THE GROUND SQUIRRELS OF CALIFORNIA. 



763 



Thus it is immaterial whether the carbon disulphid is exploded or not, 

 provided the gases reach the animal. Experiments with artificial bur- 

 rows where the vaporized gas or products of explosion have to travel 

 considerable distances before reaching the animals indicate that effective 

 work depends largely (1) on the shape of the burrow, (2) the relative 

 elevation of the squirrel and (3) the point of application of the fumi- 

 gant. That is to say, carbon disulphid gas will flow readily on the 

 level or down hill. It will not, in either its exploded or unexploded 

 form, rise over an elevation of even one to two feet, unless driven by 

 some form of pump or compressed-air apparatus. 



Our conclusions with reference to the question of " exploded versus 

 the nonexploded gas" is that it makes very little difference. Under 

 some circumstances there may be a slight advantage in favor of the 

 .exploded, since the force of explosion tends to disseminate the gas 

 somewhat more quickly, but, in so far as our observations go, under any 

 conditions we have been able to provide, there is practically no choice 

 between the two methods. Furthermore, either or both methods may 

 fail unless some form of pump is used to disseminate the gas. Any 

 form of pump which will distribute adequately either the exploded or 

 unexploded gas should be effective. 



EXPERIMENTS WITH GASES OTHER THAN CARBON DISULPHID. 



We have tried a number of other gases which are sometimes used 

 or which naturally suggest themselves because of their toxic properties. 

 Among these is chlorine, which is effective but difficult to use because 

 of its corrosive qualities. It is hardly likely, however, that the use of 

 chlorine would be less expensive than carbon disulphid, which is known 

 to be effective. 



Gasoline is not nearly so effective as carbon disulphid. Two per 

 cent gasoline vapor will kill animals in about 50 minutes. The explosion 

 of gasoline apparently does not give off a highly toxic gas. Our figures 

 indicate that the cost of killing squirrels is hardly likely to be less by 

 this method than when carbon disulphid is used and owing to the longer 

 time it takes to kill animals would probably prove less effective. 



Carbon tetrachlorid appears to kill animals in about 40 minutes, 

 but is more expensive than is carbon disulphid. 



Sulphur dioxid, formed by burning sulphur, appears to kill in about 

 40 minutes. This is twice as long as the time required by an equal 

 percentage of carbon disulphid. 



The gas known as arsine, generated by the decomposition of calcium 

 arsenide with water, appears to be quite effective. We were unable 

 unfortunately to obtain sufficient amounts of this material to carry out 

 conclusive experiments. There is some reason to believe that it might 

 be as effective and cheaper than carbon disulphid. It has one disad- 

 vantage, however, in that it is highly poisonous to man. It is possible 

 that further studies may indicate sufficient superiority on the part of 

 this substance to justify recommending its use. 



CONCLUSION. 



Our general conclusion in these experiments is that good results 

 may be obtained always by using 2 per cent carbon disulphid in air. 



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