VII EXPERIMENTS IN NUTRITION 79 



It is somewhat curious to find that potassium and calcium 

 phosphates are equally necessary ; although occurring in 

 such minute quantities they are absolutely essential to the 

 well-being of the yeast cells, and without them the organism, 

 although supplied with abundance of sugar and ammonium 

 tartrate, will not live. This may be taken as proving that 

 phosphorus, calcium, and magnesium form an integral part 

 of the protoplasm of Saccharomyces, although existing in 

 almost infinitesimal proportions. 



Lastly, magnesium sulphate must not be omitted if the 

 organism is to flourish : unlike the other two mineral 

 constituents it is not absolutely essential to life, but without 

 it the vital processes are sluggish. 



Thus by growing yeast in a fluid of known composition 

 it can be ascertained exactly what elements and combina- 

 tions of elements are necessary to life, what advantageous 

 though not absolutely essential, and what unnecessary. 



The precise effect of the growth and multiplication of 

 yeast upon a saccharine fluid, or in other words the nature 

 of alcoholic fermentation, can be readily ascertained by a 

 simple experiment with Pasteur's solution. A quantity of 

 the solution with a little yeast is placed in a flask the neck 

 of which is fitted with a bent tube leading into a vessel of 

 lime-water or solution of calcium oxide. When the usual 

 disengagement of carbon dioxide (see p. 75) takes place^the 

 gas passes through the tube into the lime-water and causes 

 an immediate precipitation of calcium carbonate as a white 

 powder which effervesces with acids. This proves the gas 

 evolved during fermentation to be carbon dioxide since no 

 other converts lime into carbonate. When fermentation is 

 complete the presence of alcohol may be proved by dis- 

 tillation : a colourless, mobile, pungent, and inflammable 

 liquid being obtained. 



