Chemical analysis is founded upon the fact that when certain chemicals are mixed together they will, or they may, unite to form quite a different chemical. This newly formed chemical is probably a different colour to the substances which were used in its making, or again the original chemicals may be soluble in water and the new chemical insoluble, in which case it will form a cloudiness known as a precipitate. An example may help to make our description clearer. Suppose we take some common table salt and dissolve it in a little water in a glass, then add to this a little solution of nitrate of silver, which is sold under the name of lunar caustic, we shall find that a white cloudiness is formed when the liquids mix, although originally they were both clear. The reason of this cloudiness is that the two substances, dissolved in water have united with one another to form a third substance which will not dissolve, therefore it settles down as a fine powder. Long experience has taught analytical chemists exactly what chemicals to add to test for all the common substances, by the formation of these precipitates; so that, if any powder were given to one of these scientists he could tell its composition by applying certain tests. In this chemical analysis considerable quantities are required and it is often necessary to test very small samples, so small that the ordinary methods are out of the question. This is where the microscope scores, because with this wonderful instrument, only drops are required and tests of corresponding delicacy may be made; in fact, by modern microscopic methods it is possible to detect the presence of as little as ten thousandth part of a grain of arsenic, or quicksilver or of the deadly poisons, Strychnine or Prussic Acid.
In testing for poisons the microscope is invaluable. Frequently only the most minute traces of the poison occur in the system and the modern microscopist who makes a study of poisons and their detection can solve mysteries which would have baffled all the scientists in the world in days gone by.
Our chemical studies with the microscope may well begin with various common crystals; they are usually easy to prepare; the process of crystallization is always interesting to watch, and as objects for the microscope it would be difficult to find anything more beautiful than these home-made gems. All crystals should be examined by reflected as well as by transmitted light. When we are working with the former lighting, a piece of black paper beneath the slide will help to show up the objects to better advantage.
The easiest method of obtaining crystals of any substance which is soluble in water is to make a saturated solution in this liquid, to put a small drop upon a slide, then to tilt the microscope slightly so that there is a thin film of solution at the upper side of the drop. The microscope must not be tilted so much that the liquid runs from the slide on to the stage. Where the film of liquid is thin crystals will be found first. A word of explanation is necessary concerning the term saturated solution, especially as we may have occasion to use it many times. When we add a solid to a liquid in which it is soluble, we shall find that the liquid will take up a certain amount of the solid and no more; when, on the addition of more solid it fails to dissolve, we have reached the saturation point. A saturated solution then is a liquid in which the maximum amount of solid is dissolved.
We have already described the most simple way in which crystals may be formed, and we may easily make objects for our microscope in this manner of all the substances we can find which are soluble in water. With some we shall find that crystals do not form easily, in which event we may modify our tactics and warm one drop of saturated solution on the slide till nearly all the moisture is driven off, then there should be no difficulty in watching the crystals in process of formation. Common salt, sugar, alum, borax, washing soda, iron sulphate, called also green vitriol and copper sulphate, or blue vitriol, are common and easily obtained substances, all soluble in water. As we carry our experiments a little further we shall find that crystals formed from cold solutions as suggested in our first method differ from those formed from hot solutions. Again, if we use some other substance than water as our solvent the crystals which separate out will differ once more. Many very interesting experiments may be tried on these lines.
Many beautiful crystals may be obtained by dissolving various substances in gelatine or gum. The method is simple. Gently warm a little gelatine, to which is added an equal volume of water, in a chemist’s test-tube. When the gelatine has all dissolved make a saturated solution of the substance, from which crystals are derived. Green or blue vitriol are good subjects for the experiment. Add a little of the saturated solution to the gelatine and stir with a glass rod, taking care to avoid the formation of air bubbles. A little of the mixture may now be placed in a thin film on a slide covered with a cover slip and left to cool. Examination when cool under the microscope will show lovely fern-like crystals, whose beauty rivals that of ice crystals familiar to us on our window panes during hard weather in the winter. Barium chloride also produces very beautiful fern-like crystals when treated in this manner. Chlorate of potash, familiar to most of us as a remedy for sore throat, forms crystals totally dissimilar to those substances we have named. Having made as many crystals as we wish from water and also from gelatine solutions, we may turn our attention to gum arabic. The method of obtaining crystals from this substance is exactly similar to that described for gelatine except, of course, that we substitute gum for gelatine. This work is of the greatest interest, for not only does it yield wonderfully beautiful objects for our microscope, but it is a hobby full of surprises. When we are about to examine a new crystal for the first time we can never so much as hazard what shape it will assume. Sugar, it may be mentioned, does not crystallize at once from a saturated solution in water, and the best method of obtaining the crystals is to warm the slide, on which we have placed a drop of solution, and then when dry to set aside for a day; at the end of that time, especially if the air be moist, the crystals will have formed.
We shall find it interesting to try experiments in mixing two different substances, then we shall probably obtain crystals totally unlike those of either of the ingredients. As an example of this method, let us make a saturated solution of a mixture of blue vitriol and magnesium sulphate in water. Place a drop of the solution on a slide, heat over a flame, not only till dry, but till the substance left on the slide begins to melt. We must use every care not to crack the slide, and this may best be accomplished by keeping it moving while over the flame. Now if we watch our object through our microscope we can witness the formation of wonderful feathery crystals, as the slide cools.
Some strikingly beautiful results may be obtained in another manner; the method is used by analysts in their so-called fusion tests. We take a small grain of some substance, say, lead nitrate, place it in the centre of our slide, cover with a cover slip, and warm over a flame till it melts. Then, taking a similar-sized grain of another substance, such as saltpetre, we place it against the edge of the cover slip on the side opposite to the lead nitrate. Further warming will cause the saltpetre to melt, and run below the slide and mix with the lead nitrate. If we watch the meeting of the two chemicals a wonderful sight will reward us. The lead compound forms a “beautiful crystalline skeleton,” whilst the saltpetre forms six-sided stars at the opposite side of the cover slip. The experiment may be repeated, using lunar caustic and saltpetre, also the iodides of silver and potassium; in fact we may try any chemical substances we have at hand, though we shall find that some do not melt very easily, and potassium chlorate is somewhat dangerous, for it forms explosive compounds with certain substances.
The number of interesting and beautiful objects which may be obtained by so-called solution tests is practically unlimited, and the enthusiastic microscopist will certainly try all the tests we describe as well as many others of his own invention. Should the chemist wish to detect the presence of aluminium in very small quantities, he relies on his microscope and proceeds in the following manner. He takes a drop of the solution, suspected of containing alum, and adds to it a drop of sulphuric acid. This mixture he puts on his slide, which he heats over a flame till dry. Now he adds a drop of water, and then a very small amount of calcium chloride is brought to the edge of the water. Beneath his microscope he can now watch the formation of beautiful colourless, eight-sided crystals which denote the presence of aluminium.