Page:Popular Science Monthly Volume 46.djvu/590

574 of all fixtures, and even then there is risk in applying it of flooding parts of the house. "The peppermint test is useful in a measure, but unless great care is taken in applying it the results are at times misleading. It is, therefore, in the hands of inexperienced or unscrupulous persons a rather dangerous and somewhat objectionable test. It is not always possible to define by it the exact position of the leak, or to determine exactly what the defect is. In the more positive smoke test, on the other hand, any leakage becomes apparent to the senses of smell and sight; in fact, in nearly all cases, except where leaks are very slight, the issue of smoke will indicate the exact point at which plumbing is unsafe. In order to have continued assurance that the plumbing and the drainage and the gas-piping remain safe, it is advisable to repeat the tests from time to time. The walls of a building settle, the pipe joints may become untight, or the joints may open by expansion when much hot water passes through the waste, or pipes may break, or traps may sag or tip over, rubber gaskets of floor joints may disintegrate and rot, leaving open cracks through which sewer air may pass, or joints made with brass couplings may become loose, and rubber or leather washers may rot; in short, there are numerous points which in a plumbing system may become defective after it is in use for some time. Hence the necessity of periodical re-inspection, which is just as desirable with plumbing work as it is with steam boilers or other machinery."

The Electric Arc.—In a lecture at the Royal Institution on Electrical Illumination Prof. J. A. Fleming exhibited the formation of an arc between carbon rods, and said that it had been experimentally proved that the arc could not be started unless either the rods were first brought into contact or the insulating power of the air between was broken down by an electric spark. An immensely magnified imago of the arc was projected on the screen, so that its interior structure was rendered visible. It was seen, for instance, that the positive carbon rod becomes most intensely hot at the extremity and hollowed out into the form of a crater, from which about eighty per cent of the total light is emitted. The negative carbon does not become so hot. The space between the two, or the true arc, is filled with vapor of carbon. In the central space a brilliant violet axis is seen, violet being the color of incandescent carbon. Outside this is an auraole of carbon vapor of yellow or golden color. With the use of a prism the central axis of the arc gave a spectrum marked by two brilliant violet bands. It was next shown that the rise of electric pressure in the arc takes place chiefly at the surface of the crater, which is in fact the place where the work is done in evaporating the carbon. The light emitted is therefore due chiefly to the incandescence of the carbon in the crater. Hence the light is not given off equally in every direction. It is most intense in that direction in which the largest area of crater can be seen.

Humming Birds as Carriers of Pollen.—The agency of humming birds in transferring pollen from flower to flower is shown in a paper by Joseph L. Hancock to be parallel in importance with that of insects. The common ruby-throated humming bird, though it is not endowed with specialized structures for the specific performance of this office, has in its mouth parts and feathers means for harboring the pollen. The anatomical peculiarities of its head permit access to flowers of a wide range of forms. The bill, by virtue of its flexibility, is capable of probing to the bottom of most of the common forms of flowers; and in the feeding process the flower is often bent over. The various ways in which pollen is carried to this bird were revealed on microscopic examination of some dead specimens. On the lower mandible just in front of the angle of the mouth, overshadowed by the nasal scale when the bill is closed, a faint yellowish line marks the deposit of pollen grains resting, clustered together, in a small groove. Pollen grains work their way free to the summit or vanes of the feathers, and are caught up by the barbs of the feathers along the sides of the chin and lores, where they remain ready to be deposited when a more suitable surface is presented. A second receiver of pollen is the deep median groove under the lower bill, the point of meeting of the rami. Four ways have been observed by the author in which pollen becomes engaged or held by the feathers. In flowers, the pollen of which is