\TATI E. ] 



OHEMISTKY. 



32J 



on evaporation, forms clouds ; and these again, being con- 

 densed as rain, full in fertilising showers, to eventually 

 form rivers which run to the ocean. Rain-water, when 

 collected at a distance from towns, is generally almost 

 pure ; but near those places it becomes contaminated by 

 matters suspended in the air such as nitric acid, sulphur, 

 and ammonia. After rain-water has percolated througl 

 strata of earth, and ia again seen as rain or spring 

 water, it will have dissolved both organic and inorganii 

 matter, and this to a very large extent. The inorgani 

 matter may vary from 2 to 100 grains per imperia 

 gallon ; and that of an organic nature differs similarly 

 in quantity. The purest water, in a chemical sense 

 supplied in Great Britain, is that obtained from Loch 

 Katrine, in Scotland ; which is conveyed to Glasgow, a 

 distance of forty miles ; whilst the wells in the neigh- 

 bourhood of London, are, generally speaking, highly 

 contaminated with all kinds of impurity. The follow- 

 ing table shows an analysis of various sources of the 

 water-supply of London, <fcc. ; and has been extractec 

 from the report of the medical officer of the city o: 

 London, to the Common Council. 



TaWe of Jmpuritiet in Water-Supply from various source* 



Total 

 pei gallon. 



Distil-.! water 



Loch K ttrine water, new supply to Glasgow (March) 



er water supply (March) 



-road well 



Cheliw (Thames War) 



houthwark ,, 



Lambeth t , 



We Middlesex , 



Grand Junction , 



East London (River Lea) 



New River 



Kent (from the Chalk tratt) 



Organic 

 per gallon. 



gr. or deg. 



0-0 



235 



3-33 

 5808 

 19-72 

 2196 

 20-44 

 21 76 

 2136 

 21-03 

 1972 

 2i 16 



i . or dei 

 0-0 

 60S 

 680 

 7-84 

 176 

 2-40 



2 in 



2-40 

 1-80 

 1-36 

 1-71 

 2-64 



Fig. 49 



Nearlv all the water supplied to London, is either 

 obtained from the New River, which rises in Hertford- 

 shire, or from the Thames near Hampton Court. Being 

 now always filtered before it is supplied for domestic 

 use, it never contains any notable quantity of suspended 

 matter. In many districts, however, turf, sand, <bc., 

 are found in considerable quantities : it is necessary to 

 filter the water before use ; and we shall describe an 

 arrangement that may be employed for the purpose 

 an engraving of which is gubjoiifil. 



To construct one of these simple and effective filters, 

 the common garden-pot is 

 the only necessary vessel, 

 into the bottom hole of 

 which a piece of sponge or 

 rag may l>e placed. On 

 this, inside the pot, a 

 layer of coarse gravel is 

 laid ; over this, one of char- 

 coal, broken into sum!! 

 pieces, but not powdered. 

 .On the charcoal a layer of 

 fine sand is placed ; and, 

 over it, one of gravel. A 

 piece of calico or flannel 

 should be placed over the 

 gravel, so that when water 

 i.s poured thereon, it may 

 not run in channels through the different substances. 

 A string may be tied round the rim of the pot, and 

 the arrangement may thus be hung from a nail. Besides 

 the filtering effect, the charcoal has the power of re- 

 taining offensive animal and vegetable matter; so that 

 the results are both mechanical and chemical in the 

 removal of the impurities. The preceding engraving 

 will fully illustrate the method of making one of these 

 filters ; and shows the succession and position of each 

 of the substances employed. 



We may hare make a few observations on the im- 

 purities of water, as generally found in that supplied for 

 d >mestio and other such purposes, so far as they affect 

 sanitary and commercial matters. A table already 

 rot. i. 



given, states the proportions of organic and inorganic 

 substances present in water from various sources ; and 

 the nature of these substances we may inquire into. 



It is very essential, for culinary and manufacturing 

 purposes, that water should be as free from lime and 

 earthy matter as possible ; and the presence of these is 

 the cause of the hardness of some kinds of water. Lime 

 itself is but sparingly soluble in water; but when the 

 latter contains carbonic acid, then chalk or carbonate of 

 lime is readily dissolved in considerable quantities, as 

 the following experiment will prove. 



Experiment 39. Pour an ounce of lime-water that 

 is, water in which slaked lime has been dissolved into 

 a glass vessel, and add to it some common "soda-water," 

 which contains a large amount of carbonic acid. A 

 white precipitate of chalk is first formed ; but eventually, 

 the excess of carbonic acid present is so great, that the 

 chalk so formed will be dissolved and held in solution. 



If water thus impregnated with lime be afterwards 

 boiled, the excess of carbonic acid will be driven off, and 

 the chalk will be deposited in the vessel. Hence hard 

 waters are mostly rendered softer by the process of 

 boiling, their earthy matters being thus separated from 

 them. It is owing to the presence of earths, &c., 

 in water, that the tea-kettle becomes so constantly 

 furred. The same result takes place, on a very large 

 scale, in boilers used for steam-engines, and becomes a- 

 source of great annoyance and positive danger at times. 

 The earthy matter settles on the bottom and sides of the 

 boiler, and there produces a solid and stone-like mass. 

 This prevents the passage of heat from the fire through 

 the metal of the boiler to the water, and thus the iron 

 occasionally becomes red-hot, when the earthy crust is 

 thereby detached, allowing the water to touch the over- 

 heated surface. On this occurring, a sudden and rapid 

 production of steam ensues, which, having no adequate 

 means of escape, frequently causes dangerous explosions. 

 Hard water can be rendered soft by the addition of 

 lime to it, provided its hardness arise from the presence 

 of an excess of carbonic acid, as we have already 

 mentioned. This seems at first sight paradoxical, but is 

 easily explained. By the addition of lime to water con- 

 taining a large quantity of lime in solution, a portion of 

 the carbonic acid united thereto is abstracted ; and the 

 result is, tliat some of the lime already present is thrown 

 down in the solid state. The following experiment will 

 illustrate this fact. 



Experiment 40. To the water saturated with lime, 

 produced in the last experiment, add some more lime- 

 water. A white precipitate of insoluble chalk will be 

 [>r<luced, and a portion of lime previously existing in 

 solution will be removed. 



In a similar manner, hard water may be rendered soft, 

 on the large scale, by the addition of lime-water to it. 

 Dr. Clark, of Aberdeen, patented this process some 

 years ago, for its application to water before its supply 

 ;o towns, when much lime was present. We believe that 

 ;he plan was long employed at the Charlton and Wool- 

 wich water-works, the water being drawn from a chalky 

 soil, and largely impregnated with lime thereby. 



Sulphate of lime, or plaster of Paris, is often met 

 nth. It is always present in both the Thames and New 

 iliver waters, and in most rivers in Great Britain. Its 

 emoval is uot possible by any chemical means on the 

 arge scale ; and the only plan to soften water of this 

 cind is to boil it, or to add small portions of carbonate of 

 soda when it it used for domestic purposes. 



The remaining inorganic impurities, such as oxide of 

 ron, sulphate of magnesia, dzc. , are either present in such 

 mall quantities as to be of no consequence ; or, on the 

 )ther hand, abound so much as to constitute what are 

 ermed mineral waters. 



Mineral springs are frequently found in rocky and 

 ither districts; and the water is charged with gases, salts, 

 te., of various kinds. Amongst the gases are carbonic 

 acid and sulphuretted hydrogen. Oxide, or generally car- 

 jonate of iron, is frequently present in considerable quan- 

 ities ; so much so as to deposit a deep-red coating on all 

 he objects it comes in contact with. Sulphate of 



2i 



