The radiations interfere in an optical sense of the word, and in some directions reinforce each other and in other directions neutralize each other, so making the resultant radiation greater in some directions than others.
The evidence for the existence of the luminiferous aether has accumulated as additional phenomena of light and other radiations have been discovered; and the properties of this medium, as deduced from the phenomena of light, have been found to be precisely those required to explain electromagnetic phenomena."
Among extinct Tertiary mammals we can actually trace the giving off of these radii in all directions, for taking advantage of every possibility to secure food, to escape enemies and to reproduce kind; further, among such well-known quadrupeds as the horses, rhinoceroses and titanotheres, the modifications involved in these radiations can be clearly traced.
Thus the history of continental life presents a picture of contemporaneous radiations in different parts of the world and of a succession of radiations in the same parts.
We observe the contemporaneous and largely independent radiations of the hoofed animals in South America, in Africa and in the great ancient continent comprising Europe, Asia and North America; we observe the Cretaceous radiation of hoofed animals in the northern hemisphere, followed by a second radiation of hoofed animals in the same region, in some cases one surviving spur of an old radiation becoming the centre of a new one.
There results from continental and local adaptive radiations the presence in the same geographical region of numerous distinct lines in a given group of animals.
Because of the repetition of analogous physiographic and climatic conditions in regions widely separated both in time and in space, we discover that continental and local adaptive radiations result in the creation of analogous groups of radii among all the vertebrates and invertebrates.
O-KovEiv, to see), that branch of physical science which has for its province the investigation of spectra, which may, for our present purpose, be regarded as the product of the resolution of composite luminous radiations into more homogeneous components.
In the opinion of the writer the latter instrument will ultimately replace the bolometer, its only disadvantage being that the radiations have to traverse the side of a vessel, and are therefore subject to absorption.
This need not necessarily be interpreted as indicating the impossibility of rendering gases luminous by temperature only, for the transparency of the gas for luminous radiations may be such that the emission is too weak to be detected.
The spectra produced under these circumstances have been studied in detail by C. de Watteville.4 Of more frequent use have been electric methods, owing to the greater intensity of the radiations which they yield.
As a single electron charged negatively; these rays can penetrate sheets of aluminium, glass, &c., several millimetres thick; and (3) the 'y rays - which are non-electrified radiations characterized by a high penetrating power, i% surviving after traversing 7 cm.
When none of the radiations which fall on a body penetrates through its substance, then the ratio of the amount of radiation of a given wave-length which is absorbed to the total amount received is called the "absorptive power" of the body for that wave-length.
A body which absorbs all radiations of all wavelengths would be called a "perfectly black body."
All bodies when heated emit the same kind of radiations which they absorb - an important principle known as the principle of the equality of radiating and absorbing powers.
But all such bodies appear to lose their distinctive properties when heated in a vessel which nearly encloses them, for in that case those radiations which they do not emit are either transmitted through them from the walls of the vessel behind, or else reflected from their surface.