PREFACE. 11 



railways, and the almost endless employments de- 

 pending upon their construction and use. About a 

 quarter of a million of persons are employed on railways 

 alone in Great Britain. The various original investi- 

 gations on the chemical effects of light led to the 

 invention of photography, and have given employment 

 to thousands of persons who practise that process, or 

 manufacture and prepare the various material and 

 articles required in it. The discovery of chlorine by 

 Scheele led to the invention of the modern processes of 

 bleaching, and to various improvements in the dyeing 

 of the textile fabrics, and has given employment to a 

 very large number of our Lancashire operatives. The 

 discovery of chlorine has also contributed to the em- 

 ployment of thousands of printers, by enabling Esparto 

 grass to be bleached and formed into paper for the use 

 of our daily press. The numerous experimental inves- 

 tigations in relation to coal-gas have been the means of 

 extending the use of that substance, and of increasing 

 the employment of workmen and others connected with 

 its manufacture. The discovery of the alkaline metals 

 by Davy, of cyanide of potassium, of nickel, phosphorus, 

 the common acids, and a multitude of other substances, 

 has led to the employment of a whole army of workmen 

 in the conversion of those substances into articles of 

 utility. The foregoing examples might be greatly en- 

 larged upon, and a great many others might be selected 

 from the sciences of physics and chemistry : but those 

 mentioned will suffice. There is not a force of Nature, 

 nor scarcely a material substance that we employ, which 

 has not been the subject of several, and in some cases 

 of numerous, original experimental researches, many 

 of which have resulted, in a greater or less degree, 



