If the surface of the water on which the camphor swims, is not sufficiently extensive to allow the liquor from the camphor to evaporate, the dissolution of the camphor is either retarded or stops altogether, or the undissipated liquor forms itself into a thin film upon the surface of the water. In like manner, the particles of the saw-dust of wood soaked in oils, moves quickly when they touch the water : but their motions do not continue, because the film of oil which they spread over the water is not dissipated.
If the water is very pure, and is exposed to a heat even so high as that of boiling water, the dissolution and the motions of the camphor are not prevented. On the con trary, they are often promoted by the application of this heat.
M. Venturi applies the preceding principles to the ex planation of the motion of the tremellx observed by Ad anson and by Corti. This aquatic plant rises to the sur face of the water during the day, and descends to the bot tom at night. If the plant is shut up in a box, whose sides are opaque, and if a pencil of light is admitted through an opening in one of its sides, the tremelli changes its situa tion in a few hours, and advances to the hole at which the light penetrates. M. Venturi observed with a microscope, that its branches have a small motion belonging to each of them, in virtue of which they sometimes oscillate from one side to another, and sometimes advance to free themselves from the pencil when they are interlaced. These appear
ances are explained by Venturi on the supposition that the water which it absorbs is decomposed by the assistance of light, and that the plant gives out the oxygen in a direc tion always opposite to the light. Hence it will follow that the plant must always move towards the quarter from which the light is admitted.
In addition to the works quoted under the article CA PILLARY ATTRACTION, the reader is referred to the follow ing : Pascal, Traitez de l'Equilibre de Liqueurs et de la pesanteur de la Masse de ('Air, Avertissement, 2d edit. Paris, 1664. Rohault, Traite de Physique, or Dr Clarke's translation of it, under the title of Rohaulti Physica. Lond. 1710, Part I. chap. xxii. § 69. 70, 71, 80, 81. Boyle', New Experiments Physico-Mechanical,touching the spring of the air, and its Effects, made for the most part in a New Pneu matical Engine, exp. 35. p. 262. Oxford, 1660. Boyle, Phil. Trans. 1676, vol. xi. p. 775. Hooke's Attempt for the explication of the phenomena observable in an experi ment published by the Right Hon. Robert Boyle, in the 35th experiment of his Epistolical Discourse, touching the air, in confirmation of a former conjecture made by R. H. Lond. 1660. Hooke On Springs, 4to. 1678. Vossius, De et aliorunz fluminum origine, Hag. 1666. Fabri, Dialogi Phy