MODERN SCIENCE. 239 



in one hour ; the quantity of heat which the earth receives is 

 only TFSTTnnnnnr f tne ^ eat emitted by the sun invented the 

 electrical pyrometer to measure temperatures so high that 

 the mercurial thermometer could not be used. 



1798 1854. Melloni discovered numerous phenomena of 

 RADIANT HEAT ; showed radiant heat to have the same 

 general properties as light i.e., it may be reflected, refracted, 

 polarised, and decomposed determining at the same time 

 the specific power of each substance for transmitting heat; in 

 other words discovered the DIATHERMANCY (transmission of 

 heat) through various media in various degrees. The term 

 diathermancy bears the same relation to radiant heat that 

 transparency does to light; and "in like manner the power 

 of stopping radiant heat is called athermancy, which thus 

 corresponds to opacity for light." Melloni invented the 

 THERMO-ELECTRICAL MULTIPLIER such a delicate instru- 

 ment to measure heat, that the warmth of the hand is enough 

 at a distance of a yard to deflect its needle. 



1800 1877. Talbot was the first to propose the bright 

 lines which appear in the spectra of artificial flames as a 

 means of detecting substances too minute in quantity to be 

 detected by any other process a suggestion which he carried 

 out in 1834, and which a few years later made the spec- 

 troscope the finest and surest of all instruments of analysis. 

 After experiments from 1832 to 1840 he applied the discovery 

 of Niepce (Daguerreotype) to paper, giving thereby the 

 finishing stroke to PHOTOGRAPHY. 



1801 1 8 . Jacob! (M. H.) discovered the art (called 

 ELECTROMETALLURGY) of precipitating metals from their 

 solutions by the slow action of a galvanic current (1840). 

 This art is divided into Electrotyping, which is used in 

 producing copies of coins, medals, seals, etc. ; and ELECTRO- 

 PLATING the art of covering base metals with a thin coating 

 of silver or gold. Jacobi has therefore given birth to a vast 

 and rich industry. He was also the first to propose and fix 

 A UNIT OF ELECTRICAL RESISTANCE (i.e., a standard of com- 

 parison of resistances) a determination of great scientific and 

 practical importance. Finally, using Arago's electro-magnets 

 (made by means of an ordinary electric machine instead of a 



