AND Till': ACTINIC CONSTITUTION OF THE ATMOSPIIEUK. 9 



1G.2 c. c. of tlie same liiiu'-wiiter and had consequently lost 6.6 c. c. of 

 tlieii- oi'iginal streiigtii. Two vessels with the new solution titrated to- 

 gether 21.7 ('. c. and had consequently lost not more than 1.1 c. c. The 

 new solution is therefore nearly 6 times less sensitive than the other. 



The next day, the weather being fine, the losses amounted to 1.5 c. c. 

 for the new solution and to 8.5 o. c. for the old. This is al)out the same 

 ratio as on the day before. 



On Se[)tember 12th, after a fine day, four trials gave me the same 

 results, losses of 9.3 c. c. for the old solution, and of 5.9 c. c. for the newly 

 made. The difference in sensitiveness was less marked than six days 

 previously. 



On Septendjer 25th, twenty days later, the losses became 8.6 c. c. for 



the first liquid, and 7.7 c. c. for the second. This is not yet equality, 



which was reached only in the month of Octobei-, after a little more than 



a month. 



The fact that two liquids of different ages reach at the end of some time the 



same degree of sensitiveness, proves that thei-e must be a maximum. We shall, 



however, soou see that this is not a maximum maximornm. However this may be, 



if the solution of oxalic acid has once reached this maximum, it diff'ei's in no way 



from wdiat it was at first, neither from a chemical nor from a physical point of 



view ; it gives by evaporation the same crystallized acid, and its acidimetric value 



is unchanged. A molecular activity, however, has been at work, upon which I 



shall not dwell just here. I will state now only two im[)ortant facts concerning it : 



one is that it requires time for its completion, and the other that it betrays itself 



by easier oxidizability under the influence of solar radiations. 



The only phenomenon which in our present state of knowledge may be com- 

 pai'ed to that which we have just discovered, is the increase of sensitiveness 

 observed iu seusitized collodion which has been allowed to rest and to grow old 

 for a few days. This fact is well known to photographers. It may make us think 

 also, by analogy, of the valuations in the rotatory power of sugar solutions, some 

 hours after their prepai-ation u[) to the moment when they become stable. It is 

 admitted that these few hours are necessary to enable the sugar molecules to spread 

 uniformly throughout the solution and to assume the orientation necessary to 

 stable equilibrium. But all these analogies are remote. The phenomenon deserves 

 being investigated by itself, and we have here only to face its practical conse- 

 quences. 



These may be summed up in a few words : that it is desirable to allow the oxalic 

 acid solution to acquii-e such sensitiveness before using. This is all the easier since 



