"The thread of the garden spider is so much finer than that of the silk-worm, that the united strength of five of the former is, according to Mr. Rolt, equal only to one of the latter; and, assuming that the weight is in proportion to the strength, and that a spider will yield twice a-year a thread 750 feet long, while that produced by a single silk-worm is 1,900 feet, it follows that the produce of one silk-worm is equal to that of 6.3 spiders. Now, as on an average it takes about 3,500 silk-worms to produce a pound of silk, it would take about 22,000 spiders to produce an equal quantity. Besides, spiders are not so easily confined as silk-worms, and whenever two come in contact, a battle ensues, which ends in the destruction of the weaker one. Spiders kept for silk must, therefore, be each in separate dens or cells; and the apparatus contrived by Mr. Rolt for this purpose, although very ingenious and well adapted to carry on a course of experiments with a hundred or two, would manifestly be wholly inapplicable to any purpose of commercial utility. Mr Rolt has, however, made some interesting additions to the history of the garden spider, and has obtained the silk in its natural state, exhibiting all its peculiar lustre; his method, likewise, of winding the silk directly from the animal is, to say the least of it, effectual and ingenious." It should be added that "the Society are of opinion that it will never be possible to employ spiders' silk, beautiful as it is, in any profitable manufacture; but have thought proper to confer a reward on this gentleman for his communication, as forming an interesting addition to the natural history of the spider." _Medicinal Employment of Holly Leaves._ Dr. Rousseau has obtained from the Medico-Botanical Society of London its silver Medal, for an essay on the effects of holly leaves in fever: he has cured several intermittent fevers by the remedy, whose alkali he calls Ilicine.--_Gazette of Health._ _Tobacco no security against Cholera._ M. Chevalier proved, from documents, that the assertion made on a former evening, that tobacco was a preservative against cholera, was erroneous. He stated that twenty-seven mechanics employed in the tobacco manufactories had died of the disease.--Ibid. _Prussic Acid a Poison to Vegetables._ The sensitive plant, when exposed to the vapour of prussic acid, instantly closes its leaves. The same plant, as well as other tender plants, such as the garden pea and kidney bean, when subject to the influence of this acid, quickly wither and die, and the laurel-water has the same effect upon them. It appears also that plants which naturally contain the acid, such as the cherry-laurel and almond tree, are not less susceptible of its poisonous action than others. Seeds, steeped for some time in the acid, lose their power of germination.--Ibid. * * * * * SPIRIT OF THE PUBLIC JOURNALS. * * * * *