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Statement showing the Fall of Rain at Bombay in the last Six Years, measured with Howard's Pluviometer.

Year. June. July. August. Septemb. October. Total.

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An Account of some Experiments with the Prism.
By S. L. Kent, Esq. MGS.

(To the Editor of the Annals of Philosophy.)

DEAR SIR,

Carpenter's Hall, July 16, 1823. IN offering to you the following details of a few simple experiments with the prism, I am not impelled by the belief that they may prove of any practical utility, or serve to throw any new light on the doctrines relating to colours, to which I have given little or no attention myself; they will, however, evince that this instrument affords the means of passing a few hours very agreeably. Leaving to others more conversant with such pursuits to look into them for any instruction they may possibly afford, I cannot however refrain from noticing the following experiment made by Dr. Wollaston, Phil. Trans. 1802, vol. 92, p. 2:

"If a beam of daylight be admitted into a dark room by a crevice 1-20th of an inch broad, and received by the eye at the distance of 10 or 12 feet, through a prism of flint glass, free from veins, held near the eye, the beam is seen to be separated into the four following colours only: red, yellowish-green, blue, and violet."

My seventh experiment, however, tends to the reduction of the prismatic colours into three primary ones, wanting the blue one observed by Dr. Wollaston. I beg to add that I am not aware that any one of these seven experiments have hitherto been made, or described by any other person, and am, Sir, Your humble servant,

S. L. KENT.

P. S. I should add that the prism used in these experiments is five inches long, and the side planes one inch broad; the lens

is six inches in diameter, having a focus of two feet three inches; and I may mention, that I found it requisite that the diameter of the lens should exceed the length of the prism in order to insure a good spectrum.

Exper. 1.-I threw the colours of the prism on a screen, eleven feet distant, and having placed the lens between them, and only two inches from the prism, 1 found the prismatic colours magnified, and in the same order, to the dimension two feet six inches in width, and one foot three inches in depth. In this case the sun's rays were admitted through a Venetian blind; but when admitted through a hole in a shutter of five inches by four, the dimension was only two feet by nine inches.

Exper. 2.-Having placed the lens at the distance of two feet six inches from the prism, the figure of the prism was clearly defined, but without exhibiting any prismatic colours whatever on the screen.

Exper. 3.-I placed the lens three feet from the prism, which produced only the figure of the prism having the violet ray at the bottom, and the yellow above.

Exper. 4.-When the lens was five feet from the prism, the figure of it was distinctly seen with the prismatic colours reversed.

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Exper. 5.I placed the lens behind the prism, and threw the sun's rays on it at its focal distance two feet three inches, when the prismatic colours were increased, both in brilliancy and magnitude, considerably more than in Exper. 1.

Exper. 6.-I put the lens within the focal distance of the screen, when a small figure of the prism was seen very bright, but without any prismatic colour.

Exper. 7.-Having placed the lens as in Exper. 2, when no prismatic rays were produced, but a perfect spectrum of the prism in a strong white light; I then placed another prism in the focus of the lens, and to my surprise it produced three colours only, viz. yellow of a greenish tint, red, and deep violet. Wishing to ascertain if those three colours were neutral, I tried them with a third prism, and found not the slightest alteration; and having placed a card so as to receive them, I found, on giving it a whirling motion, that the colours were entirely lost.

ARTICLE VI.

On the Crystalline Forms of Artificial Salts.
By H. J. Brooke, Esq. FRS.

(Continued from p. 43.)

THE crystallographical characters of natural and artificial productions appear to have received less general attention than the other branches of science connected with mineralogy. I have already alluded to the inadequate descriptions of crystalline forms contained in Dr. Henry's excellent work on Chemistry; and I may refer to another recent and valuable publication which happens to lie before me, Dr. Ure's Dictionary, for abundant evidence of the neglect which the crystallographical character has experienced among chemists of the first rank.

Crystalline forms which are incompatible with each other are frequently quoted in these works as belonging to the same substance; and sometimes those forms are described in terms to which no very definite meaning can be attached; as where Andalusite is said, in Dr. Ure's work, to crystallize occasionally in rectangular four-sided prisms verging on rhomboids.

The crystalline form of morphia is given in Dr. Henry's work, on the authority of three different chemists, as a rectangular prism with a rhomboidal base; as a regular parallelopiped with oblique faces; and as a four-sided rectangular prism; and Dr. Ure quotes the form given by Choulant, as a double four-sided pyramid with square or rectangular bases. The first of these forms is impossible, unless we suppose the base oblique to the axis of the prism, and then it is incompatible with the third and fourth. The second is not very intelligibly described. The last two are not incompatible with that which is given below.

If we inquire into the causes which have occasioned this neg lect of a science, not really difficult in itself, we shall perhaps find that it is owing chiefly to the very profound manner in which it has been treated by the late Abbé Haüy, in whose hands the subject first assumed a strictly scientific form. His complicated analytical operations were probably repulsive to most readers, and so much so, that even in France there are scarcely, as I have been very recently informed by one of his friends, a dozen persons who have followed him in his researches.

Another cause of the little acquaintance which appears generally to exist with even the forms of crystals, may, perhaps, be traced to the nomenclature which the late Abbé established to designate them; by this they were presented to the reader as

independent rather than as related forms, and the mind was thus led away from the consideration of their relations to each other, rather than assisted in comprehending them.

It is probable that the study of crystals will be much assisted by a general series of forms, serving as a type, with which all the crystals of different substances might be readily compared. This series I have attempted to supply in the volume already alluded to, which contains tables of all the modifications of which the simple crystalline forms are susceptible.

The letters placed on the figures which accompany these remarks correspond with those used in the tables here referred to; and by means of these, the reader may trace the relations of all the planes on these figures, to the simple primary form from which they are supposed theoretically to be derived. I have, therefore, omitted, in most instances, to give a figure of the primary form of the substances described.

Morphia.

These crystals are very minute, and have only one cleavage that I can perceive, parallel to the plane h. The primary form is a right rhombic prism, only the lateral planes of which appear on the crystals. For these I am indebted to Mr. R. Howard, of Stratford.

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Tartaric Acid.

The crystals from which this form has been determined, were also given to me by Mr. R. Howard. I have not succeeded in cleaving them, but the primary form is an oblique rhombic prism. Fig. 1 exhibits the crystal as usually modified, with the planes symmetrically placed. Fig. 2 exhibits the same modified form, with the planes irregularly disposed as they appear in most of the crystals, the corresponding planes in both being marked with the same letters. This affords another instance of irregularity, which renders it not easy immediately to perceive the relations of the several planes to each other.

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