274 Messrs. Langley and Very on the 



Part II. — Thermal Observations. 



To give an idea of the amount of heat at our disposition for 

 experiment, and of the actual minuteness of the radiation 

 which proceeds from even the most luminous tropical insect, 

 we may say that if that rate of radiation from a lampblack 

 surface 1 square centim. in area, which represents the amount 

 of heat necessary to raise 1 grm. of water 1° C. in 1 minute 

 (i. e. one small calorie), be taken as unity, then the luminous 

 radiation of the fire-fly's heat per square centim. of exposed 

 luminous surface, as we have found, is about 0*0004 calorie in 

 10 seconds, and the total luminous radiation from the most 

 powerfully illuminating light-spot of the insect (the abdominal 

 one) will not exceed 0*00007 calorie in the same time. But a 

 small portion of this could fall upon the bolometer, and that 

 which actually reached it during the time (10 seconds) required 

 for each observation was sufficient only to affect an ordinary 

 mercurial thermometer having a bulb 1 centim, in diameter 

 by rather less than 0°*0000023, or by less than 400W0 °f one 

 degree Centigrade. 



We have just mentioned that the total amount of heat- 

 radiation upon which we have to make our investigation re- 

 presents less than jtqoVoo ca l° r ie, while that portion of this 

 which falls upon the apparatus would, in the time of one opera- 

 tion, only raise the temperature of an ordinary mercurial 

 thermometer by less than 400V 00 degree > an d we have first to 

 notice the difficulty that in case invisible heat exists in company 

 with the light (and it certainly does exist in ordinary emana- 

 tions from the surface of any living creature independent of 

 phosphorescence), we have in this minute radiation heat of 

 two different kinds, both invisible, and which it is yet indis- 

 pensable for us to discriminate. 



We are helped to do this by the consideration that while 

 the insect, like any non-luminous one, must emit " animal 

 heat " from all its surface, its general surface-temperature is 

 certainly low, since it feels cold to the hand, whose greater 

 warmth excites it to shine. This heat, then, corresponds to a 

 temperatnre much below 50° C. ; and such temperatures must, 

 as we have shown in other memoirs, be accompanied by the 

 emission of waves whose length relegates them to quite another 

 spectral region to that in which the invisible heat associated 

 with light mainly appears. We can then discriminate the 

 rays of this invisible " animal " heat without the formation of 

 a heat-spectrum, by their inability to pass through a glass 

 which transmits with comparative freedom radiant heat whose 

 wave-length is less than 3% the latter including the region 



