colour as Sierra Leone copal, and hardly so dark as that known as Zanzibar copal. This difference in colour may be one of the distinctions between the fossil and recent resins, that exposed on the trunk, being in a measure bleached by the action of light and air. From the materials sent to Kew by Lieut.-Col. Playfair, a year or two since, it seemed tolerably clear that Zanzibar copal was the produce of Trachylobium Mossambicense, Kl., and Dr. Kirk, the present Vice-Consul in Zanzibar, has confirmed this in a recent letter. His conclusions have been arrived at from a careful examination of the plants, their position of growth, their res nous properties, and the care with which the resin is collected and sorted. These notes of Dr. Kirk are exceedingly interesting, and were made during a holiday trip on the African mainland, opposite the southern end of the island of Zanzibar, where a large creek offers a naturally formed harbour. The vegetation of the sea-shore is composed of Rhizophoras, Brugueiras, Avicennias, Sonneratias, etc. This characteristic tropical vegetation, forms a belt or line behind which, and within a few paces of the water's edge the ground suddenly rises to a height of about thirty feet, from whence a flat plain extends inland for a distance of about twenty miles. A section of this would show a formation of sand, gravel, vegetable mould, and clay, and occasionally beds of water-worn pebbles.

The vegetation of the neighbourhood of this creek of Dar Salam is very varied, and among other hard, or heavy-wooded trees and bushes, the Trachylobium Mossambicense is conspicuous from its white flowers, and rounded head of glossy leaves. This tree is known to the natives as the M'Sandarusi, or tree of copal. Dr. Kirk says, "On examining the tree more closely, the trunk and main limbs. were seen to be covered with the clear resinous exudation now brittle and hard; from the upper branches it dropped down on the ground below, but not in a fluid state. To judge by the appearance it presented, I should say that the resin soon dries and hardens, after being exuded, but must be easily broken off by violence. Pieces of various tint and form were collected, some with insects embedded, but all presented a smooth polished exterior, quite free of any pitting, or "goose-skin," known on all kinds dug up from the ground. This sort is known in trade as Sandarusi za m'ti, or copal from the tree. It is exported in considerable quantity to India, but not to Europe."

Dr. Kirk further believes that this same tree, the Trachylobium Mossambicense, is the source of the true Zanzibar copal, the halffossilized resin known in English commerce as anime, and which is

one of the most valuable sorts for varnish making imported. In the copal trade at Zanzibar alone, the merchants have three distinct kinds, which they again subdivide into classes according to the colour, form, surface, etc., for copal has many peculiarities known only to those who trade in the article, though sometimes these peculiarities affect the quality and price of the resin. One sort, the Sandarusi za m'ti, or tree copal, which we have referred to, Dr. Kirk thinks never enters European commerce. It is gathered from the trunk of the Trachylobium, which grows all along the coast from Mozambique to near Lamo, or from 3° to 15° south lat. It, however, occurs in greatest abundance between Cape Delgado and Mombas, "but becomes very rare," we are told, "at a little distance inland, and quite unknown long before the change in geologic structure offers an explanation for its absence." The sea-shore appears to offer advantages for its growth, as the plant will not flourish far inland. Another kind of copal is known to the merchants as Chakazzi, which has been by some corrupted into "jackass." This resin, unlike the last, is dug from the ground, but is apparently of comparatively modern deposit, being found either near the roots of living trees, or in that part of the country where the trees at present exist. It is, however, said that the source of this quality of resin is found inland in extinct forests. This statement Dr. Kirk receives with doubt, inasmuch as the traders constantly mix this with the more valuable produce of the interior. The third sort, known as the true Sandarusi, fetches the highest price, and is the true copal, or anime of the English markets. It forms the bulk of the Zanzibar copal, and is dug from the soil in extinct forests farther inland than where the tree is now seen. These forests extend "all along the ancient sea-beach; the maritime plain, which here fringes the continent to a depth of twenty to forty miles in general. Some spots are richer than others, and soils indicate good diggings.' When the rains which follow the north-east monsoon have softened the soil, the natives of the country commence to dig. They form small pits, searching the soil as removed, but there is no system; and, like the gold washings of Africa, so the copal regions yield not a fraction of what a little system and industry might produce. At present every clan-feud stops the search. The producer receives, even when successful, only a trifle from the Indian merchants, who again part with it, after paying enormous dues to the Zanzibar state, to the European and American traders." The quantity of copal exported from Zanzibar has been known to amount, in some years, to 800,000 lbs., valued at

£60,000. The supply, however, of the best kind, namely, the fossil resin, is unlimited. The extent over which it is spread is so considerable that almost any quantity might be collected with a little system, and if more inducement to work was held out to the labourers.

The conclusions to be deduced from Dr. Kirk's remarks are these:-That Zanzibar exports three distinct kinds of copal, one of which is always procured from the trunks of the living trees, and is, therefore, never seen in a fossilized state, and that this resin is never exported to Europe, the other two kinds alone finding their way into English commerce. The first of hese two, known in the trade as Chakazzi, though always dug from the soil, is of modern formation, as it is always found in the neighbourhood of existing trees, and though slightly marked with the impress of the sand, or goose-skin," it has evidently lain only just long enough to receive it. The second is the most valuable of all the kinds of copal known in commerce. It is harder and less soluble than any other sort, yet it produces a very elastic and brilliant varnish, and this sort is found only in extinct forests. We will, however, give Dr. Kirk's conclusions in his own words :-"If we take into account the similarity of recent and fossil resins in appearance, their near approach in physical property, the fact that the recent gum, after being imbedded in sand, takes the characteristic surface markings, and recollecting that where now the good copal is dug as a fossil, the present copal tree, in all probability, once grew, when the sea was nearer to the hills than now. I think we may be satisfied that the present Trachylobium was the source of the old copal, which is the resin only modified by time and long exclusion from air and light under the ground."

The fact of insects, and other substances, being found in copal, whether recent or fossil, is easily accounted for. The tenacity of the resin, when in a semi-fluid state, readily entraps all bodies coming in contact with it, and so with the continued flow and the quickness with which it hardens, insects, and other substances, are easily embalmed, and considering that the resin flows from the underside of the principal branches of the tree, the fruits, flowers, or twigs of the undergrowth, or lower vegetation, would be likely to be caught by the exuding resin, and so preserved.

Dr. Kirk points this out as a very probable reason why some portions of the plant of the Trachylobium itself is never found embalmed, and adds that the heavy glossy foliage would be less likely to adhere than the foliage and twigs of the smaller growing

surrounding vegetation. From Dr. Kirk's letter, together with the fine series of specimens recently received from him at the Kew Museum, much light is thrown on the origin of the East African copals, and we hope further researches will entirely clear up this, one of the many tangled knots in Economic Botany.

VELOCITY OF CEREBRAL FUNCTIONS.

THE " Archives des Sciences" for April 15, contains a paper by Dr. Adolph Hirsch, on M. F. C. Donders's experiments to determine "the velocity of the psychical functions of the brain," as detailed in the "Archives de Reichert et du Bois Raymond," and we extract from it the following passages :

"We know now that the brain requires about fifty-thousandths of a second to distinguish and signalize the distinction between two colours, and only fifty-thousandths of a second to distinguish between two vowels which are pronounced. What is more, M. Donders has succeeded in separating these two psychical acts into their components, and he has found that the brain employs about th of a second to recognize an impression, andth of a second for an act of volition to signalize that the impression has been received. With regard to the rapidity of perception in cases of hearing, sight, and touch, and the duration of the functions of the cerebral organ, M. Hirsch observes, I endeavoured to reply to the first of these questions in 1861, and the results which I obtained for the physiological times of the different sensations have been since confirmed by eminent physiologists, amongst them M. Donders, who gives as the mean of his experiments for touch, th of a second, for hearing, th, and for seeing, th. But this physiological time, as I have named the interval between the excitation, and the signal given by the manifestation of perception, comprehends a greater number-M. Donders has enumerated not less than a dozen-of acts, and divers functions of the senses, the peripheral ganglia, the nerves, brain, muscles, etc., almost all have to be accomplished in this small fraction of a second. It was important to separate as far as possible these different acts, and especially to fix the time employed in the functions of the brain for which only a maximum limit of th of a second was known, obtained by deducting from the total physiological time, the portion employed in transmission by sensor and motary nerves. But what was the minimum limit ?"

M. Donders conceived the happy thought of intercalating in the series of functions comprised in the physiologic time, certain fresh terms of purely psychical action; and this retardation evidently due to the intercalation of a new act of the brain, has made us acquainted with the duration of the latter.

M. Hirsch thus describes M. Donders's apparatus. The noematachograph is composed of a cylinder somewhat like that of the phonautograph, on which time is registered by means of a diapason making 261 vibrations in a second, and moved by electromagnetism, on the principle proposed by Helmholtz. These vibrations can be divided into fifths, and thus thousandths of a second obtained. The time at which the action which produces a sensation occurs, is registered by the machine, and likewise that of the sensation experienced by the power experimented upon.

"The mode of accomplishing this varies according to the means of excitation employed. When an inductive current is used to give a slight shock or prick to any portion of the body, or to light up suddenly different letters, or when the spark is observed through coloured glasses to produce the sensation of different colours, the current itself makes its own registery by a spark passing between the style of the diapason and the cylinder, through a sheet of blackened paper, in which it makes a little hole. The observer registers his perception by touching a key, which causes a style to mark the cylinder. To avoid the error introduced by the variable time taken by electro-magnets in attracting their armature, M. Donders prefers a purely mechanical signal. The person under observation turns aside a horizontal bar of wood carrying a point, which marks the cylinder. By holding this indicator between two fingers and turning it right or left, two signals can be given to express different sensations.

"In experiments on hearing, the sound produced by a spring striking a pin springing from the cylinder, or by a diapason put into sudden motion, or by the human voice, is registered by the phonautograph, or by a modification of König's stethescope, over which an elastic membrane is stretched communicating by two caoutchouc tubes with two embouchures. One of these serves to transmit the sound which is to be perceived, and by the other the patient reproduces the sound he hears, so that the phonautograph registers both at the same time below the chronoscopic line of the diapason. Acting alternately upon the same excitation, by the hand and the voice, we can determine and eliminate the difference of time in the two kinds of signalling."