DISCOVERY BY MEANS OF NEW EXPERIMENTS. 531 



was afterwards made a baronet. In 1822, Faraday, by 

 devising and making new experiments, discovered that 

 most of the gases might be liquefied : and Thilorier, in 

 1835, by means of an ingenious apparatus and experi- 

 ment, solidified carbonic anhydride by the cold produced 

 by the vaporisation of the liquefied gas. It was Melloni 

 who during a long series of experiments, discovered the 

 great degree of transparency of rock-salt to rays of heat ; 

 that alum is highly opaque to those rays, a-nd that the 

 order of degrees of transparency of bodies to light is very 

 different from that of their transparency to heat. It was 

 by means of a long series of well-devised and laborious ex- 

 periments, in which a weight by its falling was caused to 

 drive a paddle and stir a known weight of water in a 

 closed vessel, that Joule in 1849 discovered the mechanical 

 equivalent of heat, i.e. how much mechanical power was 

 required to produce a given amount of heat by friction. 

 He found that the mechanical force, of 1 pound weight 

 falling through 772 feet, would raise the temperature 

 of 1 pound of water one Fahr. degree ; and thus laid the 

 foundation of the dynamical theory of heat. 



Between the years 1540 and 1600, Dr. Gilbert devised 

 an experiment of bringing an electrically excited body 

 near to the end of a light needle of any metal balanced, 

 and turning freely on a pivot, and found ^hat various sub- 

 stances in addition to amber, viz., agate, diamond, beryl, 

 crystal, glass, Bristol-quartz, sapphire, glass of antimony, 

 jet, sealing-wax, gum mastic, sulphur, and other sub- 

 stances, are attracted by electrics ; and he stated in his 

 book, published in the year 1600, that all solid bodies, 

 even metals, and also water and oil, are attracted. Boyle 

 also, previous to the year 1675, discovered by experiment, 

 that warming bodies before rubbing them increases the 

 electric effect; and by means of other experiments, he 



M M 2 



