(a) Reinsch’s test. A portion of the suspected paper (two or three inches square) is cut into small pieces, and placed in a good-sized test tube; water is added until the tube is about a third full and then one or two teaspoonfuls of pure hydrochloric acid, and a small piece of pure copper foil. If the test tube is now heated for a few minutes over a spirit lamp, arsenic, if present, will be deposited as a black or dark steel-coloured coating on the copper. A mere tarnish of the copper must not be accepted as evidence of the presence of arsenic, but an almost complete obliteration of the colour of the copper.
(b) Take the copper covered with arsenic, dry it, and then heat it in a perfectly dry test tube. Crystals of white arsenic, which may be identified under the microscope, will be deposited higher up in the tube.
(c) Marsh’s test. The ordinary apparatus for developing hydrogen by the action of diluted sulphuric acid on zinc is employed, the suspected paper being inserted in the bottle. The hydrogen coming off is burnt, and a clean porcelain surface is applied to the flame. If there is arsenic in it, it is deposited on the porcelain in a black patch.
Windows are required to open directly into the external air in every habitable room. The window area according to the model bye-laws of the Local Government Board and the London Buildings Act of 1894, must be at least one-tenth of the floor area, and half of this at least must be made to open. The following rules have also been given. (B = breadth, L = length and H = height of room.)
| Area of window |  | (B × L) ∕ 10 | London Building Act |
| (B × L × H) ∕ 100 | Gwilt | ||
| √(B × L × H) | Morris |
In a room measuring 15 × 20 × 12 feet, the preceding rules would give a superficial area of window space of 30, 36, and 60 square feet respectively. Plate glass dissipates heat less quickly than sheet glass.
Objection may be taken to plate glass windows, in passing, especially for shops, banks, etc., in view of the fact that they are commonly made without any arrangement for ventilation (see also page [148]).
The hygienic necessities of Floors are that they shall be impervious to moisture and to dust. On the ground floor the ordinary arrangement is to provide a joisted and boarded floor raised about a foot above the ground. Dry rot is one of the dangers in connection with such boarded floors on the ground floor. The chief causes which tend to induce rotting, are damp walls, lack of ventilation, contact with mortar, damp earth, or vegetable mould, and worst of all, alternations of damp and dryness, or wet along with heat.
In order to avoid these dangers in connection with boarded floors, the ends of all timbers resting on walls should have a clear air-space around them, and communicate with the external air by means of perforated bricks. The larger timbers, girders, etc., should rest on stone templates, and the smaller joists on hoop-iron bonds. In all cases, the timber used should be well seasoned, and properly ventilated. The ends of oak posts, which are to be driven into the ground, should be charred, if the timber is old, or steeped in a solution of chloride of zinc.
The ends of the joists should be trimmed, so as not to come too near to chimney flues.