Page:Popular Science Monthly Volume 19.djvu/484

468  pump. In the higher mollusks the heart has generally two cavities—an auricle for receiving the blood and a ventricle for propelling it. The bivalve mollusks have generally two auricles. In the mollusks we discover a well-developed capillary system, but the venous or return circulation is still partly lacunar. The heart of invertebrates is always systemic—it forces the blood to the body, not to the breathing-organs. But some of the cephalopod mollusks, the so-called devil-fishes, have contractile cavities at the bases of the gills, which act the part of a pulmonary heart, forcing the blood through the breathing-organs on its way to the true heart. These accessory hearts are called branchial hearts.



SCIENCE, like a child, grows quickest in the first few years of its existence; and it is therefore not astonishing that, though twenty years only have elapsed since Spectrum Analysis first entered the world, we are able to speak to-day of a modern spectroscopy, with higher and more ambitious aims, striving to obtain results which shall surpass in importance any of those achieved by the old spectroscopy, to the astonishment of the scientific world.

A few years ago the spectroscope was a chemical instrument. It was the sole object of the spectroscopist, to find out the nature of a body by the examination of the light which that body sends out when it is hot. The interest which the new discovery created in scientific and unscientific circles was due to the apparent victory over space which it implied. No matter whether a body is placed in our laboratory or a thousand miles away—at the distance of the sun or of the farthest star—as long as it is luminous and sufficiently hot, it gives us a safe and certain indication of the elements it is composed of.

To-day, we are no longer satisfied to know the chemical nature of sun and stars; we want to know their temperature, the pressure on their surface; we want to know whether they are moving away from us or toward us; and, still further, we want to find out, if possible, what changes in their physical and chemical properties the elements with which we are acquainted have undergone under the influence of the altered conditions which must exist in the celestial bodies. Every sun-spot, every solar prominence, is a study in which the unknown quantities include not only the physical conditions of the solar surface, 