reference to the use of carbonate of soda, that "the hardness of water is almost exclusively caused by the presence in solution of the bicarbonates and sulphates of lime and magnesia. These salts are all decomposed by carbonate of soda, slowly in the cold, but rapidly when the water is hot, insoluble carbonates of lime and magnesia being deposited or precipitated as a fine mud, while soluble bicarbonate and sulphate of soda are formed." The process, however, of using soda is much more expensive than the ordinary lime process, above 4 times as much soda being required to produce the same results as in the case of lime. An old receipt for softening water, and useful for some domestic purposes, is as follows:-Dissolve 6 lbs. of pearlash or sub-carbonate of soda in a gallon of soft water, boil the solution, and when boiling add 2 ozs. of soap, and stir until all the soap is dissolved. When this solution is added to water to be softened, the carbonate of soda and the soap combining with the salts producing both temporary and permanent hardness, form an insoluble compound by the combination with the soap, which coagulates and rises to the surface of the water, and may be skimmed off.

As any of the earthy alkaline earths may be used instead of, or in addition to, lime, it is not surprising that, since the date of Clark's patent, numerous patents have been taken out for softening and purifying water in which lime, in combination with other alkaline earths, have been proposed. For example:

In 1849, Mr. John Horsley took out a patent for the use of calcined or caustic barytes, phosphate of soda, oxalic acid, or preparations of these substances.

In 1850, lime, in combination with chloride of barium, was patented.

In 1852, hydrate of potass and hydrate of soda, clay, and alkalies were the subject of separate patents.

In 1853 and 1854, hydrate of barytes, and hydrate of strontia formed the subject of patents.

In 1855, a patent was taken out for a powder containing

oxalate of ammonia, peroxide of manganese, and charcoal.

In 1856, silicate of soda, in combination with carbonate of soda, was patented.

In 1856, bicarbonate of soda and oxalic acid, in crystals, were again proposed to be used.

In 1856 and 1857, carbonic acid was proposed to be introduced in conjunction with hydrate of lime.

In 1860, bicarbonate of soda and silicate of soda were again twice patented.

In 1862, the ordinary lime process was re-patented. In 1863, the use of chloride of barium was patented. In 1865, hypermanganate of potash, carbonate of soda, alum and neutralised aluminite, or a solution of iron was proposed; and in another patent, sequi-sulphate of alumina was proposed as a means of purifying water. In another patent taken out in 1865 the use of soda and lime is again patented.

In 1866, a patent was taken out by a Mr. J. W. Tobin for an improvement on the Clark process for mixing the lime and filtering.

In 1866, the use of chlorine and permanganates, in combination with any alkaline earth, was patented.

In 1867, a patent was taken out for a preparation in a portable form, consisting of marsh mallow, linseed, bran, starch, gum, or any softening emollient, for the purpose of softening water for ablution purposes.

In 1868, the use of barytes was again patented, and the precipitate removed in vessels of a cellular form.

In 1869, the use of steam applied to water as a means of softening it was patented, the particles subsiding on shelves arranged in a vertical vessel.

In 1872, unslaked lime and sulphate of soda were patented as a means of purifying water.

In 1873, the lime process, in conjunction with a mixing and filtering arrangement, was patented; also lime, carbonate of soda, chloride of barium, or other reagent in conjunction with filtering.

In 1874, the treating of water with lime and carbonate. of soda in combination with filtration was proposed.

In 1875, the lime process pure and simple, and in combination with other reagents and filtering, was patented.

In 1876, the lime process in combination with filter presses was first patented by Mr. Porter.

In 1877, the use of oxide of magnesia and basic carbonate of magnesia was proposed; also the use of a carbonate of potassa, silicate of soda, and nitre cake, used separately or combined.

In 1878, a patent was taken out for combining bicarbonate of soda with the lime process, and filtering upon the Porter-Clark plan.

In 1879, Mr. Porter took out an additional patent for carrying out automatically the softening and purifying of water. In the same year phosphoric acid and phosphate of lime were proposed as a means of softening water.

In 1880, the means of automatically carrying out the softening process, and adjusting the quantities by means of an arrangement of ball valves in a cistern, was patented.

In 1881, Mr. Porter took out a further patent for the automatic regulation of the supply of the solution of lime.

In 1881, Mr. Atkins took out a patent for treating water with lime, and subsequently filtering through a specially constructed filter.

In 1882, an apparatus for softening and clarifying water was proposed, in which sloping shelves in a vertical vessel were used, the water entering the bottom of the vessel and flowing off at the top; or concentric cylinders overflowing from one to another might also be used with the same object.

In 1883, Messrs. Gaillet and Huet's apparatus for softening water was patented, consisting of sloping and V-shaped shelves in a vertical vessel, the water entering at the bottom and flowing out at the top.

The use of steam and caustic soda was also patented.

A patent was also taken out for the use of phosphate of soda, and an apparatus for measuring the water and lime to be used in the softening process.

Modern Inventions for Softening Water.-The modern inventions for carrying out the Clark process may be described as the application of machinery to the saving of time, space, and labour. Of those in general use, dealing with them in the order of date, the Porter-Clark process comes first. In the ordinary Clark process, lime-water, in known quantity, is first admitted into a tank, to which the water to be softened is added. However, in some cases the lime-water and the hard water are allowed to flow in together into the tank, but it was considered by Dr. Clark to be an advantage, in carrying out his process, for the hard water to be brought into contact with an excess of limewater in the first instance, which was let into the tank before the water to be softened was added. In the ordinary Clark process, not less than sixteen hours were required for the softening and subsidence of the matters separated from the softened water. In the Porter-Clark process, instead of allowing the particles of carbonate of lime to separate and subside, a brisk agitation is maintained, so that these particles remain in suspension to permit chemical reaction with the lime, and the purification of the hard water, which, when completed, is passed onward to filter presses, where the carbonate of lime adheres to the filtering cloth. The subsequent operations of filtering through the deposit on the cloth then takes place, and the water is passed away at once in fit state for use, so that the process is continuous in its action, and may be carried on just as quickly as water can be pumped from a well, provided the apparatus is of sufficient capacity to allow time for the water to remain in contact with the lime in passing through the machine. The lime in this case is passed through horizontal cylinders, which are termed lime-churns, and is constantly churned up with water, and the lime-water so made is, by suitable arrangements, allowed to mix with the ordinary water in proper proportions, after which it is again agitated. Mr.

Porter has also an apparatus by which, instead of using ordinary filter presses, he can use filter frames, and where power is not available, he also suggests a means of working the process without such power. In some cases the power obtained from the pressure of the water is utilised for working the apparatus. An apparatus of this description may be seen at the Camden Town locomotive sheds of the London and North Western Railway, and the apparatus may also be seen at work within this Exhibition.

The Atkins Process is also a modification of the Clark process, by which the space formerly required is reduced. The lime is put into a vessel where lime-water is formed, and this water is allowed to mix in its proper proportion with the water to be softened in a specially arranged mixing vessel, after which it passes into a reservoir of small dimensions. From this reservoir it is conveyed to filtering vessels which contain a special arrangement of filter, consisting of a series of chambers mounted upon a central hollow shaft, these disc chambers being covered with prepared canvas, upon which the deposit of chalk, &c., adheres, and through which the softened water filters. The filters are cleansed by means of revolving brushes. The apparatus does not require power to maintain it while at work, the only power used being that necessary to give motion to the brushes when the apparatus is cleansed. The system may be seen at work at the Henley-on-Thames Waterworks, and at other places.

In the Process of M. Maignen, a powder is used which the inventor calls "Anti-calcaire." This powder is made of variable composition in order to suit the special characteristics of the water to be treated, the ingredients used for ordinary hard water being lime, soda, and alum in suitable proportions. In the apparatus, which is at work at the International Health Exhibition for softening water for some of the breeding tanks, the water entering the apparatus gives motion to a water-wheel, which in its turn works an arrangement for distributing a given quantity of the softening agent, and causing it to pass into the water.