454 STUDIES IN GENERAL PHYSIOLOGY 
liters of water. Of these solutions 5, 10, 15, or 20 c.c. were 
added to 100 c.c. of a 0.7 per cent. NaCl solution. The gas- 
trocnemius muscle of a frog was laid bare without injuring 
the muscle, its surface rapidly dried between sheets of filter 
paper, and the tendon cut. The muscle was placed between 
two watch crystals, weighed, and then introduced into one of 
the solutions described above. The muscle remained in the 
solution for exactly one hour, when it was removed, again 
carefully dried between filter papers, and again weighed. 
The method is accurate to within about 5 mg. The follow- 
ing circumstances are, however, to be considered in using 
the method. As the exchange between the substances con- 
tained in the muscle and in the solution occurs chiefly at the 
surface of the muscle, it is necessary to use for the same 
series of experiments only muscles of approximately the same 
weight, which have, in consequence, about the same surface. 
A light muscle (with its relatively larger surface) will show 
a relatively greater difference in weight at the same osmotic 
difference between the muscle and solution, after remaining 
one hour in the latter than a heavier muscle (with its rela- 
tively smaller surface). Secondly, the fact must be remem- 
bered in utilizing this method that the osmotic pressure of 
the gastrocnemius of the frog is subject to not inconsiderable 
variations, depending upon whether the animal was previously 
quiet or active. If the effect of acids is to be compared, 
care must therefore be taken that frogs are used which have 
been kept for twenty-four hours under as nearly as possible 
the same conditions (light, water, temperature, oxygen sup- 
ply). Even then individual differences continue to exist. 
This difficulty can be overcome only by making a large 
number of experiments. 
2. I will first give five series of experiments which show 
that the acid effects of HNO,, HCl, and H,SO, are the same 
when the same number of hydrogen atoms are contained in 
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