venient quantity of the water (say 500 cc.), well shaken, so as to get a fair sample. The sand and organisms, after filtration, are washed into a test-tube, with from 3 to 5 cc. of distilled water. The tube is shaken thoroughly, the sand allowed to settle for a moment, and the water quickly decanted. This procedure leaves the sand, but transfers most of the organisms. 1 cc. of the liquid is placed on a counting plate, consisting of a shallow cell with a brass border, the cell holding exactly 1 cc. A fractional part of the field is now observed, and the organisms counted therein; in order to do this, the slide is ruled into millimetre squares, or a metal disc is fitted into the eye-piece with a square hole cut in its centre, the area of the square hole being such that with the powers used it just covers a square millimetre.

The Massachusetts biologists have by this method examined an enormous number of waters; some having been examined on several days in each month in the year. The method of recording and tabulating the results are shown in Table LIII., giving the biological results of the water supply of Nantucket. This water supply, owing to the increase of Anabana (one of the blue-green algae), in the months of August, September, and October smells and tastes offensively.

Mr. Dibdin's Process.-A better method than the SedgwickRafter process is that which has been invented by the chemist to the London County Council (Mr. Dibdin). A litre of the water (or less, should the water contain much suspended matter) is filtered through hard filter paper. The deposit is washed off the filter paper into what is called a "micro-filter." The microfilter is prepared as follows:-A piece of clean combustion tubing is drawn out into a capillary tube of a diameter of 2 mm. The open small end is plugged by a paste made of equal parts of air-dried clay and kieselguhr. This plugged end is dried in the Bunsen flame, and ultimately heated to redness. The residue from the water is now placed in this micro-filter, and the micro-filter is fitted into any convenient flask or bottle by means of an india-rubber plug attached by a side tube to a good water-pump, and the superfluous liquid drawn through, until only about 1 to 15 cc. remain. The sediment and suspended matter are thus collected in the form of a compact cylinder just above the porous filtering substance, and are now carefully measured. The results being expressed in millimetres per litre. The cylinder of deposit is removed by scratching the tube with a sharp file about half an inch from the filter plug and breaking it off. A platinum wire is pushed in so as to loosen the deposit from the porous plug.

1893.

Upon inverting the tube, open end downwards, the deposit falls away from the plug towards the open end. The tube is then cut close to the plug and the plug removed. On holding the little tube, now open at both ends, and giving it a downward shake, the whole of the deposit can be transferred in about a drop of water to a microscope slide and examined in the ordinary way, afterwards portions of the deposit may be stained with aniline dyes, cultivated and otherwise investigated.*

1. Lifeless Forms.

1. Mineral Matters, especially sand, clay, and not unfrequently fine spicula of glass derived from the glass pipette, &c.

2. Vegetable Matters.-In shallow pools, in rivers, reservoirs, and, in fact, all open waters, the microscopist seldom fails to find vegetable débris in the shape of dotted ducts, spiral vessels, parenchymatous cells, bits of cuticle with the hair still adhering, the down of seeds, roots of duckweed, bits of chara, &c. It depends on the amount as to what conclusions are to be drawn; but this is certain, that a water showing these matters is not likely to be from a deep spring, but one over which the atmosphere more or less freely plays.

3. Dead Animal Matters-(a.) Purely Animal, such as hairs from domestic or wild animals, striped muscular tissue, the scales of moths, butterflies, or other lepidoptera, eggs of entozoa (which, of course, may, for aught we know, be living).

(b.) Human Débris.-Human hair, human epithelium. (c.) Manufactured Matters. Wool, silk, &c. All animal matters, whether derived from insect, human, or domestic animal life cannot be considered a favourable indication; and even the presence of cotton, silk, hemp, and the like, though innocuous in themselves, yet afford evidence that the water is in such a position as to be liable to accidental contamination.

2. Living Forms.

The lower forms of vegetable and animal life spring from a common point, so that it is in certain cases impossible to definitely ascribe life-forms to either kingdom; nevertheless, it is convenient to divide provisionally the microscopic life-forms into (a.) vegetable, (b.) animal.

* Analyst, 1895.

(a.) Vegetable. The most common vegetable forms are what have been called the water fungi :-Beggiatoa, Crenothrix, Cladothrix-then there are confervæ, oscillatoria, volvocinæ, desmids, diatoms, and bacteria. To these may be added the green, or sometimes red, cells of palmellæ, and the moving reproductive spores of confervoids, charæ, &c. Those forms which possess cells holding "chlorophyll" denote water which is exposed to daylight. Beggiatoa. There are There are several species of Beggiatoa; the commonest being Beggiatoa alba. Beggiatoa forms long flexible threads, 1 to 5 mm. in diameter, containing a number of round bodies. It grows on dead vegetable and animal matters, sometimes in enormous quantities. The author has seen the bottom of a polluted brook lined with it for the extent of half a mile. The attached part of the thread is thinner and shows divisions into segments with but few particles, but towards the free end the segments become less obvious and bright round coloured particles more numerous. These particles consist mainly of sulphur. The free ends are occasionally liberated, and give rise to a swarm of spiral threads which (under the name of Ophidomonas) were formerly classed as a distinct species.

The growth of Beggiatoa is favoured by organic matter and sulphur compounds; hence its presence usually denotes sewage pollution.

Leptothrix is another thread form met with in waters. Α few botanists consider it to be simply a stage in the life of Crenothrix and Cladothrix; but the majority consider it a distinct species.

Leptothrix ochracea forms thin threads stained yellow or yellow-red by a deposition of iron oxide in the sheath.

Crenothrix forms a brown slimy precipitate consisting of fine threads from 1.5 mm. to 5 mm. in diameter. The threads are composed of oblong cells, the cell divisions being evident. Old cells are often coloured by iron oxide. Some of the cells develop spores which form masses of brownish scum. These grow to fresh threads. Sometimes the spores grow within the cell to new threads, and then the new threads break through the cell, forming a bundle of fine filaments. Old threads may show no sign of division into cells.

Cladothrix is similar to Crenothrix, but the threads are of less diameter. It may always be, however, distinguished from Crenothrix by the threads dividing dichotomously, hence the name Cladothrix dichotoma.

Fresh-water sponges occur as a thin incrustation on objects;