Page:Popular Science Monthly Volume 4.djvu/785

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yeast in beer; and, if so, the seeds of the fungus would be likely to develop rapidly, if they came in contact with milk, or water containing nitrogenous matter. 'A little leaven would leaven the whole lump,' and, as it appears to me, in this way much disease may be accounted for. The microscope, then, will enable us to make out the presence or absence of this fungoid or confervoid matter in foul water, and my own observations confirm me in the view that, being present, it is highly dangerous, and, if its cause can be removed, and the water made pure, all danger from this source at once ceases, while if it cannot it should be at once disused, and pure water be sought for elsewhere."

A Sun-driven Engine.—G. A. Bergh, writing in Poggendorff's Annalen, on the application of solar heat as a motive power, says that the engine which is to serve for this purpose must employ some liquid with a very low boiling-point. There are several such liquids—sulphurous acid, methylic chloride, methylic ether, etc. Of all of these, sulphurous acid best deserves attention, as it has several useful properties for the end in view. It is not too difficult to condense, and it can be got at a moderate price. Now we have got the principle on which we must construct our solar engine. Conceive a vessel, filled with sulphurous acid, exposed to the sun's rays; the tension of the sulphurous-acid vapor, if the temperature of this vessel A exceeds that of the surrounding air by at least 10° to 20°, must be from one to three atmospheres higher than that of the sulphurous-acid vapor in another vessel, B, similarly filled with sulphurous acid, but which has only the temperature of the surrounding air. We can thus arrange an engine which agrees in principle with the steam-engine, with merely this difference, that the water is replaced by sulphurous acid, and the fuel by the solar heat; while the vessel exposed to the sun's rays represents the steam-boiler, the vessel kept at ordinary temperature may represent the condenser. The sulphurous acid condensed, after doing work in the vessel B, could easily be driven back, by a force-pump, into the vessel A. The capability of work of such a machine must naturally increase with the amount of heat communicated to vessel A or be proportional to the surface exposed to the solar rays.

An establishment, furnished with a machine like this, might carry on its work while there was sunshine, but, in default of this, would be brought to a stand-still. True, the solar heat might be replaced by the heat of the air, if the temperature of the air were pretty high, and one had at hand a refrigerating substance like ice. But, as this is not always the case, the establishment should have, besides the sun-machine, an apparatus which might "store up" some of the work done by this. As such, Natterer's apparatus for condensing carbonic acid might be used. If a supply of carbonic acid were kept in a large gasometer, the Natterer apparatus might be fed from this. In a wrought-iron vessel, thus filled with liquid carbonic acid, we should thus have an enormous store of mechanical force, which might be made to replace the action of solar heat in the sun-machine, partially or wholly. After work done, the carbonic acid, become gaseous again, might be collected in the gasometer. Or, again, the sun-machine, while in action, might drive an ice-machine, and might, in default of sunshine, profit by the ice it had produced, for maintenance of its working.

The Movements of Drosera.—Prof. Asa Gray, commenting in the American Journal of Science on a paper by A. W. Bennett, on the movements of the glands of Drosera (sundew), remarks that the author's description of these movements does not do justice to the facts, as observed by Dr. Gray himself. Mr. Bennett observed not only the bending in of the glands upon the body of the insect which lights on its leaf, but that "the sides of the leaf had also slightly curved forward, so as to render the leaf more concave." With us, says Dr. Gray, the leaves do much more than that. As well in Drosera rolundifolia as in D. longifolia, the end of the leaf folds over upon the base, or nearly like a shut hand, thus fairly inclosing the captive insect.

He adds that, when Mrs. Treat's account of this infolding of the leaf was published, in 1871, the discovery was thought to be new. But he has since found that the in-