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 formulae 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
t