HYGROSCOPICITY 149 



per cent. It was the work of Jodin that led Becquerel in the 

 paper before quoted to make the critical distinction in a seed's 

 water-contents between the water of hygroscopicity and the 

 water concerned in the latent life of the embryo. 



Whilst several years have passed since Jodin directed his 

 attention to the ordinary hygrometric variations experienced 

 by peas, Berthelot in more recent times has opened up the 

 whole subject of the hydratation of vegetable matter, and in 

 so doing has thrown an important light on the nature of 

 hygroscopicity in plants (" Recherches sur la desiccation des 

 plantes et des tissues vegetaux ; conditions d'equilibre et de 

 reversibilit6," Annales de Chimie et de Physique^ April 1903). 

 He shows that the peculiar property possessed by air-dried 

 vegetable matter of regaining from the air the water it has 

 been made to lose by heat and other artificial means is a 

 function of the hygrometric condition of the atmosphere. It 

 is not easy for me to state Berthelot' s principle tersely, and Berthelot's 



" 1 t 



accordingly I have above followed Maquenne in his reference reversibility, 

 to this subject in Comptes rendus^ October 1905. Nor is it easy 

 to grasp its full significance at first, since, as is natural in such 

 abstruse inquiries, much will seem pointless that does not cross 

 the boundary of one's own researches. To me perhaps it is 

 not so hard, since the principle has cast a flood of light upon 

 the results of my studies of permeable and impermeable seeds. 

 A plant, says Berthelot, does not dry entirely in air like 

 porcelain or metals (see Note 21 of Appendix). It retains 

 after being thus dried a certain amount of water, which varies 

 in response to the changes in the hygrometricity of the atmo- 

 sphere. When this water has been driven off by exposure to a its applica- 

 temperature of 110 C., it is gained back little by little from tissues ui a 

 the air up to a limit practically the same as that reached when & eneral - 

 the plant was dried in air. In a word, the water which the 

 air-dried material loses in the oven is regained in the air. 

 This is Berthelot's principle of reversibility, and it is 

 characterised by him as essentially a physico-chemical process 

 independent of life. It applies equally to the plant that has 



