36 TRANSMISSION OF LIGHT AND HEAT. PART i. 



, coloured by these reflections . Gold is yellow by reflection ; 

 | red cloth is red by absorption. In the same sense, phy- 

 I sically speaking, in which the red cloth is red, gold is 

 not yellow but blue or green ; such is in fact the colour 

 of gold by transmission through gold leaf, and therefore 

 gold is greenish blue by absorption. In this case we 

 see that while the substance copiously reflects and in- 

 tensely absorbs rays of all kinds, it more copiously re- 

 flects the less refrangible rays with respect to which it 

 is more intensely opaque. In general absorption and 

 radiation are independent of colour. 



There is a vast diversity in the property which sub- 

 stances possess with regard to the transmission of 

 radiant light and heat; glass, for instance, transmits 

 light abundantly, but is impervious to heat from non- 

 luminous sources ; while other substances, which are 

 altogether opaque to light, transmit heat copiously, as 

 the bisulphide of carbon, which of all liquids is the most 

 diathermic, while water in all its forms is almost im- 

 pervious to heat. 



Sir William Herschel discovered that invisible rays 

 of high heating power exist beyond the red end of the 

 solar spectrum, and Mr. Tyndall has shown that the 

 reason of a substance being impervious to the light of 

 the most brilliant flame and at the same time pervious 

 to these extra red rays is, that the intercepted rays of 

 light are those whose periods of recurrence coincide 

 with the periods of oscillation possible to the atoms of 

 the substance in question. The elastic forces which 

 separate these atoms are such as to compel them to 

 vibrate in definite periods, and when their periods syn- 

 chronize with those of the ethereal waves, the latter are 

 absorbed. Thus transparency in liquids as well as in 

 gases is synonymous with discord, while opacity is syno- 

 nymous with accord between the periods of the waves 

 of ether and those of the body on which they impinge. 



