Most of us have kept silkworms, those who have not may find it worth while to expend a few pence in some of these insects for the sake of examining the raw silk. A cocoon, as the work of the caterpillar is called, consists of three layers, an outer layer of floss, a middle layer and a so-called layer (the inner layer) of parchment. Only the middle layer is used in commerce, the floss is too fine and weak and the parchment is so impregnated with silk glue as to be useless. If we examine some raw silk, taken from the middle layer of a cocoon, we can easily see the two parallel fibres of silk and the outer wrinkled covering of silk glue. Now, magnifying our object more highly, we shall see that each fibre is a solid rod, with a smooth lustrous surface and without any sign of lumen or cell structure; the rods too are continuous and this alone distinguishes silk from all other fibres animal or vegetable. Two tests are worth trying for they are characteristic of silk. On the addition of a little strong sulphuric acid we observe that the silk rapidly dissolves, on the other hand, if a few fibres are boiled in hydrochloric acid, the silk dissolves but the envelope of silk glue remains unchanged and appears beneath the microscope as a cracked and wrinkled tube. The caterpillar of another moth spins coarser greyish coloured fibres, which are spun into the well known Tussore silk. As with the common silkworm these caterpillars spin two fibres and glue them together with silk glue. In this case, however, under a high magnification we shall notice that the fibres are marked with a number of very fine lines running lengthways, whilst every now and then there are fairly deep indentations. The former markings are natural to the fibres, the latter are caused by one fibre being pressed against the other. In countries where the production of silk is of great importance, the microscope is not only pressed into service for examining the product of these useful little insects, but also in keeping watch for a very deadly disease which attacks the caterpillars. It is called “pebrine” and the great scientist Pasteur, whose name is world famous for his work on bacteriology, discovered that it was caused by a tiny fungus. Artificial silk is an important article of commerce. It is made in several different ways and of various substances. Some artificial silk is made of collodion, some again is made of cellulose the substance of which the cell walls of young plants is composed; gelatine is also used in making this commodity. As a rule it is easy to distinguish artificial from real silk, for usually the imitation consists of flat fibres or at least fibres quite different to the smooth rods of real silk. The iodine test is often sufficient indication, for with this chemical true silk is coloured brown.