Page:Popular Science Monthly Volume 8.djvu/264

252 is necessary to the most perfect health of the trout, it is still a fact of great value that they can live, and grow, and fatten, on a vegetable diet.

Changes in the Skin of Fur-bearing Animals.—The obvious difference between the fur of animals in summer and in winter is found by Dönhoff to be associated with an equally striking difference in the texture and thickness of their skin. Thus, the average weight of an ox-hide in winter is seventy pounds; in summer, fifty-five pounds; the hair in winter weighs about two pounds, and in summer about one pound; leaving fourteen pounds to be accounted for by the proper substance of the skin. These differences are quite as decided in fœtal animals as in adults. Calves born in winter have a longer and thicker coat than those born in summer; moreover, there is a difference of more than a pound in the weight of their skins after the hair has been removed. Similar facts may be observed in the case of goats and sheep. That these differences are not to be ascribed to any corresponding change in the diet and regimen of the parent animals, is proved by the fact that they are equally manifest in the young of individuals kept under cover, and on the same food all the year round.

Intensity of Solar Radiation.—In a letter to Ste.-Claire Deville, Soret alludes incidentally to some recent optical observations which show the great intensity of solar radiation. If we look at an ordinary flame through plates of glass colored blue with cobalt, we observe that with a certain thickness of glass the flame presents a purple color, as the glass transmits the extreme red rays, and the highly-refrangible blue and violet rays, while it intercepts the rays of intermediate refrangibility. If the source of light have a high temperature, and therefore emit highly-refrangible rays, the flame appears blue, and it requires a number of superposed plates in order to develop the purple tint. Thus it was found that, at the temperature at which platinum fuses, two plates would give a purple color; at the fusion of iridium three plates were required, and on observing the sun the purple color was not developed even with half a dozen plates.

Extinction of Animals in Rodriguez.—Alphonse Milne-Edwards, in a communication to the Paris Academy of Sciences, shows from documentary evidence that the solitaire and the other gigantic birds of the Island of Rodriguez became extinct between 1730 and 1760. Reports addressed to the Compagnie des Indes show that the island was regarded as a sort of provisioning-store, not only for the Isle of Fiance and the Island of Bourbon, but also for the ships frequenting these parts. One object of their visits was the collection of land-tortoises, and efforts were made by the compagnie to put some restrictions on this business. The land-tortoise has long since disappeared from the island. As for the great birds of Rodriguez, owing to their undeveloped wings they were easily captured, while the delicacy of their flesh caused them to be much sought after.

Terrestrial Radiation.—Prof. Thiselton Dyer, at a recent meeting of the British Horticultural Society, made the following communication upon the phenomena of terrestrial radiation and its effects on vegetation, basing his remarks upon the observations of Buchan. The effects of radiation, he said, are at the maximum when the air is calm and very dry, and its temperature rather low. If, however, the cold air produced through the influence of terrestrial radiation be allowed to accumulate close to the ground, no small amount of damage may be done by a comparatively light frost. On sloping ground such accumulation of cold air cannot go on, because, cold air being heavier than air which is warmer, as soon as the air in immediate contact with sloping ground is cooled it flows down to a lower level, just as water would do, and its place is taken by the warmer current of air immediately above. In this way a higher night temperature is maintained in situations where the ground slopes down to lower levels, and accordingly such situations should be chosen for those plants which, at any stage of their growth, are peculiarly liable to be injured by frost. If the air be not calm, but a wind—even a slight wind—be blowing, the different layers of air are thereby mixed; and thus the air cooled by contact with the cold ground is not suffered to rest thereon, but is mixed with the air