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DYEING AND CALICO PRINTING.

All colours are produced indirectly by colouring substances, and owe theirorigin to light. We perceive objects and colours by means of rays oflight proceeding from them to the visual organs. Light, as it proceeds fromthe sun, contains the elements of all colour, and can be analysed or decom-posed by a glass or crystal prism, which will separate it into seven differenthues. Only three of these appear essential, the others being produced bycombinations of these three. The three primary colours are red, yellow, andblue, their union producing green, violet, orange, and indigo. The chemical sub-stances, prussian blue and vermillion for instance, are not in themselves blue orred, but appear so because they possess the power of decomposing light. Prus-sian blue absorbs and destroys the red and yellow rays, leaving only the bluevisible; while vermillion, on the contrary, nullifies the blue and yellow raysand reflects the red ones. It is the same with all colours ; a colouring mattershould be looked upon as a body having the power of reflecting undulations ofa certain speed ; colour cannot exist in solids, but it is an action of the undula-tions of the luminiferous ether on the organs of sight. Ail colours are alike inthe dark; but they are not always the same in daylight as in artificial light,and chemical substances may change colour, or lose it altogether, withouttheir chemical composition being affeCted. Effects of colour may be producedby light in two different ways, either by reflection from a surface which appearscoloured—the more common phenomenon—or by transmission of light through acoloured transparent body, such as stained glass or liquid. Some substancesvary in colour when seen by transmitted or reflected light; crystals ofmurexide are green by reflected and red by transmitted light, and there are anumber of similar cases. This is an additional proof that colour is not dueto a body itself, but to the position or physical arrangement of its particles.

It is not intended to enter into any other details of either physics orchemistry; numerous excellent works exist upon those subjects, and a briefresume would necessarily be too incomplete to be of any real utility.

Water—Tests for Impurities.

This fluid is of no less importance from an industrial and manufacturingpoint of view than in its relations to man and the whole organic creation.Although it exists in great abundance and covers by far the largest partof the surface of our globe, it is never met with in a state of absolute purityit must be remembered that we are now considering water more especially inrelation to dyeing and printing. Perfectly pure water, even if it could beobtained upon a large scale without much cost and trouble, would not in allinstances answer the practical purposes of the dyer and printer. Experienceteaches that in madder dyeing, for instance, especially with some preparationsof that substance, the presence of carbonate of lime in water is considered tobe very advantageous.

The following substances have been detected in natural waters: carbonic,nitric, nitrous, sulphuric, sulphurous, phosphoric, boracic, silicic, hydrochloric,and hydrosulphuric acids. The presence of the following basic substances,potassa, soda, lithia, ammonia, strontia, baryta, lime, magnesia, alumina,oxides of iron, manganese, zinc, copper, lead, silver, arsenic, nickel, andcobalt has been ascertained. Chlorine, bromine, iodine, fluorine, sulphur,and carburetted hydrogen have also been discovered in natural waters.