EXPERIMENT STATION BULLETINS. 445 



were given. The working difference which might occur between the two 

 systems of the apparatus was about 0.2 c. c. When a test to obtain the 

 influence of gasoline-vapor or carbon disulphide vapor upon the 

 hydroquinone-oxidase activity of insect tissue extract was to be 

 run, equal samples from the same tissue-extract were placed in each 

 of two sterile stender dishes of 36 m. m. diameter, and the dishes were 

 introduced into the two gas-containers ''c." ^'c". Fig. I. Mercury seal 

 was made at the mouth of each gas-container, and the U-tubes were con- 

 nected with the hydroquinone burettes. Then, by means of the mercury- 

 mug and the free opening of one of the two-way cocks of one gas-con- 

 tainer, as much insecticide vapor as was wanted could be introduced 

 from a separate stock-container of concentrated insecticide vapor. After 

 that, the cocks of the gas-containers Avere opened to the outside an 

 instant until both mercury manometers read level. The temperature 

 was recorded. Then the cocks were all properly set and the motor 

 started, so that in the two systems the gas or air was made to circulate 

 until it was uniformly mixed. The motor was stopped and started in 

 this way several times (pressure adjustments being made between times, 

 if necessary) until the manometers stood level after each short interval. 

 The cocks of the gas-containers were then quickly opened to the outside, 

 and hydroquinone solution was carefully let down from the burettes 

 until it reached the openings of the hooked U-tubes above the stender 

 dishes in the two containers "c." "c." A measured amount, usually 2 

 c. c. of hydroquinone solution to 5 c. c. of the tissue extract, was meas- 

 ured into each extract stender dish. Test was again made to see that the 

 pressure manometers stood level and then both cocks of each gas-con- 

 tainer were closed. The temperature was again noted. The barometer 

 reading and the readings of the tAvo gas-burettes Avere recorded. The 

 apparatus then stood during the desired period of the test in a room 

 where the temperature varied but little — and only very slowly — and one 

 could be sure that both systems of the apparatus were at the same temper- 

 ature w^hen readings of the gas-burettes were made. At the end of the 

 test period, the cocks at the top of the gas-containers were turned so as 

 to connect each container with its own system. The mercury in each gas- 

 burette Avas quickly manipulated so as to bring the mercury manometers 

 level, and the readings of the tAvo gas-burettes AAere taken. Then the 

 cocks "st" and "st"' Avere closed and the pumps were started. The gas 

 or air in each system was thus made to circulate through the absorption 

 flasks until trial showed that the readings of the gas-burettes (after 

 short runs of the pumps) were constant, Avhen the tAvo manometers ''e" 

 and "e" stood at zero. The reading of each gas-burette Avas then again 

 recorded. Thus there were three gas-volume readings for the system con- 

 taining the insecticide-treated extract and three for the check. The first 

 reading in each set shoAved the constant volume of the gas under certain 

 recorded conditions of temperature and barometric pressure; the second 

 showed the volume after oxygen absorption (i. e. by the extract contain- 

 ing hydroquinone solution), before any carbon dioxide had been re- 

 moved; and the third reading gave the volume after the carbon dioxide 

 l)roduced during the experiment period had been circulated through the 

 hydroxide flask and absorbed. The readings Avere all reduced to the 



