28o SCIENCE PROGRESS 



the benzene nucleus required for tyrosine although it is quite 

 unable to produce indole. 



With regard to the question as to whether particular amino 

 acids are associated with any special function, some such con- 

 nection has been established by the author between arginine 

 and histidine on the one hand, and purines on the other, for he 

 has been able to show in the case of rats that the excretion of 

 allantoin fell off markedly when these two acids were removed 

 from the diet, but rose again as soon as they were included. 

 The relation between these particular acids and the purines is 

 not far to seek on chemical grounds, since they are characterised 

 by the peculiarity, not found in any other amino acids, of having 

 a carbon atom between two nitrogen atoms, which arrangement 

 is of course typical of the purines. 



The causation of the staleness of bread forms the subject of 

 an investigation by J. R. Katz {Zeitsch. Physiol. Chem., 191 5, 

 95, 104). The staleness of bread according to this author is 

 not merely due to drying, since bread kept in an atmosphere 

 saturated with moisture also becomes stale. What actually 

 occurs is that the starch grains harden and become less able to 

 absorb water and the soluble polysaccharides contained in them 

 are rendered insoluble, with the result that the bread becomes 

 harder and less sweet. The crumbly nature of the stale bread is 

 due to the transfer of water from the starch grains as they 

 harden to the gluten, which lessens the adherence of the particles 

 to each other. Exactly the reverse process takes place in 

 baking : the absorptive power for water and the proportion of 

 water soluble polysaccharides is increased. Equilibrium is 

 apparently attained at the baking temperature, and as the 

 temperature falls the reaction goes in the reverse direction. 

 In support of this view it has been shown that bread may be 

 kept fresh for twenty-four hours if maintained at a temperature 

 of 6o°C. 



GEOLOGY. By G. W. Tyrrell, A.R.C.Sc, F.G.S., University, Glasgow. 

 Regional and Stratigraphical Geology. — Two papers on Welsh 

 stratigraphy appear in the current Quarterly Journal. The 

 geology of the district between Abereiddy and Abercastle 

 (Pembrokeshire) is dealt with in detail by Dr. A. H. Cox {Quart. 

 Journ. Geol. Soc. 1916, 71, 273). The region is extensively 

 folded, and a complete sequence of Ordovician rocks from the 



