If we have a photographic friend who will supply us with quite a small quantity of gold chloride, we shall be in a position to try three most interesting experiments and to obtain some curious crystals. We require a very weak solution of gold chloride in water, not more than 3.5 per cent. for our first experiment. Mix one drop of this solution with the same quantity of hydrochloric acid on a slide and heat over a flame till dry. The microscope will now show us probably the most curious crystals we have ever examined; some are long, some short, and some a zigzag in form; mixed with these there will be a few flat plate-like crystals with rectangular projections. All these curious crystals are yellow. If instead of hydrochloric acid we use a solution of common salt in water and repeat the experiment as before, we shall obtain pale yellow prisms and some crystals of common salt. Gold is costly, so it is perhaps lucky that one of the tests for this rare metal is one of the most delicate known to chemists; it is possible, in fact, to detect very minute quantities of gold. For this experiment we may take an exceedingly weak solution of gold chloride and place a drop on our slide; we also require a solution of the chloride of tin, known as stannous chloride, in an evil smelling liquid called chlorine water. If now we watch our drop of gold solution under the microscope, and while watching mix with it a drop of stannous chloride solution, a strikingly beautiful purple colouration is produced--this purple has been named the purple of Cassius. [Illustration: _By the courtesy of Messrs. F. Davidson & Co._ 1. THE EYE OF A COCKCHAFER This section shows the eyelashes, the convex lenses, and, in the lower portion of the plate, the nerve fibres leading from the brain to the eye. 2. HOOKS ON BEE’S WING The row of hooks on the margin of the bee’s wing are clearly shown. By their aid the fore and hind wings are fastened together when the insect flies. ] If we desire further experiments in the testing of common substances, and incidentally in the production of beautiful crystals, we might do worse than try the effect of adding a solution of platinum chloride to any solution containing a compound of potassium. Charmingly beautiful crystals will result.