BROWN: WooDSs FOR MICROSCOPIC STUDY 147 
hydrochloric acid, but is unaffected by acetic acid. Calcium 
carbonate, which is of comparatively rare occurrence in crystalline 
form, dissolves in either acid with effervescence. 
The fact that each mineral crystal is often closely incased in a 
resistant, nearly impermeable membrane may make it necessary 
to free the face of the mineral from its coating before the test can 
be applied. This may be done by holding a thin section in a 
pair of forceps, igniting, and allowing to burn or char only suffi- 
ciently to free the crystal,or a portion of it. Care must be exercised, 
for the calcium oxalate, with sufficient heat, is changed to calcium 
ca bonate and finally to calc um oxide. 
: H. CHEMICAL GROWTH-RINGS 
Some woods without any visible structural growth-rings, may 
yet exhibit what appear to be definite seasonal variations in the 
chemical composition of the wood, so that distinct chemical rings 


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Chemical growth-ring in Xylosma hawaiense Seem. Arrow indicates 
dicection of growth. Z, lignified zone. A slight retardation of ewe is indicated 
by somewhat shortened ray-cells at R, and occasional slightly compressed fibers (F)- 
may be demonstrated by microchemical means. The term chemi- 
cal growth-rings is here suggested for such zonal variations in the 
