss is necessary to take place in order to reach an equilibrium, must take place by conversion into heat and by radiation. As we have already seen, this radiation may be either instantaneous, as in the case of most bodies, or it may require minutes, hours, or even days, as in the case of phosphorescent bodies. This fact is of the utmost importance in the attempt I here make to explain the phenomena of photo-chemistry.

In their influence upon combustion and decomposition, the phenomena of light and heat exhibit a striking parallelism. Each tends in some cases, to promote combination, but in the vast majority of cases, to dissociate elements already combined. Such especially is the action of light in the cases which I propose to consider.

I have shown elsewhere, that, contrary to long-established opinion, perfectly pure iodid of silver, isolated from all other substances, is sensitive to light, and this fact, now I believe universally admitted, must form the corner-stone of photo-chemistry. For iodid of silver is precisely the only substance fitted to give us a clear view into the action of light upon matter in general, by which I mean that this action is so much more evi