A word of warning is necessary concerning this and the following experiments; they may not always succeed, for success largely depends upon having the solutions at the right strength, and experience alone can teach us what is correct. Interesting and easily formed crystals may be obtained from barium sulphate, and the experiment may also be made to show the phenomenon which we have already mentioned, that the form of the crystals depends on the nature of their solution. A little barium sulphate should be dissolved in strong sulphuric acid. Here, by the way, another warning: every care should be taken in the use of all acids; they should never be allowed to come into contact with face, hands or clothes, nor should they touch any part of the microscope. Some acids give off fumes, and these should not be allowed to reach the eyes or nose, and the microscope must be protected from them. To continue our experiment: While hot a drop of the solution of barium sulphate in sulphuric acid should be transferred by means of a glass rod to a slide and allowed to cool. Examination with the microscope will show that the barium sulphate has formed small rectangular scales. With the remainder of the sulphuric acid we now make a saturated solution of barium sulphate, and find, on repeating the method described above, that the chemical has formed curious x-shaped crystals. Similar experiments may be tried with calcium sulphate with the certainty of interesting results.
Many crystals of calcium, in the form of calcium oxalate, may be found in plants, and they are well worth looking for. They may best be seen in sections of the plants, but, if we have not mastered the art of cutting sections, we may find them by teasing the plant cells apart with our mounted needles. In the stems of rhubarb there is the substance in bundles of long needle-shaped crystals, to which the name of raphides has been given. In the seed of the garden poppy, just below the skin, there is a layer known as the crystal layer, where crystals of calcium oxalate occur as tiny balls called crystal sand. In the leaf stalks of begonias very beautiful and occasionally very large crystals of this substance may be found, whilst in shapes they are strikingly varied. In orris root there are enormous crystals of calcium oxalate; in fact it is common in many plants.
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.
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.