‘Plaster-casts are made by pouring plaster of Paris, mixed to a creamy consistence with water, into a mould made of many pieces, which pieces are again held together in an outer or “case-mould” of two or three pieces. When the mould is removed piece by piece, fine ridges stand up on the cast where the plaster has flowed between these pieces. These ridges are removed by rubbing them obliquely with the surface of the stem of the dried equisetum. It cuts away the plaster as rapidly as a file, but without leaving any visible file-marks. The surface left is much smoother than from fine emery or glass-paper, and the rush does not clog nearly so fast as the paper. ‘In order to find the explanation of this, I carefully burned some small pieces of the equisetum stem, mounted the unbroken ash on microscope slides with Canada balsam, and examined its structure. This displayed a flinty cuticle, a scale-armour made up of plates of silica, each plate interlocking with its neighbours by means of beautifully regular angular teeth, forming myriads of microscopic saw-blades, which become loosened from each other and crumpled up in drying, and thus present their teeth obliquely to the surface. These teeth supply the image-maker with a file of exquisite fineness, and harder than the best Sheffield steel. Their comparative freedom from clogging I think must be due to their loose aggregation while held by the dried and shrivelled woody tissue of the sub-cuticle. ‘This natural file is used for other purposes, such as the polishing of ivory, hard woods, and metal, but is only understood in certain obscure industrial corners. I here commend it to the attention of my readers, because I have just discovered a new use for it. Like many others, I have been occasionally troubled by minute irregularities of the teeth, lacerating the tongue, and producing small ulcerations, which, I am told, are dangerous to those who have passed middle age, being provocative of cancer. A friendly dentist has ground down the offending projections with his emery-wheel, and thus supplied relief. But in course of time other sharp angles have stood forth, but so trivial that I felt ashamed of visiting the torture-chamber for their removal. I tried emery paper; but it was ineffectual and unpleasant, as the emery rubbed off. Then I tried the Dutch rush, rubbing its surface crosswise and obliquely against the offending angles. The success was complete, both grinding down and smoothing being effected by one and the same operation.’ LIGHTNING-STROKES IN FRANCE.