iv.] YEAST. 75 



was in all probability possessed by the prehistoric 

 populations of the globe ; and it must have become a 

 matter of great interest even to primaeval wine-bibbers 

 to study the methods by which fermented liquids could 

 be surely manufactured. No doubt, therefore, it was 

 soon discovered that the most certain, as well as the 

 most expeditious, way of making a sweet juice ferment 

 was to add to it a little of the scum, or lees, of another 

 fermenting juice. And it can hardly be questioned that 

 this singular excitation of fermentation in one fluid, by 

 a sort of infection, or inoculation, of a little ferment 

 taken from some other fluid, together with the strange 

 swelling, foaming, and hissing of the fermented sub- 

 stance, must have always attracted attention from the 

 more thoughtful. Nevertheless, the commencement of 

 the scientific analysis of the phenomena dates from a 

 period not earlier than the first half of the seventeenth 

 century. 



At this time, Van Helmont made a first step, by 

 pointing out that the peculiar hissing and bubbling of a 

 fermented liquid is due, not to the evolution of common 

 air (which he, as the inventor of the term " gas," calls 

 "gas ventosum"), but to that of a peculiar kind of air 

 such as is occasionally met with in caves, mines, and 

 wells, and which he calls " gas sylvestre." 



But a century elapsed before the nature of this " gas 

 sylvestre," or, as it was afterwards called, "fixed air," 

 was clearly determined, and it was found to be identical 

 with that deadly " choke-damp " by which the lives of 

 those who descend into old wells, or mines, or brewers' 

 vats, are sometimes suddenly ended; and with the 

 poisonous aeriform fluid which is produced by the com- 

 bustion of charcoal, and now goes by the name of 

 carbonic acid gas. 



During the same time it gradually became clear that 



