In this country, such great practical results have been obtained by means of invention, that many persons suppose a sufficiency of inventive skill will enable us to effect every possible scientific object, and are surprised that no one can invent a plan of utilising the entire heat of coals, or a mode of overcoming the sewage difficulty, or prevent the great leakage of coal gas, or arrest epidemics, or produce a steam engine which shall work without waste of power. The progress of invention however depends upon that of discovery, and these various inventions, etc., wanted by manufacturers and others probably cannot be perfected until suitable _new_ knowledge is found. Every new invention has its own appropriate discoveries, by means of which alone it can be perfected; it was not possible to perfect the idea of an electric telegraph before the discoveries of Volta and Oersted were made. According to scientific laws, out of everything proceeds everything, and out of nothing, nothing can come, even ideas are not created. An unlimited number of inventions cannot be made by means of a limited amount of scientific knowledge; and our present stock of such information applicable to {73} invention, is very insufficient. One great reason why only a small portion of patents are of practical value; and so many useless ones are taken out is, that in consequence of our so-called "practical" spirit, we overestimate the power of invention and under-value the discovery of new abstract truths; because also invention has done so much, we think it will continue to do so, but the latter depends upon a continued supply of discoveries. Nearly every manufacturer is aware by painful experience of the great and almost incessant variation that occurs in the quality and properties of the materials used in his trade, and the frequent risk of failure of his process. In the manufacture of iron, for example, the presence of much phosphorus, sulphur, or silicon in the ore is liable to be very detrimental to the quality of the iron produced from it; in the manufacture of glass, the least quantity of iron in the materials will seriously injure the colour of the product; in the selection of copper for telegraph wire, if it contains the least trace of arsenic, the wire will not conduct the electricity properly. The difficulties experienced in procuring suitable materials for a manufacturing process are in some cases very great; and when they are procured, additional difficulties arise from the inability of the manufacturer or his manager to analyse them.