172 ANNUAL OF SCIENTIFIC DISCOVERY. 



concave lens, the plane side having been formed of one of the original 

 faces of the six-sided crystal of quartz, as he had ascertained by its 

 action on polarized light, this was badly polished and scratched. 

 The convex face of the lens had not been ground in a dish-shaped 

 tool in the manner in which lenses are now formed, but was shaped on 

 a lapidary's wheel, or in some such manner. Hence it was unequally 

 thick, but its extreme thickness was 2-10ths of an inch, its focal length 

 being 4^ inches. It had twelve remains of cavities which had originally 

 contained liquids or condensed gases ; but ten of those had been 

 opened probably in the rough handling which it received in the act of 

 being ground ; most of them therefore had discharged their gaseous 

 contents. Sir David concluded by assigning reasons why this could 

 not be looked on as an ornament, but a true optical lens. 



Sir David then exhibited specimens of the decomposed glass found 

 in the same ruins. The surface of this was covered with iridescent 

 spots more brilliant in their colors than Peacock copper ore. Sir 

 David stated that he had several years since explained how this pro- 

 cess of decomposition proceeded, on the occasion of having found a 

 piece of decomposed glass at St. Leonard's. It had contained man- 

 ganese, which had separated from the silex of the glass, at central 

 spots round which circles of most minute crystals of true quartz 

 had arranged themselves ; bounded by irregular jagged circles of 

 manganese, these being arranged in several concentric rings. When 

 this process reached a certain depth in the glass it spread off laterally, 

 dividing the glass into very thin layers, and new centres seemed to 

 form at certain distances, and thus the process extended. 



RECENT INVESTIGATIONS ON HEAT. 



THE theory of Heat has made great advances within the last ten 

 years. Mr. Joule has by his experiments confirmed and illustrated 

 the views demonstrated about the end of the last century by Davy 

 and Rumford regarding the nature of heat, which are now beginning 

 to find general acceptance. He has determined with much accuracy 

 the numerical relation between quantities of heat and of mechanical 

 work. He has pointed out the true principles upon which the mechan- 

 ical value of any chemical change is to be estimated, and by very care- 

 ful experiments he has arrived at numerical expressions for the 

 mechanical equivalents in some of the most important cases of chemi- 

 cal action, in galvanic batteries, and in combustion. These researches 

 appear to be laying the ground work for the ultimate formation of a 

 Mechanical Theory of C/temistri/j by ascertaining experimentally the 

 mechanical equivalents expressed in absolute motive force of the ther- 

 mic, electric, and magnetic forces. Mathematical developments of 

 the theories of heat and electro-dynamics, in accordance with these 

 principles, are given in various papers by MM. Helmholz, Rankine, 

 Clausius, and Thompson, published principally within the last two 

 years. 



Mr. Hopkins, F. R. S., is at present engaged in investigating the 



