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ther the atmosphere be dry or moift; and inftantly close when certain chemical ftimuli, fuch as the vapour of vol. alkali, or the fumes of burning fulphur, are applied to them. Thefe conclufions were founded on the recollection of experiments which I made more than twenty years ago. But the Abbè Barthalon de St. Lazane, in a late treatife on the electricity of vegetables, has adopted an oppofite hypothefis, and adduced the following trials in fupport of it. When the fenfitive plant, fays he, is touched with a piece of polished metal, terminated at each end by a round knob, its leaves fhrink back and shut. When it is touched with a piece of glass, of the fame form, it remains infenfible. But if this piece of glass be electrified, and the plant be touched with it in this state, the leaves inftantly close themselves. Hence he infers, that the plants called Mimofæ are endued with a much greater portion of electrical fluid than others; that this fluid efcapes when touched by a foreign body, capable of conveying it away; and that they fhrink by being thus deprived of what is effential to their health and vigour.*

I have lately procured a fenfitive plant, with the defign of repeating the Abbè's experiments.

*See Abbè Barthalon de St. Lazane D'Electricité des Vegeteaux: Alfo Appendix to Monthly Review, vol. XVII. P. 135.

But

But at the prefent feafon of the year, I find this vegetable in a very languid ftaté; fo that my trials have not afforded me much fatisfaction. I could not, however, perceive any difference, whether the leaves were touched with a piece of polifhed iron, or a stick of fealing wax. And the following well authenticated facts, feem to refute the Abbè's hypothefis, concerning the electrical œconomy of this plant.

I. The branches of the fenfitive plant have two motions, the one natural, the other artificial. By the first it progreffively increases, in the morning, the angle which it forms with the ftem; and retreats in the fame gradual manner, in the afternoon. By the fecond it contracts its leaves, when forcibly touched or shaken.

II. The fenfibility of the plant feems, chiefly, to refide in the articulation of the branches of the common foot stalk, or of the particular foot ftalk of each wing.

III. No motion enfues from cautiously piercing the branch with a needle, or other sharp inftru

ment.

IV. A ftroke, or an irritation, produces a more forcible effect, than an incifion or even an entire fection.

V. A flight irritation only acts upon the neighbouring parts, and extends its influence according to its force.

VOL. II.

K

VI. Plung

VI. Plunging the plant in water seems to have no other effect, than that of diminishing its vigour.

VII. A piece of wax, ftrongly electrified, made the leaves of the fenfitive plant clofe quickly, by attracting them to it with confiderable force.

VIII. The motions of the fenfitive plant are owing to a strong contraction. Each foot ftalk seems to be terminated with a kind of joint, on which the leaves turn, with furprizing facility.*

NOVEMBER 9, 1784.

• Confult Milnes's Botanical Dictionary; the Encyclopædia Britannica; and Whytt on Vital Motions.

An

An EXPERIMENTAL INQUIRY into the CAUSE of the PERMANENT COLOURS of OPAKE BODIES. By EDWARD HUSSEY DELAVAL, F. R. S. of the ROYAL SOCIETIES of UPSAL, and GOTTINGEN, of the INSTITUTE of BOLOGNA, and of the LITERARY and PHILOSOPHICAL SOCIETY of MANCHESTER. Communicated by Mr. Charles Taylor. Read May 19, 1784.

TH

HE chief defign of this inquiry is, to investigate the nature, origin, and cause, of the permanent colours of opake bodies.

I was led to the purfuit of it, from a perfuafion of its utility, to thofe interefting and elegant Arts, whofe object is the preparation, and use, of colouring fubftances.

The discovery of this principle is the foundation, on which alone all the parts and materials of the knowledge, relative to thofe Arts, can be raised and supported.

It should be the office of experimental philosophy, to examine the powers and properties of all the materials, requifite to technical ufes. Nor fhould its views be confined to the theories, which refult from those researches, but directed to the practical application of them.

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In proportion as the Principles of any Science are unknown or mifconceived, the advancement of the Arts and Manufactures, which depend on them, muft of course be impeded: for, without thofe guides, no addition or improvement can be attained, except fuch as arife from mere accidental obfervation.

But when scientific Principles are disclosed to the Artist, he becomes enabled to draw, from thofe original fources, an ample store of ufeful inventions, by which his Art is conftantly enriched; and from thence confiderable advantages may be derived. And thus the speculative Sciences, by their extenfion to practical purposes, become objects of great public utility.

The truth of this obfervation is, in no instance, more confpicuous, than when applied to the Science of Optics, and to the Arts and Manufactures, which are connected with, and dependent upon it. For, the invention and improvement of the feveral operations, which owe their rife to that Science, appear to have kept an equal pace with the philofophical difcoveries, by which they were fuggefted: and their progrefs feems to have been conftantly retarded, in proportion as the correfpondent branches of the parent Science have remained in a state of deficiency or imperfection.

The experiments and obfervations, which have been made by means of Tranfparent Colourless Substances,

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