filtering paper, and throw away the first 300 c. c. passing through the paper. The residue must be filtered into a clean dry glass beaker. If not quite bright and clear, the liquid must be refiltered through the same paper. Measure off 250+51=255 c. c. of the clear filtrate into a porcelain basin, and add a good pinch of precipitated chalk free from chlorine (the amount is not important), and stir well. Then run in from a burette the iodide of starch solution, stirring all the time. The point at which the blue colour no longer disappears is noted, and the number of c. c. of the starch solution read off. If now we know how many c. c. of the iodide of starch correspond to each c. c. of silver solution, we can state, with great accuracy, the number of c. c. of silver remaining in solution in 255 c. c. of our filtrate (i.e., 250 of water), and this multiplied by gives the number per litre of water. We have, then, simply to subtract the c. c. of silver in excess from the added quantity in order to find the number used up in precipitating the chlorine in one litre of the water.

Took

Example.-Took one litre of well-water, added 2 drops of nitric acid, and treated with 20 c. c. of silver solution. Found excess of silver in a portion tested with common salt. This portion was not added to the bulk, but was thrown away. Filtered and rejected first 300 c. c. 255 c. c. subsequently of clear filtrate, treated with chalk, and added iodide of starch-6 c. c. of the starch liquid decolorised. Therefore, 24 (6X4) c. c. would be required for silver in solution in the whole 1020 c. c. Now 8 c. c. of our iodide of starch exactly represent 1 c. c. of the silver solution; therefore, 24-8=3 c. c. of silver in excess. Hence, in our experiment, 17 c. c. of the silver solution were used up in precipitating the chlorine in a litre of water; therefore, 17X1=1.7 centigrammes of chlorine are present in 1 litre of the water,2

If a sample of water happens to contain sulphuretted hydrogen, alkaline sulphides, or iron in solution, it is necessary to treat it as follows before proceeding as above:-Boil a litre for half an hour with a few drops of strong nitric acid, allow to cool completely, and make up to a litre again by addition of distilled water. Then treat with silver, as usual.

Nitrates and Nitrites.-Take 1 c. c. of a solution of nitre containing 1.8 centigrammes of the salt and evaporate to

15 c. c. is the fourth part of the 20 c. c. of silver added.

2 This was easily ascertained by adding 5 c. c. of silver solution to 250 c. c. of pure water, mixed with a little chalk. 40 c. c. exactly of the iodide of starch were decolorised, and 8 c. c. of the iodide are decolorised by 1 of the silver solution.

3 Prepared by dissolving 180 centigrammes of potassium nitrate in 100 c. c. of water.

dryness in a glass capsule. Place the capsule on a sheet of white paper, allow to cool, then add 2 c. c. of pure concentrated sulphuric acid, and stir well after throwing in a small clear crystal of green vitriol (ferrous sulphate); a dark brown tint will be produced.

Take the glass capsule containing the residue of the evaporation of 250 c. c. of water (see ante, page 230), and treat it in exactly the same way with sulphuric acid and green vitriol. If the water contains 1 centigramme of nitrogen per gallon as nitrate or nitrite, the tint developed will be of the same intensity as that caused by the nitre used above as a standard for reference.

This is a very rough but useful plan. When results of precision are required, the process we prefer is that of Thorpe, which will be found fully described at page 315 of Thorpe's Quantitative Analysis; but since the estimation is not one of much practical importance to the Sanitary Officer, we content ourselves with giving the reference to a good description of the process.

238

CHAPTER XX.

HOUSE CONSTRUCTION.

SITE-Of Town and of Country Houses-Aspect-Subsoil-Shelter. WALLS-Solid and Hollow-Warmth and Dryness, how secured in construction of Walls. AccoMMODATION. SANITARY ARRANGEMENTS.— Water-closets-House Drains-Precautions to be observed in LayingSewer-gas-Modes of its Entrance into Houses-Removal of Sewergas-Ventilation of Sewers.

Site. In towns sites can seldom be chosen by the person who purposes building, as the site is to a great extent, if not altogether, determined by the laying out of the streets of the town. In laying out new streets or new towns, care should be taken to accommodate them to the sites of future houses. Sufficient space should be secured both in front and rere for any houses which may be built. This space should be sufficient, not only for free traffic, but also for free circulation of air; and care should be taken that sufficient space is preserved between opposite houses to prevent the houses on one side from intercepting the direct sunlight from the other.

For country houses or cottages sites should be elevated, so that a fall from the building may be secured in one direction at least. The house should be exposed to the south or southeast when possible. The following Table, taken from Mr. Eassie's papers in the Sanitary Record,' will show why these aspects are selected :—

It is doubtful whether a south-west is really the "worst aspect." Probably east is equally bad or worse. It is too common to sacrifice the aspect of a house with the object of obtaining a good prospect.

The subsoil upon which the house stands should be porous; retentive soils, or gravels which may be retentive, should be avoided where possible; and where a house must be built on a retentive soil, great precaution must be taken effectually to drain the subsoil, and to obviate the dampness of the site as much as possible by the use of concrete. The site must also be so chosen that sufficient facilities shall be secured for drainage and water supply. Shelter should also be secured; this is best effected by trees planted between the house and the points of the prevailing cold or moist winds. It is a bad plan to calculate on steep hills for shelter, as these, in many cases, create currents, which are almost as injurious as the regular prevailing winds. If the house is to be a large one, surrounded by pleasure-grounds, the relations between the house, lawn, and gardens, must be taken into consideration in the choice of site.

Walls. In the building of walls of all houses, especially those for dwelling in, the first objects to be obtained, from a sanitary point of view, are warmth and dryness. If the walls are too thin, they will be easily affected by changes of temperature, which will not only tend to produce sudden chills of the internal air of the house, but will also produce deposits of moisture on the internal surface when sudden falls of temperature occur, with a previously damp atmosphere. Thus the mere thinness of a wall may produce a damp interior, although the wall may be quite impervious to moisture. Thin walls are the great drawback to most modern houses. We think walls should never, under any circumstances, be less than twelve inches thick. With a view of preventing dampness of walls, two points must be attended to-first, to prevent the wall being directly penetrated by drifting rain from the outside; and, secondly, to

prevent the saturation of the wall by ground moisture rising from below. The first object will be attained if the walls be sufficiently thick, well constructed, and of good materials. The surest way of effecting this object, with reasonable economy of materials, is by the use of hollow walls—that is, by, as it were, building two walls and attaching them to one another by ties of brick or iron placed at intervals. This plan is strongly recommended by Mr. Bridgeford, of this city, as the most effective. The intermediate space should be ventilated by perforated bricks. By this means sudden cooling of the inner wall is prevented by the air-jacket placed between the walls, and the possibility of drifting rain permeating to the inside is completely avoided.

To accomplish the second object-namely, to prevent the rising of damp in the walls-many plans have been proposed, but the only effectual one is the interposition of a dampproof course at some distance above the ground. The distance of the damp-proof course above ground should be such as to prevent the possibility of the wall being wetted above by the gradual accumulation of the soil or gravel of the adjoining flower-beds or walks, and also sufficiently high to prevent much soakage of water from the splashing of rain from the ground in wet weather. Many materials are employed for making these damp-proof courses-bricks set in cement, cement itself, tiles, lead, bricks or tiles set in coal tar, felt, glazed tiles, and asphalt. The last is, we believe, most to be recommended, and is stated to be preferable by Mr. Henderson, in his Lecture on the Construction of Dwellings.1

Accommodation.-We have next to consider the amount of accommodation required for each individual or family.

The greater the amount of accommodation the better, provided the size of the house or apartments does not exceed the means of the occupier to maintain the premises in a

1 Lectures on Public Health, Royal Dublin Society. P. 153.