The filtering material used could be gradually prepared from the sludge obtained after electrical treatment, unless it could be more profitably sold as a manure, and I am not a believer in the value of sewage sludge in large quantities.  This sludge, a waste product, is converted into magnetic oxide of iron, of which I have here two small samples.  This magnetic oxide is a good filtering material, but, like every other filtering material, it would of course require renewal.  There would, however, always be a supply of the waste product—­sewage sludge—­on the spot, and the spent magnetic oxide recarbonized could be used indefinitely.

The annual cost for dealing with the Salford sewage is estimated at in round figures L2,500 for coal, labor, maintenance of engines, boilers and dynamos.  To this must be added the consumption of iron and its replacement, which would have to be written off capital expenditure.

If a colorless effluent were required, absolutely free from suspended matter, the additional cost is estimated at from L1,200 to L1,500.

* * * * *

LAVENDER AND ITS VARIETIES.

By J. CH.  SAWER, F.L.S.

Lavender—­technically Lavandula.  This name is generally considered to be derived from the word lavando, gerund of the verb lavare, “to wash” or “to bathe,” and to originate from the ancient Roman custom of perfuming baths with the flowers of this plant.

The general aspect of the various species which compose this genus of labiate plants, although presenting very characteristic differences, merges gradually from one species to another; all are, in their native habitat, small ligneous undershrubs of from one to two feet in height, with a thin bark, which detaches itself in scales; the leaves are linear, persistent, and covered with numerous hairs, which give the plant a hoary appearance.

The flowers, which are produced on the young shoots, approximate into terminal simple spikes, which are, in vigorous young plants, branched at the base and usually naked under the spikes.

As a rule, lavender is a native of the countries bordering on the great basin of the Mediterranean—­at least eight out of twelve species are there found to be indigenous on mountain slopes.

The most commonly known species are L. vera, L. spica and L staechas.  Commercially the L. vera is the most valuable by reason of the superior delicacy of its perfume; it is found on the sterile hills and stony declivities at the foot of the Alps of Provence, the lower Alps of Dauphine and Cevannes (growing in some places at an altitude of 4,500 feet above the sea level), also northward, in exposed situations, as far as Monton, near Lyons, but not beyond the 46th degree of latitude; in Piedmont as far as Tarantaise, and in Switzerland, in Lower Vallais, near Nyon, in the canton of Vaud, and at