67 



As the apparatus was now calibrated it was possible conversely, 

 by reading the deflection of the spot of light, to calculate the 

 magnitude of any source of heat, which was in the apparatus. For 

 such an unknown source of heat germinating wheat-grains were 

 used. (The number of these was always 500). 



In the course of the experiments however it became plain that 

 in this case the deflection of the light spol could not be looked 

 u[)on as showing exclusively the lieat-evolulion which took place in 

 germination. Foi' when 500 germinated wheat-grains, which had 

 previously been killed by heating to \()(f, were j)laced in the 

 apparatus, then it was seen that (he spot of light inevitably 

 passed the zero; in various experiments of this kind a deflection of 

 about 8 centimeties was always found. 



In order to ascertain whether the dead seedlings did not after 

 all give off some heat possibly as a i-esult of a continued enzyme- 

 action, the apparatus was filled by way of control with quantities 

 of filterpaper pre\'iously soaked in water. In this case there could 

 be no question of heat-evolution by (he filterpa[)er. Also with this 

 arrangement of the experiments the spot of light invariably passed 

 the zei'O, reaching finally a maximum deflection corresponding to 

 that obtained when dead seedlings were [)laced in the apparatus. 

 The extent of this deflection was independent of the temperature of 

 the surrounding water (fixed at 25° and 85°;, in other words, with 

 this arrangement of the experiment there arose always a constant 

 difference of temperature between the in- and out-going current of air. 



Since in these cases no direct evolution of heat by means of the 

 substances used was possible, another cause for the rise of temperature 

 in the experiment described had to lie found. The most [)robable 

 thing was that condensation of water-vapour must have taken j)lace 

 in some way and that the heat thus set free caused an increase of 

 temperature in the out-going air and in consequence of this of the 

 upper thermal needle. In the calibration-experiments the spot of 

 light had remained at zero when there was no heat-source in the 

 apparatus ; the difference in conditions then and during the experiments 

 just described was, that the space within was in the latter case for 

 a great part filled with a completely imbibed mass. 



The many efforts made to eliminate this irregularity were practically 

 without results; I was therefore compelled, in experimenting with 

 living seedlings, to adopt a correction, the amount of which was 

 experimentally fixed while theoretically it had to be left |)artly 

 unexplained. 



Since it was therefore found that by filling the apparatus with 



5* 



