quiring the art of the optician to reveal their beautiful diversity of forms, and their extreme delicacy of structure. They are inhabitants of the water, and so widely distributed, that there is scarcely a pool, a ditch, or a water-course where they are not to be found, attached to submerged weeds, or mixed with floating debris, or deposited on old piles subject to tidal influences, or forming a covering on the surface of the mud, arranged in patches, and varying in colour from a yellowish brown to a dark chocolate. Some species are indigenous to fresh, some to salt, and some to brackish water, but those common to the one are never found in a lively and healthy state in the other.
In the early history of Diatomaceæ, they were placed by naturalists in the animal kingdom, in consequence of their curious movements; a more intimate acquaintance with their nature and habits, acquired by the aid of improved microscopes, and confirmed by chemical experiments, has resulted in their being transferred to the vegetable kingdom by the unanimous consent of all competent judges. It is now ascertained that the power of locomotion is not peculiar to the Ditomaceæ, but that many of the simple plants, such as Oscillatoria, the Desmidiaceæ, Protococcus, and others, possess this power in common with the Diatoms, and some of them, in certain stages of their existence, are much more active in their movements than any of the Diatomaceæ.
The characteristic feature of the Diatom is its silicious envelope covered with a net-work of fine markings, diversified in pattern, and in some of the species so exquisitely delicate as to require the best objectives and the highest powers to resolve them. Being composed of silica, these frustules are indestructible by the usual agents of decomposition; when, therefore, they are cleansed by being boiled for a few minutes in hydrochloric or nitric acid, they become objects of permanent interest, and may be preserved by mounting, either dry or in Canada balsam.
In order to secure clean and good specimens for mounting, it is absolutely necessary to separate the Diatoms from all foreign matter, which unavoidably becomes mixed with them in collecting. The following is a plan I have tried with great success. On returning home allow your bottles to stand for an hour, by which time the Diatoms and debris will have settled at the bottom, pour off the greater portion of the water, then shaking the bottle briskly, empty its contents into a soup-plate, place the plate in a window for an hour or two, when it will be found that the Diatoms, attracted by the light, will have arranged themselves on the surface of the mud; by gently rotating the plate they become loosened and may be poured off in a pure state, and are ready for being examined alive, or for boiling in acid.
In collecting Diatoms, half a dozen wide-mouthed bottles, a large-bowled spoon as thin as possible, and a stick with a hook at one end, are all the apparatus required. The spoon for carefully skimming the mud, the stick for pulling in any submerged plants or floating rubbish, and the bottles to hold your gatherings, taking care to place the different kinds in separate bottles, being furnished with a Coddington lens to examine your specimens on the spot.
The marine forms of Diatomaceæ may frequently be found attached to seaweed; but some of the rarer kinds must be sought for from the stomach of lobsters, oysters, whelks, and other mollusks.
The favourite habitats of those common to brackish water are marsh-ditches exposed to tidal influences, where they may generally be obtained in great quantities and many varieties.
The best localities near London are the marshes at Erith on the opposite side of the road from the entrance to the Railway station, and Swanscombe Salt Marsh, lying towards the river, about half a mile from Northfleet Railway Station. At both of these places I have always been able to get many interesting forms, together with fine specimens of living Gromia, and a variety of beautiful animal Infusoria. Others may be procured from the marsh ditches at North Woolwich, immediately beyond the Gardens by the Railway Station.
Fresh water Diatoms, in one form or other, are almost ubiquitous. They may be found deposited in brown tufts on the sides and bottom of nearly every clean ditch and every running stream. They line the sides of the locks up the river, fringe the leaves of the larger water-plants, and float in every collection of scum. Pinnularia cuspidata, Stauroneis, &c., may be obtained from a rill running across Keston Common, near Bromley, Kent, and all these forms, together with the beautiful Surirella biseriata and Surirella splendida, abound in the boggy pools on Winter Down, lying to the right of the main road one and a-half or two miles through Esher, opposite to Claremont Park.
I must reserve a few remarks upon the subject of illumination, and the best mode of
