Page:Popular Science Monthly Volume 39.djvu/734

716 imposed upon the teacher should be enthusiasm, life, growth. When these are absent, the teacher is dead. Knowledge, to be of any value to the teacher, must become a permanent, increasing, living force in his work and character. Knowledge which is non-productive is dead. Knowledge which is alive, which strengthens the memory, which guides the judgment, which enlightens the reason, which fortifies the will—this is the knowledge which, acting through his individuality, makes the teacher a power in the school.

Hand-marks.—M. Bertillon, of the Paris police, has devised a method of identification by photography of parts of human bodies. The hand being the part that is usually most affected by the occupation, series of photographs of hands have been taken, which may be compared with whole figures of the same workmen at their work. They show the effect upon the organ of friction from tools in use. From the hands of the navvy all the secondary lines disappear, and a peculiar callosity is developed at the spot rubbed by the spade-handle. The hands of tin-plate workers are covered with little crevasses produced by the acids employed. The hands of lace-makers are smooth, but they have blisters full of serum on the back and callosities on the front part of the shoulder, due to the friction of the straps of the loom. The thumb and the first joints of the index-finger of metal-workers show large blisters, while the left hand has scars made by the sharp fragments of steel.

Tea-culture.—According to a Society of Arts lecture by Richard Bannister, tea is derived from the cultivation of two species of tea plants, the Chinese and the Assamese. Hybrids of various degrees between these two form a great proportion of the plants usually grown. In the tea-garden the plant is kept down to from three to six feet in height; in a state of nature it reaches thirty or forty feet, with a stem one foot in diameter. The seed, which is inclosed in a hard, round shell, ripens about one year after the flower has faded. Planting is done either direct from the seed itself, or from nurseries where the young plants can be watched carefully and tended till they are strong enough to take their places in the plantation. Tea grows on almost all soils, but one that is light, friable, and rich is necessary for complete success. Close planting is recommended—viz., four feet apart—equivalent to 2,722 shrubs per acre. On steep slopes the Chinese variety may be planted closer—two feet by three and a half feet, or 6,223 plants per acre. A good deal of care must be devoted to pruning, with the object of keeping the shrub well spread and at a convenient height for picking. A tea plant is picked as the successive "flushes" occur. A flush is the throwing out of new shoots and leaves, the latter of which form the tea of commerce. The average flushing period is from seven to nine months, and the intervals between flushes vary from seven to fourteen days. The number of flushes ranges from eighteen, where no manure is used, to twenty-five in good soil. To a certain extent, the harder a tea plant is picked, the more it becomes stimulated to reproduce new shoots in the place of those lost. When the season is over, the tea bush is from three and a half to four feet in height and about five feet in diameter; pruning down, its height is reduced to two feet and its diameter to three feet. In this state it remains during hibernation. In the spring the buds at the base of the leaves develop into shoots, the buds of which develop themselves in the same way. The first shoot from the branch becomes the nucleus of subsequent flushes on that part of the bush, and is therefore carefully preserved. The youngest leaves give the best tea.

Making Incandescent Lamps.—An incandescent or glow lamp consists, according to Major-General Webber, from the manufacturing point of view, of the filament, the wire mount, or conductor, and the glass bulb. Inventors, seeking a highly refractory substance out of which to make the filaments, have all ended in using carbon of either a fibrous or an amorphous consistency. The form of the filament has been governed by the need to hold within a bulb of given size a carbon of a given length. The filament must have a uniform section, and that is most certainly obtained in those which are formed by squirting a viscous solution of cellulose into a precipitating solution. The filament material is wound upon blocks