k upon the palm of the hand, at the point where the
hole appears to be, and again observe as before. Unless the attention be
strongly concentrated upon objects seen through the tubes the ink-spot
will be visible within the tube (apparently), but that part of the hand
upon which it rests will be invisible, unless special attention be
directed to the hand. Ordinarily the spot will appear opaque. By
directing the tube upon brilliantly illuminated objects, it will,
however, appear transparent, and may be made to disappear by proper
effort. By concentrating the attention upon the hand, it may also be
seen within the tube (especially if strongly illuminated), that part
immediately surrounding the ink spot appearing first.

3. Substitute for the hand a sheet of unruled paper, and for the ink
spot a small hole cut through the paper. The small hole will appear
within the tube, distinguishing itself by its higher illumination, the
paper immediately surrounding it being invisible. Many other curious
experiments will suggest themselves. For example: if an ink spot
somewhat larger than the tube be observed, the lower end of the tube
will appear to be blackened on the inside.

ICE MACHINES.

Ice machines are constructions designed to employ the heat generated
from coal in extracting the heat stored up in water at the ordinary
temperature. One ton of coal will make 15 tons of ice, and yet only
about 1 per cent. of the power used is utilized, these machines being
especially wasteful of heat. The work is done through the medium of some
volatile fluid, like ether or ammonia, or by the use of previously
cooled air. Raoul Pictet, who advocates the employment of another
fluid--sulphurous acid solution--says that every machine must comply
with five conditions: 1. Too great pressure must not occur in any part
of the apparatus. 2. The volatile liquid employed ought to be so
volatile that there will be no danger of air entering. 3. It is
necessary to have a system of compression which does not require the
constant introduction of grease or of foreign materials into the
machine. 4. The liquid must be stable, it must not decompose by the
frequent changes of condition, and it must not exert chemical action on
the metals of which the apparatus is constructed. 5. Lastly, it is
necessary, as far as possible, to remove all danger of explosion and of
fire, and for this reason the liquid must not be combustible. The only
substance, in his opi