386 EPOCHS IN THE HISTOIIY OP THE CONTEMPLATION OP 



alterations which terrestrial matter undergoes, are con- 

 nected with modifications of heat, light, and electricity, 

 either in repose or moving in currents, and as the pheno- 

 mena of temperature operating by expansion are most acces- 

 sible to visible perception and cognizance, it follows that, 

 as I have elsewhere observed, the invention and improve- 

 ment of thermometric instruments marks an important epoch 

 in the progress of the general knowledge of nature. The 

 field of application of the thermometer, and the conclusions 

 founded on its indications, are commensurate with the 

 domain of those forces or powers of nature which exert their 

 dominion alike in the aerial ocean, on the dry land, in 

 the superimposed aqueous strata of the sea, and in inorganic 

 substances, as well as in the chemical and vital processes of 

 organic tissues. 



More than a century previous to Scheele's extensive 

 labours, the action of radiant heat was also investigated by 

 the Florentine members of the Academia del Cimento, by 

 remarkable experiments made with concave mirrors, towards 

 which, non-luminous heated bodies, and masses of ice of 

 5001bs. in weight, radiated actually and apparently. ( 518 ) 

 Mariotte, at the close of the 1 7th century, investigated the 

 relations of radiant heat in its passage through glass plates. 

 I have here recalled these detached experiments, because, 

 since that period, the doctrine of the " radiation of heat" has 

 thrown considerable light on the cooling of the ground, the 

 origin of dew, and many general climatic modifications, and' 

 through Melloni's admirable sagacity, has even conducted 

 to the contrasted diathermism of rock salt and alum. 



With investigations on the variations of atmospheric 

 temperature, coincident with changes of latitude, season and 



