1917] 
ZELLER—DURABILITY OF YELLOW PINE 103 
block from the top in the column. Each culture block was 
labeled with a soft lead pencil, this proving to be the most 
satisfactory method of labeling. 
After the culture blocks were sawed and labeled they were 
placed in an oven at 65° C. and dried to a constant weight 
and then weighed in grams, accurately to the second decimal, 
and estimated to the third. The period of time for kiln-drying 
to constant weight proved to vary according to the porosity 
of the wood, the lighter wood drying in 3 to 4 days, the heavier 
in 6 to 7 days. 
After the weights were obtained, the volumes of the culture 
blocks were determined by immersion in mercury. А grad- 
uate cylinder calibrated to 2-се. divisions was cut off, so that, 
when filled with mercury up to within 2.5 inches of the top, 
blocks could be inserted and removed with ease. This method 
is sufficiently accurate and practicable where a large quantity 
of volume determinations are to be made. Its main inaccuracy 
lies in the large surface of mercury exposed where the read- 
ing is taken. The volumes were taken in cubic centimeters. 
From the weight and volume obtained the specific gravity was 
determined for the individual culture blocks. 
The percentage of summer wood was determined for each 
column of blocks in a sample by measuring in millimeters the 
width of the layers of summer wood on a radial line 2.5 em. 
long and multiplying this value by four. These measurements 
were made on a smoothly planed cross-section of the whole 
sample. The values for percentage of summer wood cannot 
be considered absolute. It will be noticed that even within 
the individual columns of culture blocks the specific gravity 
may vary considerably. At first it was thought that this might 
be due to error in determining the specific gravity, but upon 
examination of the individual blocks it was found to be due to 
another cause. It is difficult while rip-sawing a sample to fol- 
low the grain of the wood exactly. Thus, whenever the length- 
wise sawing is at all oblique to the grain, there is a change in 
the apportionment of summer wood for the neighboring 
blocks, changing the specific gravity proportionately. 
The number of growth rings per inch were counted on the 
