e inside, and may be used over a common fire, or on a stove. A is
the body, which may be made to hold from one to four gallons of water,
which is introduced at the opening b, which is then stopped by a cork.
The tube d connects the neck a of the still with the worm tub, or
refrigerator B, at e, which is kept filled with cold water by means of
the funnel c, and drawn off as fast as it becomes warm by the cock f.
The distilled water is condensed in the worm--and passes off at the
cock b, under which a bottle, or other vessel, should be placed to
receive it. The different joints are rendered tight by lute, or in its
absence, some stiff paste spread upon a piece of linen and wrapped
around them will answer very well; an addition of sealing wax over all
will make them doubly secure.

[Illustration: Fig. 16 (hipho_16.gif)]

HYGROMETER.--This is an instrument never to be found, I believe, in the
rooms of our operators, although it would be of much use to them, for
ascertaining the quantity of moisture floating about the room; and as
it is necessary to have the atmosphere as dry as possible to prevent an
undue absorption of this watery vapor by the iodine &c., and to procure
good pictures,--its detection becomes a matter of importance. Mason's
hygrometer, manufactured by Mr. Roach and sold by Mr. Anthony, 205
Broadway, New York is the best in use.

It consists of two thermometre tubes placed, side by side, on a metalic
scale, which is graduated equally to both tubes. The bulb of one of
these tubes communicates, by means of a net-work of cotton, with a
glass reservoir of water attached to the back of the scale. Fig. 17
and 18 represent a front and back view of this instrument.

Fig. 17 is the front view, showing the tubes with their respective
scales; the bulb b being covered with the network of cotton
communicating with the reservoir c fig. 18, at d.

[Illustration: Fig. 17 (hipho_17.gif)]

[Illustration: Fig. 18 (hipho_18.gif)]

The evaporation of the water from this bulb decreases the temperature
of the mercury in the tube b in proportion to the dryness of the
atmosphere, and the number of degrees the tube b indicates below that
of the other, shows the real state of the atmosphere in the room; for
instance, if b stands at forty and a at sixty-one the room is in a
state of extreme dryness, the difference of twenty-one degrees between
the thermometers--let a stand at any one point--gives this result. If
they do no