| Citrate iron and ammonium | 1-7/8 oz. |
| Red prussiate potash (C.P.) | 1-1/4 oz. |
Dissolve separately in 4 oz. distilled water each, and mix when ready to use. But by putting mixture in dark bottle, and that in a tight box impervious to light, it can be kept two or three weeks.
In some frames used at the School of Mines for making large blue prints a similar device has been in use for several years. Instead, however, of the heavy and cumbrous back used by Mr. Parsons, a light, somewhat flexible back of one-quarter inch pine is employed, covered with heavy Canton flannel and several thicknesses of newspaper. The pressure is applied by light pressure strips of ash somewhat thicker at the middle than at the ends, which give a fairly uniform pressure across the width of the frame sufficient to hold the back firmly against the glass at all points. This system has been used with success for frames twenty-seven by forty-two inches, about half as large as the one described by Mr. Parsons. A frame of this size can be easily handled without mechanical aids. Care should be taken to avoid too great thickness and too much spring in the pressure strips, or the plate glass may be broken by excessive pressure. The strips used are about five-eighths of an inch thick at the middle, and taper to about three-eighths of an inch at the ends.
The formulæ for the solution given by Whittaker, Laudy, and Parsons are practically identical so far as the proportions of citrate of iron and ammonia and of red prussiate of potash, 3 of the former to 2 of the latter, but differ in the amount of water. Laudy's formula calls for about 5 parts of water to 1 of the salts, Whittaker's for 4 parts, and Parson's for a little more than 2 parts. The stronger the solution the longer the exposure required. With very strong solutions a large portion of the Prussian blue formed comes off in the washwater, and when printing from glass negatives the fine lines and lighter tints are apt to suffer. The blue color, however, will be deep and the whites clear. With weak solutions the blues will be fainter and the whites bluish. Heavily sized paper gives the best results. The addition of a little mucilage to the solution is sometimes an advantage, producing the same results as strength of solution, by increasing the amount adhering to the paper. With paper deficient in sizing the mucilage also makes the whites clearer.—H.S.M., Sch. of M. Quarterly.
HOUSE DRAINAGE AND REFUSE.
A course of lectures on sanitary engineering has been delivered during the past few weeks before the officers of the Royal Engineers stationed at Chatham, by Captain Douglas Galton, C.B., D.C.L., F.R.S.
The refuse which has to be dealt with, observed Captain Galton, whether in towns or in barracks or in camp, falls under the following five heads: 1, ashes; 2, kitchen refuse; 3, stable manure; 4, solid or liquid ejections; and 5, rainwater and domestic waste water, including water from personal ablutions, kitchen washing up, washings of passages, stables, yards, and pavements. In a camp you have the simplest form of dealing with these matters. The water supply is limited. Waste water and liquid ejection are absorbed by the ground; but a camp unprovided with latrines would always be in a state of danger from epidemic disease. One of the most frequent causes of an unhealthy condition of the air of a camp in former times has been either neglecting to provide latrines, so that the ground outside the camp becomes covered with filth, or constructing the latrines too shallow, and exposing too large a surface to rain, sun, and air. The Quartermaster-General's regulations provide against these contingencies; but I may as well here recapitulate the general principles which govern camp latrines. Latrines should be so managed that no smell from them should ever reach the men's tents. To insure this very simple precautions only are required:
1. The latrines should be placed to leeward with respect to prevailing winds, and at as great a distance from the tents as is compatible with convenience. 2. They should be dug narrow and deep, and their contents covered over every evening with at least a foot of fresh earth. A certain bulk and thickness of earth are required to absorb the putrescent gas, otherwise it will disperse itself and pollute the air to a considerable distance round. 3. When the latrine is filled to within 2 ft. 6 in. or 3 ft. of the surface, earth should be thrown into it, and heaped over it like a grave to mark its site. 4. Great care should be taken not to place latrines near existing wells, nor to dig wells near where latrines have been placed. The necessity of these precautions to prevent wells becoming polluted is obvious. Screens made out of any available material are, of course, required for latrines. This arrangement applies to a temporary camp, and is only admissible under such conditions.
A deep trench saves labor, and places the refuse in the most immediately safe position, but a buried mass of refuse will take a long time to decay; it should not be disturbed, and will taint the adjacent soil for a long time. This is of less consequence in a merely temporary encampment, while it might entail serious evils in localities continuously inhabited. The following plan of trench has been adopted as a more permanent arrangement in Indian villages, with the object of checking the frightful evil of surface pollution of the whole country, from the people habitually fouling the fields, roads, streets, and watercourses. Long trenches are dug, at about one foot or less in depth, at a spot set apart, about 200 or 300 yards from dwellings. Matting screens are placed round for decency. Each day the trench, which has received the excreta of the preceding day, is filled up, the excreta being covered with fresh earth obtained by digging a new trench adjoining, which, when it has been used, is treated in the same manner. Thus the trenches are gradually extended, until sufficient ground has been utilized, when they are plowed up and the site used for cultivation. The Indian plow does not penetrate more than eight inches; consequently, if the trench is too deep, the lower stratum is left unmixed with earth, forming a permanent cesspool, and becomes a source of future trouble. It is to be observed, however, that in the wet season these trenches cannot be used, and in sandy soil they do not answer. This system, although it is preferable to what formerly prevailed—viz., the surface defilement of the ground all round villages and of the adjacent water courses—is fraught with danger unless subsequent cultivation of the site be strictly enforced, because it would otherwise retain large and increasing masses of putrefying matter in the soil, in a condition somewhat unfavorable to rapid absorption. These arrangements are applicable only to very rough life or very poor communities.