Page:Hardwicke's Science-Gossip - Volume 1.pdf/133

lime. This analysis, however, only indicates the ultimate elements, and not the mineral composition of the rock; and, to a certain extent, granites must be regarded as mixtures of crystalline minerals in various proportions. Thus it is also useful to observe that an average granite may be expected to contain from two to three-fifth parts of crystals of quartz or crystalline quartz, about the same, more or less, of felspar, also partly crystalline and chiefly in definite crystals, and the remainder (one-tenth part) of mica. But the mica may form two or three-tenths and the quartz three-fifths, or more, while the proportion of felspar, as well as the particular composition of the felspar, as well as the particular composition of the felspar, both vary extremely. Good ordinary granite weighs 166¼ lb. the cubic foot, and the cubic yard as nearly as possible two tons. Fourteen cubic feet to the ton is the usual estimate. Its toughness or resistance to crushing weights is very great, and is believed to vary a great deal in different samples. For practical purposes it is safe to assume that fresh unweathered granite will bear any direct crushing weight to which it can be exposed. Granite contains a certain amount of water capable of being driven off by continued exposure to a heat insufficient to produce any other change. This quantity differs much in different specimens, even of the same stone, but it may be said to be represented by the "loss" in the table of analysis. It averages, therefore, about 0.8 per cent. Taken in its ordinary state, and containing this quantity of water, it is still sometimes capable of absorbing about one-fourth more (or 0.2 per cent.), when placed in water for a few hours. This quantity is, however, rather larger than could be expected of a good sample. Expressed in another way, I may say that a cubic yard, or two tons weight of granite, contains, in its ordinary state, something more than 3½ gallons of water, and some specimens can absorb nearly a gallon more on being placed in pure water for a short period. It is important to notice this fact, as the influence of frost on stone is in proportion to the water it takes up, and determines its durability.—Ansted's Lectures on Practical Geology.

Festoons of Driver-ants.—First a single ant clung tightly to a branch, and then a second insect crawled cautiously down its suspended body, and hung to its long, outstretched limbs. Others followed in rapid succession, until they had formed a complete chain of ants, which swung about in the wind. One of the largest workers then took its stand immediately below the chain, held firmly to the branch with its hind-limbs, and dexterously caught with its forelegs the end of the living chain as it swung past. The ladder was thus completed, and fixed ready for the transit of insects; and in a similar way the whole tree was covered with festoons of ants, until it was blackened with their sable bodies.—Homes without hands.

Red-leaved Sea-weed (Delesseria sanguinia).—The eight leaves of this alga, sent herewith, are taken from specimen which grew in the aquarium of the Zoological Gardens, Hamburg, in the winter and spring of 1865. Is any other instance known of the plant having thus grown from a very young state, or from seed, in a tank? It is the first time I have known it do so. I found it by accident in a deep and nearly dark hold in a large tank in which specimens were placed in the summer of 1864, but which specimens died. The present specimen is parasitic on the stem of some long alga (probably a Feas); but it is also so much covered with conferva that not much of it can be seen. The conferva does not grow on the Delesseria leaves. Last night, on clearing out the tank in question, the alga was for some hours exposed to the air, and this morning some of the leaves were found of an orange colour—the colour of death, and these leaves are those now send; but the orange (decayed) parts are not so apparent as when they were in water. But there are still plenty more of much smaller leaves on the same stem, and these will be watched carefully.—W. Alford Lloyd, Keeper of the Aquarium.

Aquarium and Fernery.—The salt-water aquarium before me is about 16 inches diameter and 8 inches deep, covered with a bell-glass about 2 feet high, but not fitting closely. The lower vessel is two-thirds filled with sea-water; in it, besides other living organisms, are various anemones, of all sizes and ages, gathered from different parts of our coast; fine shingle, bits of rock, coral, &c., are strewn on the bottom, many of the latter are covered with bright emerald green vegetation of spontaneous growth. In the centre stands a tripod supporting a wooden basin. The former consists of three strong zinc wire legs; both these and the basin are concealed by specimens of rocks, &c., collected from various places and firmly fixed with good Roman cement. In the basin filled with cocoa-nut fibre we have found almost any kind of common fern will grow (Adiantum capillus-veneris, cuneatum, pubescens, pedatum, Asplenium viviparum, gracile, and Sandersonii, with many others, grow well). Last year some sowed themselves in the crevices of the rocks, forming a pretty fringe just above the water. The brushwood in this miniature forest is any kind of Lycopodium, except cæsium, which does not succeed well with us. I think succulent ferns do the best. The water is in a good state, and has not been changed since last October. I have grown ferns in pots in the room for several years in unmixed cocoa-nut fibre with charcoal for drainage, and the "mould" never appeared.—Leonora.