150 DESTRUCTIVE INSECTS OF VICTORIA : 



The carbonic acid gas does not injure the foliage of orange trees 

 confined in it ; when sufficiently pure, it stupifies the scale insects 

 confined in it for half an hour, but they wholly recover from the effects 

 of the gas after the lapse of a few hours. 



I noticed that when the trees were treated with the cyaniole and 

 soda process in the hottest part of a very hot day the foliage was almost 

 as severely injured as when the hydrocyanic acid gas had been used 

 alone. We may conjecture that this results from the fact that at a high 

 temperature the carbonic acid gas is freed from the aqueous vapour, 

 leaving the latter in a proper condition for again uniting with the 

 hydrocyanic acid gas. When these two gases are reduced to the liquid 

 state by pressure or by great cold, it is found that the liquid carbonic 

 acid gas boils at a much lower temperature than the liquid hydrocyanic 

 acid does. A given quantity of water will dissolve about its own 

 volume of carbonic acid gas, but all of this gas may afterwards be 

 expelled by boiling. 



Remarks. Of the three processes described above, it is evident that 

 the dry gas process is preferable to either of the others. Not only is 

 there less labour in its manipulation, but it is also much cheaper than 

 either of the other processes. 



After the tree has been confined in the gas the proper length of time 

 the tent should be entirely removed from it. On two different occasions 

 I simply opened the tent to allow the gas to escape, after which the 

 tent was again placed on the tree and the doorway of the tent left 

 partially open ; it remained on one of the trees for seven consecutive 

 days, while on the other tree it was allowed to remain only for a day 

 and night, but in both instances the trees were nearly killed. 



The generator used in the production of the hydrocyanic gas is as 

 shown in the foreground in Fig. 17, it was originally devised by Mr. 

 Alexander Craw and myself, and has given perfect satisfaction. 



This generator consists of two leaden vessels placed one above the 

 other and connected by a brass stop-cock ; to the end of the valve of 

 this stop-cock is firmly soldered an L-shaped piece of an iron rod, to be 

 used in opening and closing the stop-cock. The lower vessel is entirely 

 closed above ; near one side of the top is a screw-cap, covering the 

 opening through which the proper chemicals are to be introduced into 

 the vessel, while on the opposite side is an opening over which is firmly 

 soldered the end of a leaden pipe, through which the gas passes on its 

 way from the generator to the tent. When it is intended to pass the 

 gas through sulphuric acid this leaden pipe is made to enter one side of 

 an upright leaden vessel, and as near the bottom of the vessel as 

 possible ; to the top of the leaden vessel is attached a tin or leaden pipe 

 which conducts the dried gas into the tent. Of course, if it is not 

 desired to pass the gas through sulphuric acid, the leaden acid-vessel 

 can be dispensed with, the leaden pipe from the generator passing 

 directly into the tent.* 



* Mr. Coquillett writes later as follows : " In speaking of the gas generator I recommend passing 

 the gas through sulphuric acid ; a better way is to pass it into the acid, the leaden pipe which 

 conducts the gas from the generator entering the upright leaden vessel above its middle, and 

 curving downward in the vessel until the mouth of the pipe nearly reaches the bottom of the vessel 

 and is covered by the acid." C. V. R. 



