ny of the acid adheres to it, it is very subject to the action of
light. From 100 grains of mercury, about 130 of the powder are
obtained.

This powder, when struck on an anvil with a hammer, explodes with a
sharp stunning noise, and with such force as to indent both hammer and
anvil. Three or four grains are sufficient for one experiment.

_The Iron Tree._

Dissolve iron filings in aqua fortis, moderately concentrated, till
the acid is saturated; then add to it gradually, a solution of fixed
alkali, (commonly called oil of tartar per deliquum.) A strong
effervescence will ensue, and the iron, instead of falling to the
bottom of the vessel, will afterwards rise so as to cover the sides,
forming a multitude of ramifications heaped one upon the other, which
will sometimes pass over the edge of the vessel, and extend themselves
on the outside, with all the appearance of a plant.

_To make any Number divisible by Nine, by adding a Figure to it._

If (for example) the number named be 72,857, you tell the person who
names it to place the number 7 between any two figures of that sum,
and it will be divisible by 9; for if any number be multiplied by 9,
the sum of the figures of the product will be either 9, or a number
divisible by 9.

_Arithmetical Squares._

An arithmetical magical square consists of numbers so disposed in
parallel and equal lines, that the sum of each, taken any way of the
square, amounts to the same.

Any five of these sums taken in a right line make 65. You will observe
that five numbers in the diagonals A to D, and B to C, of the magical
square, answer to the ranks E to F, and G to H, in the natural square,
and that 13 is the centre number of both squares.

_A Natural Square._ _A Magical Square._
A G B A B
+--+--+--+--+--+ +--+--+--+--+--+
| 1| 2| 3| 4| 5| |11|24| 7|20| 3|
+--+--+--+--+--+ +--+--+--+--+--+
| 6| 7| 8| 9|10| | 4|12|25| 8|16|
+--+--+--+--+--+ +--+--+--+--+--+
E |11|12|13|14|15| F |17| 5|13|21| 9|
+--+--+--+--+--+ +--+--+--+--+--+
|16|17|18|19|20| |10|18| 1|14|22|
+--+--+--+--+--+ +--+--+--+--+--+
|21|22|23|24|25| |23| 6|19| 2|15|
+--+--+--+--+--+ +--+--+--+--+--+
C H D C D

To form a magical square, first transpose the two ranks in the natural
square to the diagonals of the magical square; then