TOXIG ACTION OF CERTAIN GASES. 3 
The apparatus used to maintain concentration and humidity con- 
sisted of a gas flow meter and a gas-air flow meter, both calibrated 
so that the exact amount of gas and gas air flowing at any one time 
was known, and could be maintained throughout the experiment. 
The flow of gas-air mixture was kept at approximately normal 
atmospheric pressure by having the inflowing air withdrawn as fast 
as it entered. The blast and suction were kept constant by pressure 
and suction regulators in the form of heads of water through which 
the excess air was allowed to bubble. The gases of fairly high vapor 
pressure (those whose pressure was sufficiently great to force them 
through the gas flow meter) were kept in small steel cylinders. 
In order to follow the air through the apparatus and to make 
more clear the production of a given concentration of a definite 
humidity, it will be necessary to trace the air from the blast cock 
through the apparatus. First, the air was passed over the pressure 
regulator, then into three bottles of sulphuric acid solution haying 
a density which would produce the humidity desired. From there 
it was forced through glass wool to take up any acid spray; thence 
to the mixing chamber, where it was joined by the gas. The mixture 
passed through a calibrated flow meter and into the fumigation jar, 
which was connected with the suction lne that drew off the air and 
caused the concentration to rise to the desired strength of gas air; 
that is, the suction line took care of all of the excess gas air. 
The valve on the gas cylinder being open, the gas pressure was 
kept constant by a head of liquid, and the gas was forced through 
a calibrated flow meter into the mixing chamber. As the humidity 
of the blast varied from day to day, an arbitrary humidity (40 per 
cent relative, which is common in Washington) was adopted. 
In most of the experiments a 5-liter jar constituted the fumiga- 
tion chamber. Inasmuch as the rate of flow of gas air was about 
two liters per minute, and the minimum exposure was seven minutes, 
ample time was allowed for the change from air to gas air of the 
proper concentration. . 
In the case of chloropicrin (which has a low vapor pressure), 
it was necessary to draw the air through the fumigant for the high 
concentration. For lower concentrations only part of the air was 
drawn through and the rest by-passed, and then both reunited as 
they entered the line to the gas-air flow meters. 
The exact concentration of gas was usually determined by absorp- 
tion in alkaline solution. The details for the concentration deter- 
mination for each gas are given under the heading of that gas. 
The following gases were investigated: Phosgene, cyanogen 
chlorid, arsine, chloropicrin, illuminating gas, carbon monoxid, 
and hydrocyanic acid. ‘The last, which is a standard fumigant 
against many insects, was used for comparison. 
