Page:Proceedings of the Royal Society of London Vol 1.djvu/238

Rh in alcohol, a partial solution is formed, leaving an elastic substance, which is said to possess the properties of caoutchouc, but which be- comes hard by exposure to the air.

The author has remarked, that the portion of mastic dissolved in the alcohol may be precipitated from it by water, and that this pre- cipitate possesses the properties of a pure resin; but when a stream of oxymurintic acid gas was passed through the solution, a tough elastic substance was thrown down, which became brittle when dry: this precipitate was soluble in boiling alcohol, but separated from it as the solution became cool. Its properties, therefore, approached in some measure to those of the original insoluble part.

It is, Mr. Knight observes, very well known, that in whatever position a seed is placed to germinate, its radicle always makes an eﬁ'ort to descend towards the centre of the earth, whilst the elon- gated germen takes a precisely opposite direction : and it has Men proved by Du Hamel, that if a seed, during its germination, be fre- quently inverted, the points, both of the radicle and germen, vu'll return to their ﬁrst direction. These opposite eﬁ‘ects have, by some naturalists, been attributed to gravitation; and Mr. Knight con- ceived, that if they really proceeded from that cause, those effects would take place only whilst the seed remained at rest, in the same position with respect to the attraction of the earth, and that the operation of gravitation would be suspended by a constant and rapid change of position in the germinating seed, and might be counter- acted by the agency of centrifugal force. In order to determine how far the above opinion was well founded, he made the following experiments :'—-

Having a strong rill of water passing through his garden, he con- trived, by its means, to give motion, vertically, to a wheel of eleven inches diameter. Round the circumference of this wheel, several seeds of the garden-bean, which had been previously soaked in water, were bound in such a manner that their radicles were made to point in every direction. The wheel made rather more than 150 revolu- tions in a minute.

In a few days the seeds began to germinate, and Mr. Knight had the pleasure to see that. the radicles, in whatever direction they were protruded, turned their points outwards from the circumference of the wheel, and in their subsequent growth receded still further from it. The germens, on the contrary, took the opposite direction; and in a few days their points met at the centre of the wheel. Three of these plants were suﬂ'ered to remain on the wheel; their stems soon ex- tended beyond its centre, but their points returned, and met again at the centre.