472 STATE BOARD OF AGRICULTURE. 



comparatively large. The prime condition was (it will be remembered) 

 that the lecithin cover (or lard, etc.) closed the preparation dish "lecithin 

 tight." No oxygen could pass into the air space of the dish except it 

 passed through the lecithin itself. When the rate at which oxygen passed 

 through the membrane from air had thus been determined, the mercury 

 mug "b" was raised, cocks "a" and *'c" were opened and as much of the 

 air as possible was carefully expelled.* Then, from a stock preparation, 

 gasoline-air was drawn into the container as the mercury mug "b" was 

 lowered. At first, gasoline-vapor was absorbed rapidly by the lecithin 

 membrane. By making tests (as already described) at intervals, it was 

 found that the rate of decrease in volume would finally settle down to 

 a fairly uniform rate. That is, the membrane would finally become 

 saturated with the gasoline-vapor, and the decrease in volume after that 

 was due to the loss in oxygen absorbed by the lime-sulphur solution. 

 This last rate was then determined as accurately as possible (just as has 

 been explained when the cup was in air.) Here also, the direct method 

 of measuring the loss in volume of oxygen could be checked by taking 

 samples of gasoline-air at the beginning and at the end — making de- 

 terminations of the percentage of gasoline and of oxygen present 

 at each time and thus figuring the rate at which oxygen had 

 been absorbed. Now this last rate in the loss of oxygen, it will be 

 observed, was the rate at which oxygen passed through the same lecithin 

 membrane after the latter had become saturated with gasoline-vapor in 

 the presence of gasoliue-vapor-air. The rate at Avhich oxygen passed 

 through the lecithin membrane in air might therefore be compared with 

 the corresponding rate when the membrane was under the influence of 

 gasoline-vapor. In many of the experiments, the covered preparation dish 

 was taken from the container and aired until as much as possible of 

 the gasoline-vapor had evaporated from the lecithin cover. After that 

 the dish was carefull}^ replaced in fresh air in the container and the 

 rate at which oxygen passed through the membrane again determined. 

 This was done, as a check, to learn whether the lime-sulphur solution 

 was still in condition to absorb oxygen at a practically uniform rate 

 when the membrane was exposed to pure air.. Further precautionary 

 checks were made by finding the rate at which the uncovered dish of 

 lime-sulphur absorbed oxygen in pure air and in gasoline air. These 

 checks showed conclusivel}^ that at the end of the series of tests the 

 lime-sulphur solution was still active enough to absorb oxygen much 

 more rapidly than the lecithin or other membranes could take up that 

 gas and pass it through to the absorbing solution, either in pure air or 

 gasoline-vapor-air. In all cases, the temperature during any series of 

 tests was kept as uniform as possible. When the thermometer or bar- 

 ometer changed appreciably, or whenever the tests extended over more 

 than three hours, all gas-volume measurements were changed to 0° C, 

 and 7G0 m. m. mercury pressure before comparisons were made. 



Over 35 trial experiments were made Avilh the apparatus in learning 

 the best thickness of membrane to use, and the time-limits (in order that 

 the lime-sulphur solution should not be weakened too much), as well as 

 many other details. Thereafter, twelve series of tests were run to 

 determine the influence of gasoline-vapor upon the rate of oxygen transfer 



•The edge of the metal band, which was used around the top of the preparation dish to 

 fasten the cover-membrane tightly, projected above the surface of the latter enough to 

 entirely protect it from damage by the lop of the container when the air was being expelled. 



