OF ARTS AND SCIENCES. 311 



" To measure the current, a leaden pipe (the material most readily 

 at hand), 1.25 inches in diameter and 53 feet in length, is placed near 

 and a few inches below the mouth of the blowing-machine. This 

 pipe is coiled, as it leaves the manufactory, into a circle of about 2.5 

 feet in diameter, of which it makes eight turns. In the mouth of the 

 trunk, before described as attached to the blowing-machine, is a tube 

 of tinned iron, of the same diameter as the pipe, and bent at a right 

 angle ; the upright branch, about six inches long, reaching to the mid- 

 dle of the mouth, while the horizontal portion, about five inches in 

 length, reaches within 2.5 inches of the end of the leaden pipe. Each 

 ventilator, when examined and tested, is placed upon the upright por- 

 tion of this tube. For this purpose the ventilator has through it, or 

 attached to its side, a corresponding tube of the same diameter. The 

 connection between these two tubes is completed by a glass tube 4 

 inches long and 2 inches in diameter, and the fitting made close by 

 means of cotton-wool fastened loosely around the extremities of the 

 tvi^o metallic pipes. In this compound pipe the current is induced, and 

 its velocity noted. To effect this last object, advantage is taken of the 

 well-known action of iodine upon starch.* Iodide of potassium is dis- 

 solved in a strong solution of starch in hot water, in the proportion of 

 three grains or more of the iodide to an ounce of the solution. A 

 piece of paper wetted, or rather smeared, with the prepared starch is 

 suspended within the glass tube, which can be readily removed for this 

 purpose, by means of a wire hook attached to the metallic pipe. A 

 current is now induced by the action of the blast upon the ventilator, 

 and chlorine gas allowed to enter the opposite end of the pipe, which 

 is kept carefully removed from the influence of the blast. The chlo- 

 rine is carried along with the current until it reaches the starched 

 paper, which it instantly dyes a deep blue ; the chlorine, by its superior 

 affinity for the potassium, seizing upon it, and leaving the iodine free 

 to act upon the starch. 



" Chlorine is conveniently obtained for this purpose from Labar- 

 raque's solution of chloride of soda, and its liberation quickened, if need 

 be, by adding a few drops of sulphuric acid. When the vial contain- 

 ing the chlorine is closed by the finger, and held a few seconds in the 



* The action of hydrosulphuric acid upon moist carbonate of the oxide of lead 

 was first suggested for this purpose, but the chlorine and iodide were judged 

 most convenient. 



