It is obvious from an examination of the preceding table, that thunder storms prevail most frequently in the equatorial regions and diminish as we proceed towards the poles. It is also evident that the frequency of thunder showers depends upon other circumstances than simply latitude; but without stopping to enquire what these circumstances are, we will take the average of the observations included between different parallels of latitude. We thus find that:

Maury's storm and rain charts of the Atlantic Ocean furnish most important information on the same subject. The following table presents a summary of the results of these charts. The ocean is divided into squares by parallels of latitude drawn at intervals of five degrees from each other, and meridians of longitude at intervals of five degrees. Each square of the following table contains three numbers. The first shows the number of days of observation within the given square; the second shows the number of days of lightning reported, and the third is the number of days of lightning which would occur in one year, as computed from the numbers actually observed. Thus in the square included between the parallels of 30 and 35 degrees of north latitude, and between the meridians of 65° and 70°, the first number is 548, which shows that 1644 observations have been obtained in that square. Each observation represents a period of 8 hours, so that 1644 observations represent 548 days. During this period, lightning was reported on 44 days, which is at the rate of 29 cases for one year.

SECOND SERIES, VOL. XXX, No. 88.—JULY, 1860.

55

50

40

30

25

We see in this table abundant evidence that the frequency of lightning depends upon other circumstances than simply latitude. Throughout the western half of the Atlantic Ocean, lightning occurs three times as frequently as throughout the eastern half of that ocean. If we take the average of all the observations upon the same parallel of latitude, we shall find the number of days of lightning to be as follows:

North Atlantic.

South Atlantic.

20

5

10

17

The average frequency of lightning in the North Atlantic is two and a half times as great as in the South Atlantic; also the average frequency of lightning on land, at least in the northern hemisphere, appears to be more than twice as great as in the North Atlantic. The bearing of the preceding facts upon the theory of auroras will be considered hereafter.

ART. XII.-On the Products of the Distillation of Common Rosin; by JAMES SCHIEL, of St. Louis.

THE chemical process taking place in the distillation of common rosin or colophony, and the two different liquids which are obtained by that distillation, have as yet escaped the attention of chemists; the whole process is almost completely wrapped in darkness. The two liquids just mentioned are distinguished as essence of rosin (in Europe "German oil of turpentine") and rosin oil; the former is thin, of a light yellow color, and a strong, almost aromatic odor; the latter is a thick liquid of a somewhat disagreeable odor.

The essence of rosin is a mixture of two substances, which may be separated by fractional distillation if often repeated. During this distillation the liquid passing over sometimes assumes a milky aspect, produced by the formation of a small portion of water and a trace of acetic acid; a small piece of lime or baryta instantly renders the liquid perfectly transparent. two liquids into which the essence of rosin separates, the first is very thin, perfectly colorless and transparent, and of a strong, refracting power. I call it colophonon. The specific gravity of

Of the

colophonon is 0.84 at 14° C., its boiling point 97° C. (Barom. 756); with concentrated sulphuric acid it forms a brown liquid from which water separates a green oil, having much the smell of Ol. anthos. Chlorhydric acid produces a similar transformation. With potassium it gives rise to a lively emission of gas and coagulates into a brown-yellow mass. Heated in a closed vessel above the boiling point it colors brown and assumes an odor of peppermint.

The composition of colophonon is expressed by the formula €11H1802,

The determination of the specific gravity of (vapor of) colophonon gave the number 5.1, which however I consider merely as an approximation. It is remarkable, that this substance, whose composition is distinguished from that of phoron by ¤¿¤ ̧ should have a boiling point as low.

The second constituent of the essence of rosin has its boiling point at 160° C., and possesses all the chemical properties of oil of turpentine. The analysis of it showed it to be composed of

As it does not seem to have any effect on the plane of polarization, it may be identified with the therebène of Deville.

The oil of rosin, with a boiling point above that of mercury, does not seem to have a constant composition; at least, there is sometimes a difference between the raw oil and the refined oil. The raw oil, which has the property of fluorescence, loses this property in a great degree by a very simple refining process. This process consists simply in placing the earthen vessels containing the raw oil on the hot wall around the neck of the iron retort, from which the rosin is distilled, and allowing them to stand there for some hours, keeping them covered up. A portion of the raw oil that had been heated in the water-bath in contact with caustic lime showed the composition €20H2802 of the resineine of Deville and Frémy, viz:

Carbon,
Hydrogen,

Oxygen,

Calculated.

Found.