ACTION OF TEMPERATURE 249 



Such effect of temperature on photochemical reactions can be inter- 

 preted by a modification of the absorption spectrum of the absorbing 

 molecules. A rise in temperature increases the molecular oscillations 

 and shifts the entire absorption spectrum toward the red. Such radia- 

 tion, which at a low temperature can only excite the molecule, may 

 dissociate it at higher temperature, the molecule then being more prob- 

 ably in a state of predissociation. The temperature coefficient of the 

 reaction then depends on the wave length. In Schreiber's experiment, 

 in the case of 3025 A, which is of low efficiency, this efficiency increases 

 steadily with temperature; in the case of 2560 A, w^hich is more efficient, 

 the energy of oscillation must reach a noticeable value (15°) in order to 

 manifest itself in respect to the actual energy of the photon. 



If, as we have just seen, the primary effect is sometimes sensitive to 

 temperature, the latter always accelerates the dark reactions and pre- 

 cipitates the appearance of the lesion. But the earlier appearance of the 

 lesion does not mean a change in sensitivity. What is important here 

 are the frequency and the gravity of the lesions, which temperature 

 may also influence. The unknown processes involved in the dark reac- 

 tions interfere and compete with the cellular metabolisms, some of 

 which, although more or less disturbed by irradiation, may secure dif- 

 ferent types of recovery. These recoveries are influenced by temperature 

 in so far as it affects differently the speeds of the chemical reactions 

 put into play, and in the sense of this difference. When there is no 

 such difference, no temperature recovery takes place (6, 24). It may 

 happen, however, that a cooling off during the latent period slows down 

 the process of lesion more strongly, so that it favors recovery and lowers 

 sensitivity. 



Strangeways and Fell (28) observed that degenerative modifications 

 produced by x-rays in chicken embryos are lessened and sometimes 

 stopped if embryos are kept for 24 hr at 5° C after irradiation. They 

 concluded that slowing the metabolism gave to the tissues the possi- 

 bility of repairing the lesions. 



Cook (5) confirmed this observation by submitting several groups of 

 Ascaris eggs to an x-ray dose of 5000 r. The controls incubated at 25° 

 immediately after irradiation gave 2 per cent normal embryos. The 

 other groups were kept at 5° for increasing periods, then incubated. 

 This exposure to cold increased progressively the proportion of normals : 

 4 per cent after a period of 1 week, 15 per cent after 4 weeks, 45 per 

 cent after 8 weeks, after which no more recoveries took place. 



By irradiating, either with x-rays or with ultraviolet rays, a bacterium 

 {B. dysenteriae) and a yeast (S. ellipoideus), I noticed (14) that a stay 

 at 5° C before incubation favors recoveries among the irradiated yeast 



