ohap. IV.] OF LIFE. 29 



aerian or aquatic medium determines the cessation or suspension 

 of the vital movement. We can even at will suspend and 

 reanimate life in certain organised beings. 



M. Vilmorin succeeded in reviving, by means of moistvu-e, a 

 dried fern sent from America. By drying and then moistening 

 certain infusoria we may arrest and revive the course of life in 

 them. In America and Northern Russia frozen fishes, brought 

 from great distances,- are revivified by being plunged into water 

 of the ordinary temperature. 



In Iceland, in 1828-9, Gajrmard in ten minutes revivified 

 frozen toads in tepid water. 



In the case of dried organisms, the organic substances have 

 been deprived, by evaporation, of their water of gelatinisation, 

 and thereby of their molecular mobility, the instability indis- 

 pensable to the realisation of atomic changes ; in fact, they have 

 been separated from the exterior world, yet without decompo- 

 sition ; whence their easy revival. 



In congealing organisms, an analogous result is obtained. By 

 the solidification of water substances lose their colloidal state. 

 They are, in some degree, chemically paralysed, but can never- 

 theless revive, if congelation has produced neither chemical 

 decomposition of the substances, nor morphological destruction 

 of the tissues and of their anatomical elements. 



These facts suffice by themselves to prove that the principal 

 condition of life is the interchange of materials between the 

 living body and the exterior world ; but, fortunately, we are not 

 limited to such commonplace demonstrations. Vital activities 

 have been minutely scrutinised, watched, and followed step by 

 step, as we shall see further on. We ha,ve been enabled to note 

 the incessant amalgamation with the organism of substances 

 derived from the exterior world, to observe the modifications 

 and transformations which these substances undergo and promote 

 in the midst of living matter; the results of all these biological 

 operations have been summed up, and establish approximately 

 the balance of gain and loss. In short, it is now known that 



