xvi 



early as in 1834 another train of researches on the same general subject 

 was set on foot by Mr. Harcourt, when he made arrangements for trying 

 the effects of long-continued heat on rocks and minerals. For this purpose 

 the furnaces of Low Moor, in Yorkshire, were employed by placing under 

 them a large and varied series of rocks and minerals, and artificial mixtures 

 selected and weighed to represent various natural crystals interesting to 

 geologists in connexion with metalliferous veins and metamorphic rocks. 

 The furnaces being usually kept in action for several years, it was expected 

 that the long- continued and moderate heat would be sufficient to fuse 

 some metals, and probably cause new combinations of oxides, silicates, or 

 sulphides. 



These operations gave indeed such results ; and though, contrary to ex- 

 pectation, the direct influence of high temperature was found to be too 

 great, the conglutination of sand to quartzite, the formation of iron veins, 

 the occurrence of cavities lined with a web-like silicate, and what was 

 almost a chaos of changes and displacements, showed the power of the 

 agencies employed. It is much to be desired that those curious experi- 

 ments should be repeated with precautions which are suggested by those 

 on record*. 



Accustomed to the use of the gas-furnace, Mr. Harcourt turned it to 

 experiments on transparent compounds of fusion, which might be made to 

 have refractive indices beyond the ordinary ranges, combined with scales 

 of dispersion more favourable to achromaticity. In this he was guided by 

 the trials of Faraday to prepare glass for optical purposes. Many years 

 since, the writer, who was often helpful in this way, ground one of the 

 earliest of the Harcourt glasses into a lens, and found it indeed a highly 

 refractive clear substance, but too much traversed by striae to be of 

 practical use. 



"When, some years since, Mr. Harcourt removed his residence to the 

 family seat at Nuneham, near Oxford, he constructed furnaces of a 

 different kind for the carrying on of these experiments, and followed them 

 with the zeal, resolution, and patience which had always characterized his 

 firm and well-regulated mind. At an age when most men cease from 

 continuous literary and scientific work, he with failing sight, but perfect 

 memory, was indefatigable in training an assistant and superintending his 

 work ; making many new combinations with substances untried before, 

 and now selected for quality of fusion, resistance to atmospheric vicissi- 

 tudes, range of refraction and specific action on different rays of the 

 spectrum. Thus it was hoped finally to acquire glasses of definite and 

 mutually compensative dispersions, so as to make perfectly achromatic 

 combinations. After innumerable trials, and the production of glass of 

 extremely various quality, Mr. Harcourt, continuing his inspection to the 

 last, had the satisfaction of believing that, though he could not remain to 

 witness it, a good result had been assured, and that Professor Stokes, to 

 * British Association Reports. 1860. 



