pringsheim's researches on chlorophyll. 87 



lighting. This does not at first affect the tissue examined, 

 immersed, as it is, in a drop of water, but as the degree of 

 temperature gradually rises, and the drop begins to evaporate, 

 the object may be acted on and eventually killed. In this way, 

 then, there may be a heat effect from the temperature of the 

 water in addition to that resulting from the illumination of 

 the object. It is necessary to distinguish the thermal from 

 the photochemical effects, although it is not always easy to 

 do so, especially when the heating is very great, as it is in 

 white and red light, and when the exposure is long and unin- 

 terrupted ; and it may be necessary, in such cases, to resort 

 to a cooling process in the course of experiment. This 

 can be readily carried out by using a metallic slide, with a 

 glass disk let in for the object and drop of water, and by 

 inserting into the drop a small metallic ring with radiating 

 arms. Ice laid on the slide or on these arms rapidly cools 

 the water drop to the extent required. 



It is important, then, to determine the temperature 

 reached in the water drop during the short time — five to 

 fifteen minutes — which suffices for the completion of the 

 photochemical changes. This temperature naturally depends 

 upon the colour of the intense light, upon the size of the 

 drop and its original temperature, and upon the extent of 

 cooling in the research. 



Two methods were employed in these researches for the 

 temperature determinations, (a) The insertion of a thermo- 

 electric couple of iron and nickel into the drop, the results 

 being read off by a galvanometer ; and (b) the introduction 

 of small crystals of substance of known melting point. For 

 this latter purpose two substances, azoxybenzol, which melts 

 at 45° C, and mint-camphor, with its melting point 35° C, 

 were found most convenient. 



The following are the determinations so made in this 

 series of experiments : 



In white light, the original temperature being 20° C, it 

 rose in one to three minutes to 45° C. 



In red light (fig. 28, spectra y and S), produced by a 



5 mm. thick solution of iodine in bisulphide of carbon, 

 allowing passage of red rays up to those of wave-lengths 

 •G0061 mm,, and also traces of violet rays of wave-lengths 

 '00043 — '00041 mm., a temperature 45° C. was attained in 

 three to five minutes, — a most injurious one for plant tissues. 



In both these lights the effects may thus be thermal as 

 well as photochemical. 



In green light (fig. 28, spectra ^, rj, 0), produced by a 5 — 



6 mm. thick copper chloride solution, allowing passage of 



