PART I.] Methods of Estimating the Carbonic Acid of Soil Air. 187 



observations at a lower level were not attempted, as it did not seem to be desirable 

 to go deeper into a soil such as that of Calcutta where the water-level is for a 

 considerable portion of the year so superficial as to cause saturation of the soil at 

 a short distance beneath the surface. 



The method adopted for conveying the air from the soil at these depths was 

 very simple. Two lead tubes were procured, and at one end of each a hollow 

 perforated bulb was soldered. A pit was dug in the soil — the ordinary alluvial soil of 

 Calcutta — perfectly free from all sources of surface pollution, and which had probably 

 not been disturbed for a quarter of a century. One of these tubes was passed through 

 the bottom of an ordinary flower pot, inverted, and perforated in numerous places. 

 Below and surrounding this pot fragments of earthenware were arranged so as to keep 

 the earth from plugging the orifices in the bulbous extremity of the leaden tube. 

 The pit was now filled up to within 3 feet of the surface and the other tube intro- 

 duced and similarly protected from being plugged by the fine soil ; the earth was 

 then heaped up and well beaten down, until it reached the level of the surface. 



The other pit was of a similar kind, and the leaden pipes were introduced and 

 protected in the same manner. The observations in each case were not under- 

 taken until a considerable period had elapsed, so as to allow the soil to regain its 

 ordinary condition. 



The tubes were then conducted into a room and attached to an aspirator 

 capable of holding thirty-eight and a half litres. 



The remarks made by Dr. von Pettenkofer with reference to the ease with 

 which air could be made to pass either way through the tubes which he had 

 introduced into the earth apply with equal force to our own tubes. Air could be 

 blown through the tubes with the greatest ease, so much so that we could not for 

 certain distinguish the pipes which had been lodged in the earth from a pipe of 

 similar length placed alongside them, but with both its ends opening into the free 

 air by blowing alternately through them. This fact of itself testifies to the readiness 

 with which intercommunication occurs between the atmosphere and the sub-soil air. 



Attached to the aspirator — intervening between it and the pipe leading into the 

 soil — were the usual appliances for estimating the amount of carbonic acid by the 

 Baryta process, as devised by von Pettenkofer many years ago, and which is fully 

 explained in all modem treatises on chemistry. Briefly described, the method 

 consists in causing the air under examination to pass through a flask containing 

 a solution of baryta of known alkalinity, and subsequently ascertaining how much of 

 the alkalinity has disappeared (by the passage though it of air containing carbonic 

 acid) by means of a standard solution of oxalic acid — turmeric paper being employed 

 in preference to litmus for ascertaining the precise stage when the solution becomes 

 neutral. ' 



This information having been obtained, the precise amount of carbonic acid 

 was calculated by the method usually adopted in connection with volumetric 



