carbonate of lime in solution, and forms with it a fresh portion of carbonate of lime; and as this salt is insoluble in water, it falls down in the form of a white powder; and not only the carbonate of lime that is formed, but that which was held in solution is deposited also ; and thus the water loses all its carbonate of lime. Now this is the philosophy of Dr. Clark's process for softening hard water. He adds lime to it, and the water is deprived of its hardening ingredient, the carbonate of lime. Dr. Clark's process not only renders the water much softer for washing and cooking purposes, but, whilst the carbonate of lime is falling, it entangles, as it were, the organic matters, and renders it much purer for drinking purposes. for drinking purposes. Great objections have been urged against Dr. Clark's process; but my conviction is, that all hard waters are improved by it; and, although it has been extensively employed, I am unacquainted with a single drawback to its employment.

There is another salt of lime found in water,-sulphate of lime. This may be easily detected by nitrate of baryta. The sulphuric acid unites with the baryta and forms an insoluble precipitate. Wherever there is any considerable quantity of sulphate of lime, we can easily detect it by the agency of the nitrate of baryta. Again, if we want to know whether our sulphate is of lime or not, we must add a solution of oxalic acid or oxalate of ammonia, which will throw down the lime. By these tests we may judge of the quantity of sulphuric acid approximatively; but, in order to get a knowledge of the exact quantity of these substances, we must collect the precipitates and weigh them.

Another substance of some importance in water is the chloride of sodium, or common salt, which can only be accounted for in our surface-well waters by the fact of the salt being constantly used by man in cooking, &c., and thus passing into the drains and sewers, is washed into the wells. When we find this substance in waters, we should be careful of using them dietetically. The best test for chloride of sodium is nitrate of silver. It throws down the chloride in the form of a chloride of silver. It is a creamy-looking precipitate, at first white, and gradually becomes bluish-black by the agency of light. It is, in fact, one of the salts used by the photographer to produce his pictures on paper.

I might continue these illustrations, but I hope I have shown you sufficient to indicate that a very little chemistry will enable an intelligent person to detect whether water contains large quantities of the more common impurities of water.

There is another method of extracting approximatively the quantity of saline impurities in water. If we take a quantity of water and boil it in an evaporating basin, the water will at last entirely disappear, and the inorganic matters will be left at the bottom of the vessel. By taking two or three waters, and thus treating them, we can judge, within a little, of their relative amount of impurity. We may judge also to some extent, by this test, of the quantity of organic matters present in water; for, according as these are present, will the precipitate be of a dark and dirty colour.

You must not suppose, however, that the saline impurities I have mentioned are all that are to be found

Fig. 4.-From Water of Well at Sandgate (on Mr. George's premises).

a. Rotifer.

b. Bursaria?

c. Paramecium.

d. Acineta tuberosa?

e. Vorticella.

f. Actinophrys Sol.

k. Earthy and Organic matter.

g. Filament of Conferva.

h. Stems of Anthophysa? i. Slender Fungus.

in water. There is chloride of calcium, which sometimes occurs in such large quantities as to produce very disastrous results on the system. Iron frequently occurs in such quantities as to flavour the water which contains it, and when taken in large quantities, it seems to act injuriously on the system. Nitrates occur as the result of the decomposition of animal and vegetable

matter, and sometimes in sufficient quantities to act in a depressing manner on the system. The soluble phosphates have also all been found in surface-wells, and evidently come from decaying animals or animal refuse of one kind or another.

I now come to speak more particularly of the organic constituents of water. These are of two kinds-living and dead. The living are sometimes contained in water in very large quantities. I do not know that the living things are so objectionable as the dead. I believe it to be a much more healthy practice to swallow oysters while alive than to wait until they are nearly putrid; and that is just the difference between swallowing living and dead animal matter in water. There are two sorts of living things in water-plants and animals. I have before drawn your attention to the forms of plants and animals in Thames water. Fig. 4 is a drawing of the living organic constituents of a well-water that was known to have produced disease; and Fig. 5 is another drawing of the living creatures which inhabit sewerwater. Of course, these things are all microscopic.

Although fish, snails, and shrimps live in water, I need not warn you against these, as everybody can avoid them if he choose. Now, if you look at the contents of the two last waters I have mentioned, you will find amongst the plant-like bodies certain filamentous bits, which are really half-developed forms of some low fungus. I do not know if these are themselves poisonous, but I do know that when present they indicate that a water is dangerous. In 1854 I was requested by the Vestry of the Parish of St. James, Westminster, to examine the water from the pump in

Fig. 5.-Sewer Water (taken from the Sewer in Silver Street).

a. Anguillula fluviatilis.

b. Oxytricha.

c. Paramecium.

d. Vibriones.

e. Filaments of Slender Fungus.

f. Fragments of Muscular Fibre.

g. Cells of Potato.

h. Starch granules of Wheat.

i. Hairs and integuments of Wheat.

k. Spiral Vessels.

1. Dead and Decaying Organic matter, as dotted ducts, hair of animal,

grit, and débris.

Broad Street, Golden Square. The cholera had broken out there, and killed five hundred people in less than a week, and the late Dr. Snow had accused the pump of doing all this mischief Now I detected nothing remarkable in that water but the filaments of a fungus.