122 YORKSHIRE NATURALISTS' UNION. 



Professor Tyndall, namely, the importance to the organic world 

 of the ultra-violet invisible rays of the spectrum on account of their 

 chemical energy. * It has been shown by the same author ' that 

 the invisible rays of the sun show a preference for black, which 

 diminishes the reflection.' It is, of course, no new discovery 

 that among colours, black is the greatest absorber of heat. In 

 Craven's ' Recreations in Shooting,' f the following passage 

 occurs, ' Colour is well known to influence the rate by which 

 bodies acquire, reflect, or part with heat, and as white is the 

 colour which most readily and perfectly reflects it, and which most 

 difficultly [sic] parts with it, so a body clothed with that colour 

 shall retain heat longest, and therefore be better fitted to exist in 

 the coldest latitudes.' Applying this to the winter plumage of 

 Ptarmigan, he continues quoting from Daniel, (the original 

 passage I have been unable to find) ' If two animals, one of a 

 black colour and the other white, be placed in a higher tempera- 

 ture than that of their own body, the heat will enter the one 

 that is black with the greatest rapidity, and elevate its tempera- 

 ture considerably above that of the other; but when these 

 animals are placed in a situation, the temperature of which is 

 considerably lower than their own, the black animal will give 

 out its heat by radiation to every surrounding object colder 

 than itself, and speedily have its temperature reduced, while the 

 white animal will part with its heat at a much slower rate.' 

 Birds and animals living through the winter naturally require 

 to retain in their bodies a suflicient amount of heat to enable 

 them to maintain their existence with unreduced vitality against 

 the severities of the climate. Insects, on the contrary, require 

 rapidly to take advantage of transient gleams of sunshine during 

 the short summer season, and may be content to sink into a 

 dormant condition so soon as they have secured the reproduc- 

 tion of their species ; only to be revived in some instances by a 

 return of exceptionally favourable conditions. 



* ' Fragments of Science,' Tyndall, vol. i., p. 32. 

 + ' Recreations in Shooting,' Craven, 1846, p. lor. 



Traiis.Y.N.U., 1883 (pub. 1885). Series D 



