266 THE BEGINNINGS OF LIFE. 



changes of a putrefactive or fermentative character. 

 A slight evolution or liberation of gas generally takes 

 place as the first obvious stage of the process 1 , and 

 after a variable time (hours or days, according to the 

 temperature, the nature of the solution, and other 

 modifying conditions) during which the infusion has 

 gradually become more and more turbid, a slight whitish, 

 though semi-translucent, scum or pellicle, that soon 

 thickens into a membrane, makes its appearance on 

 the surface of the fluid. This constitutes the c primor- 

 dial mucous layer' of .Burdach, or the c proligerous 



the same importance to the influence of solar heat in the evolution of 

 Life which modern science now allots to it. We have already quoted 

 one passage to this effect, but here is another : 



4 Ergo ubi diluvio tellus lutulenta recenti 

 Solibus setheriis, altoque recanduit aestu, 

 Edidit innumeras species.' 



1 This may be well seen by adding to the fermentable infusion suffi- 

 cient isinglass to 'set' the fluid slightly. The bubbles of gas liberated, 

 are for a long time retained in the slightly gelatinous liquid, and may be 

 seen throughout its substance. Very contradictory opinions prevail as 

 to the order of appearance and cause of this gaseous evolution. M. Pas- 

 teur believes that the evolution of gas takes place after the appearance 

 and on account of the changes induced by the presence of organisms. 

 In his opinion all fermentations are brought about by the presence and 

 development of organisms (derived from the atmosphere) in the fer- 

 menting fluids. His opponents, however, maintain that the organisms 

 are results of chemical changes brought about by physical conditions 

 in the molecularly mobile and unstable matter of an organic infusion, 

 and that the gaseous evolution is dependent upon some of these ante- 

 cedent, or formative, chemical changes. The gases most commonly 

 liberated in fermentations and putrefactions are hydrogen, carbonic acid, 

 sulphuretted hydrogen, or ammonia. 



