of a larger quantity.
If it is wished to get a somewhat more definite idea of the amount, it is
easy to make up a series of standards for comparison, by dissolving known
weights of common salt in distilled or rain water, and testing samples of
them side by side with the water to be examined.

Organic matters may be detected by adding a little nitrate of silver to the
water, filtering off from any precipitate of chloride of silver, and
exposing the clear liquid to sunlight; a clean stoppered bottle being the
most convenient vessel to use. The extent to which a blackening takes place
may be regarded as approximately proportionate to the amount of organic
matter present.

Filtration on a small scale is not altogether a satisfactory mode of
purifying water, as organic impurities often accumulate in the filter, and
enter into active putrefaction when hot weather sets in.--_Photo. News._

* * * * *

IMPROVED FIRE-DAMP DETECTER.

According to the London _Mining Journal_, Mr. W.E. Garforth, of Normanton,
has introduced an ingenious invention, the object of which is to detect
fire-damp in collieries with the least possible degree of risk to those
engaged in the work. Mr. Garforth's invention, which is illustrated in the
diagram given below, consists in the use of a small India rubber hand ball,
without a valve of any description; but by the ordinary action of
compressing the ball, and then allowing it to expand, a sample of the
suspected atmosphere is drawn from the roof, or any part of the mine,
without the great risk which now attends the operation of testing for gas
should the gauze of the lamp be defective. The sample thus obtained is then
forced through a small protected tube on to the flame, when if gas is
present it is shown by the well-known blue cap and elongated flame. From
this description, and from the fact that the ball is so small that it can
be carried in the coat pocket, or, if necessary, in the waistcoat pocket,
it will be apparent what a valuable adjunct Mr. Garforth's invention will
prove to the safety-lamp. It has been supposed by some persons that
explosions have been caused by the fire-trier himself, but owing to his own
death in most cases the cause has remained undiscovered. This danger will
now be altogether avoided. It is well known that the favorite form of lamp
with the firemen is the Davy, because it shows more readily the presence of
small quantities of ga