Michael Faraday, his Life and Works. 417 



wifeh a view to its optical use, that Faraday was called upon to 

 occupy himself with it. Whilst he pursued the chemical part of 

 these investigations, Dollond worked up the glass, and Herschel 

 subjected it to the test of experiment. At the end of long and 

 difficult experiments, Faraday ascertained that the greatest diffi- 

 culty in the way of the fabrication of a good flint glass (that is 

 to say, a very refractive glass) was the presence of streaks and 

 stria3 proceeding from a want of homogeneity, due, in its turn, 

 to differences of composition between the contiguous portions of 

 the same glass. The employment of oxide of lead in the com- 

 position of flint glass was the cause of this defectiveness, which 

 could not be avoided even by making use of the most efficacious 

 means of rendering the mixture perfect while in a state of fusion. 

 Among the combinations tried, that of borate of lead and silica 

 furnished a glass endowed with optical properties still more 

 strongly marked than those of flint glass, and at the same time 

 presenting a very uniform structure. This glass, which, on ac- 

 count of its great density (double that of flint glass), has been 

 named heavy glass, is found unfortunately to have a slight yel- 

 lowish coloration which renders it unfit for optical purposes : but 

 the labour which Faraday devoted to its fabrication has not been 

 lost; for, as we shall see hereafter, this same glass, in the hands 

 of the talented experimenter, became the instrument of one of his 

 most beautiful discoveries. 



In the long and curious memoir which he published upon the 

 fabrication of optical glass, Faraday gives a minute description 

 of all the processes employed by him — of the construction of 

 furnaces, selection of crucibles, means of heating, various artifices, 

 such as the injection of platinum in powder into the fused glass 

 to cause the disappearance of bubbles, &c. It is a genuine in- 

 struction in chemical manipulation, and, as it were, a complement 

 to his Treatise on this subject, which was published in 1827, 

 and has since gone through three editions. Only those who 

 are called upon to experiment in the domains of physics and 

 chemistry can appreciate the immense service which this treatise 

 has rendered to them, by teaching them a multitude of processes 

 of detail so valuable for them to know, and of which a description 

 was previously nowhere to be found, so that every one was obliged 

 to undergo an apprenticeship to them on his own account. It 

 was necessary that a savant who for so many years had been 

 struggling with the difficulties of experimentation, and who had 

 been able to surmount them in so ingenious a manner, should 

 give himself the trouble to describe the means which he had em- 

 ployed, so that his experience might be of service to others. 

 Faraday was this savant, and his object was completely attained. 



Here, perhaps, before proceeding to another set of subjects, 



