Molality of the Positive Ion in Flames. Ill 



by the following example ; under certain conditions, the 

 author found that in a pure flame the minimum gradient 

 was 6*5 volts per cm., in a salted flame only 0'7, whilst an 

 assumption of uniform gradient led to 22. The magnitude 

 of the error will depend on the temperature of the flame ; at 

 temperatures below 1150° absolute it was found that the 

 three cases quoted above would give similar results. 



(2) Experimental Details. 



The method employed in the present series of measurements 

 was that adopted by Wilson in 1899. Two electrodes are 

 placed horizontally above one another in the flame, and the 

 current between them is measured for different potential- 

 differences — the upper plate being charged positively. A 

 small bead of salt is then introduced into the flame just 

 beneath the upper electrode, and the current measurements 

 are repeated. When the potential-difference is great enough 

 to drive back the positive ions against the uprush of the gas, 

 the current suddenly increases. The exact potential required 

 is obtained by comparing the two current curves. Knowing 

 this critical voltage, one can obtain the minimum electric 

 force in the flame ; if then the upward velocity of the flame 

 gas is known, a simple calculation gives the ionic mobilhVv. 



For a full account of the experimental arrangements 

 reference must be made to a preliminary paper by the 

 author *. The alteration in temperature was effected in 

 three ways : — (1) By altering the relative proportions of 

 gas and air ; (2) by raising or lowering the burner ; and 

 (3) by altering the mesh of the lower electrode. By com- 

 bining these three devices it was found possible to secure 

 a flame at temperatures ranging from 1150° to 1950° 

 absolute. 



(3) Account of Observations. 



At the very fir?t temperature investigated, viz. 1450°, a 

 search was made to see whether the mobility of the ions pro- 

 duced by the salts of a given metal depended on the particular 

 salt used. Potassium was chosen, because its conductivity is 

 high and its salts are easily procurable. The following salts 

 were carefully tested : — 



K 2 C0 3 ; KOH; KC1 ; KBr ; KI Simple salts. 



K 2 Mn 2 8 ; KNaC0 3 ; K 2 Fe(CN) 6 Complex salts. 



Other salts, e. g. the nitrate, were tried, but had to be 

 * Camb. Phil. Soc. xvii. 1, p. 26 (1910). 



