218 On Practical Scientific Instruction. [April, 



The selection of materials for manufactures by the "rule of 

 thumb" alone leads to equally imperfect results. 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 consequent 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 very 

 detrimental to the quality of the iron produced from it ; in the ma- 

 nufacture 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 experi- 

 enced in getting 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 analyze them. 



Every manufacturer is also aware that the difficulties encoun- 

 tered in manufactures are not limited to the substances employed, 

 but extend to all the different processes and stages of processes 

 through which these substances have to pass, and to all the forces, 

 tools, machinery, and appliances employed in those processes; in 

 the manufacture of glass, for example, the greatest care has to be 

 exercised in the making and gradual heating of the pots in which 

 the glass is melted, the proportions of the materials, the construc- 

 tion of the furnaces, the management of the heat, and a whole host 

 of minor conditions too numerous to mention, all of which must be 

 attended to with the greatest care. In the manufacture of iron and 

 steel, the smelting of copper, the refining of nickel, the preparation 

 and baking of porcelain, and in many other trades, innumerable 

 difficulties, all having their origin in the properties of matter and 

 forces, continually beset the manufacturer. In some cases difficul- 

 ties occur which perplex both the workman and the scientific man 

 called in to his aid, and so far from an unscientific workman being- 

 able to overcome them, even with the aid of the scientific man, he 

 is unable to do so. 



The phenomena and changes which take place in matter and 

 forces in nearly every manufacture are far more complex than men 

 in general have any conception of; for instance, simply in heat- 

 ing a bar of iron to redness, a whole series of changes occur in 

 its structure, its magnetism, its dimensions, its cohesive power, its 

 specific heat, and its electric conducting-power, in addition to its 

 absorbent power (" occlusion ") of gases from the fire, and its super- 

 ficial oxidation ; the internal changes which occur in even so simple 

 a phenomenon as this are so numerous, as to produce the impres- 

 sion that the substance is endowed with vitality. All these pheno- 

 mena are doubtless only a very few of the number which really 



