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 3 , HC1, and H 2 SO 4 are the same 

 when the same number of hydrogen atoms are contained in 



