ON THK BISTORT OF TERRESTRIAL PHYSICS. jT'SI 



illustrated, by experiments and calculations, the various opinions which have 

 been entertained on this subject; and few chemists, from the times of Boer- 

 haave, Stahl, and Scheele to those of Priestley and other later authors, have 

 left the properties of heat wholly unnoticed. 



The elegant hypothesis of Aepinus, respecting magnetism and electficitj-, 

 founded in great measure on the theory of Franklin, was advanced in 1759: 

 our venerable countryman, Mr. Cavendish, had invented a similar theory, and 

 had entered in many respects more minutely into the detail of its conse- 

 quences, without being acquainted with Aepinus's work ; although the publi- 

 cation of his paper on the subject was 12 years later. Lambert, Mayer, 

 Coulomb, and Robison have also pursued inquiries of a similar nature, both 

 theoretically and experimentally, with great success. The electrophorus of 

 Wilke, and the condenser of Volta, are among the earliest fruits of the cul- 

 tivation of a rational system of electricity, and Mr. Cavendish's investigation 

 of the properties of the torpedo may sei-ve as a model of accuracy and |)reci- 

 sion in tlie conduct of experimental researches. • 



The speculations of Boscovich respecting the fundamental properties of 

 matter, and the general laws of the mutual action of bodies on each other, 

 have been considered by some candid judges as deserving the highest com- 

 mendation; they remain however almost in all cases speculations only; and 

 some of the most intricate of them, being calculated for the explanation of 

 some facts, which have perhaps been much misunderstood, must consequently 

 be both inaccurate and superfluous. 



The attention of several experienced philosophers, who are now living, has 

 been devoted, with much perseverance, to the ditficult subject of hygrome- 

 try. Deluc's experiments have offered us a very useful comparison of the 

 feygrometrical qualities of various substances: Saussure has investigated, with 

 great labour, the indications of the hjgrometer and the thermometer, a^ 

 connected with the presence of a certain portion of vapour, contained in air 

 of various densities; and Pictet has ascertained some similar circumstances 

 respecting vapours of different kinds wholly unmixed with any air. The hy- 

 potheses, which have usually accompanied the relation of most of these cxpe- 



