LUMINOSITY OF FLAMES CONTAINING SALT VALOURS. 
89 
(15.) The percentage of the salt which is ionised when the concentration is very 
small has the following values :— 
Caesium salts.91 
Rubidium salts ..69 
Potassium salts.58 
Sodium salts. 1 '6 
(16.) The constant, b, is proportional to the molecular weight of the salt. 
According to the theory this shows that all salts give negative ions having the 
same velocity due to an electric field. 
(17.) The conductivity due to solutions containing a mixture of salts agrees with 
that to be expected on the ionic theory. 
It is probable that the luminous salt vapours are not appreciably moved by an 
electric field because very few of the salt molecules which become luminous, while 
the salt passes through the flame, also get ionised. 
If f denotes the fraction of the salt molecules which are ionised in the flame 
at any instant, then fJcX. will be the average velocity of a salt molecule over a 
long time due to an electric field. Now f for sodium is very much smaller than 
for csesium, yet both give positive ions, having about the same velocity. We 
conclude, therefore, that the ionic velocities observed are .not of the values of 
fkK, but the maximum possible velocities 7cX. It follows that when a salt mole¬ 
cule ionises, the ions sometimes last long enough for their velocity to be measured 
before they recombine. The method used to measure the ionic velocities indicates 
the maximum velocities, not the average velocity for a large number of ions. 
According to the most recent theories of ionic velocity, the velocity depends 
chiefly on the nature of the gas in which the ions move, and not much on the 
mass of the ions. We should, therefore/expect Jc to be about the same for ions of 
different salts all moving in the same flame. 
The fact, first discovered by Gouy and here confirmed, that the luminosity due 
to sodium salts varies nearly as the square root of the concentration indicates that 
the luminous molecules are produced by a binary reaction from the sodium salt. 
If the luminosity is due to atoms of sodium, a possible reaction is Na 2 0 + C0 = 
2Na + C0 2 . 
CO and C0 2 are both present in large quantities in the flame, so that their 
concentrations may be regarded as constants, and therefore the concentration of 
the Na will vary as the square root of the concentration of the Na 2 0. Probably 
nearly all the salt is converted into Na 2 0 in the flame so that the concentration 
of the salt is proportional to the concentration of the Na 2 0, when the concentration 
is not very small. 
It seems probable that adding HC1 to the flame does not prevent the conversion 
of NaCl into Na 2 0 by the flame gases. Otherwise, HC1 should diminish the 
