No. 398. ] SPECIFIC GRAVITY OF ANIMALS. 97 
influence of the reagents. Flemming’s solution (weak) gave 
the best results. Many Stentors kept their extended shape 
and upon microscopic inspection appeared little altered. 
I used gum arabic to regulate the density of my solutions, 
since, as suggested by Dr. Davenport, it is not injurious to 
animals immersed in it. It is more convenient to take a solu- 
tion of high specific gravity and lower this gradually by adding 
water, than to increase the density by adding known weights 
of gum. Since different pieces of gum vary noticeably in 
density, two solutions made in exactly the same proportions 
may vary perceptibly in their buoyant effect. Consequently, 
to get a consistently acting medium which can be used through- 
out a series of experiments and which, each time it is made up, 
acts in about the same way, I made up a stock solution of 15 
grams gum arabic and 30 cc. of distilled water. With a bit of 
camphor to keep out bacteria and a glass-stoppered vessel to 
prevent evaporation, there is no appreciable change in a month's 
time. A long pipette and a large test-tube, both marked in 
cubic centimeters on the outside, are the only apparatus neces- 
sary. The marking on the outside of the test-tube is absolutely 
essential to avoid uncertainty as to how many cubic centimeters 
have been added to the mixture. 
In making a series of gum-arabic solutions I found that a 
gram of gum arabic when dissolved occupies between 0.66 and 
0.72 cc. According to Landolt und Bórnstein's Tabellen (94), 
the specific gravity of the gums which are included under the 
name “gum arabic” varies between 1.31 and 1.45. The space 
that one gram occupies varies, then, between 0.763 and 0.689 cc. 
Comparing this with the observed volumes, I feel justified in 
assuming as an average the space value 0.7 cc. for one gram 
of dissolved gum arabic. On this basis the two ene which 
follow (p. 98) have been computed. 
To make my stock solution I used 15 grams gum, which at 
0.7 cc. space per gram occupies 10.5 cc. Adding 30 cc. of (dis- 
tilled) water, the whole volume is 40.5 cc. The total weight is 
45 grams, and weight divided by volume gives 1.111 + as the 
Specific gravity of the mixture. The second number in the 
table is found by adding to 1 cc. of this mixture I cc. of pure 
