The experiments we have described as well as hundreds of others are used by analysts every day in the testing of various substances. We have started in every case by knowing what our solutions contain; the duty of the analyst is to discover what he has before him. Given an unlimited quantity of a substance for testing purposes it is not always easy to determine its composition. With very small quantities, perhaps less than a tea-spoon full in all, the difficulties of the analyst are increased tenfold and without the assistance of the microscope his efforts would be unavailing, he deals in drops and every drop is precious. Sad to relate this form of testing, known to science as micro-chemical analysis, has been practised to a far greater extent on the continent than in this country. Those of our readers who wish to try the experiments we have described for themselves can obtain all the necessary substances, except the poisons, at any chemists and the quantities required will only be the smallest that can be obtained, in fact any reasonable-minded chemist would probably let a microscopist whom he knew have a few grains of a large number of chemical substances, suitable for this work, at the outlay of a few pence. The chlorides of gold and platinum we fear no one will give away. All the experiments we have described thus far have necessitated the use of what are called inorganic substances, they may be described in every day language as substances derived from the inanimate world. There are many equally interesting tests which may be carried out with animal and vegetable products. Formic acid is the substance which renders the sting of ants so painful; it may, however, be prepared artificially and if a little, dissolved in water, is mixed with a solution of silver nitrate we shall obtain flat plate like crystals also some resembling fine fibres. Probably the most curious of the easily obtained crystals from vegetable products, may be made from citric acid which occurs in lemons. If a little of the acid be mixed with a solution of caustic soda and boiled with calcium chloride, a drop of the liquid after boiling placed on a slide will give crystals readily. When viewed from above they are an elongated oval shape, described by some authorities as resembling a whetstone. Viewed from the side they have a striking similarity to small sheaves of wheat. With the recognition of poisons under the microscope we need not trouble ourselves here. It would be useless to describe any of the experiments, for few of us could obtain such deadly substances as nicotine, strychnine, aconite, morphia and the like. Nevertheless the recognition of these and similar substances in very minute quantities is rendered easy, to those who have the necessary knowledge, by means of the microscope. CHAPTER XIX THE USE OF THE MICROSCOPE IN MANUFACTURES