356 Dr. R. A. Smith. Measurement of the [Apr. 29, 



mixture is brought into clear light ; iodine is set free and the solution 

 becomes yellow. 



2. The amount of iodine freed can be titrated with great exactness 

 by the use of hyposulphite, as is well known. 



In these two facts lies the whole process, the first is the new part, 

 the second makes the first quantitative, and its use is of course part 

 of the novelty. 



3. It is known that strong acid liberates iodine. Weak acid does 

 so after a long time, but the process is hastened by light. 



4. Heat even to the boiling point does not act so well as light 

 (experiments being made in sealed tubes to prevent loss of iodine, and 

 with a considerable volume of air)* 



5. Heat assists the action of light. 



6. A solution may be exposed day after day so as to give the 

 accumulated effect of sunlight, in a measurable condition at the end 

 of the time. 



7. The solution of iodide of potassium as hitherto obtained is subject 

 to change. An old solution, that is, one nearly a month old, was found 

 more sensitive than a new one in all cases tried. 



8. The result of No. 7 is, that a certain allowance may require to be 

 made for this, in those cases where the periods of observation with one 

 solution are long. 



9. The amount of allowance to be made for temperature is not 

 made out. It is not certain that any is required in the cases when 

 weak acid is used. The weather has not allowed any combined action 

 of great light and heat, but with heat and light in the rays from an 

 electric light with a parabolic reflector, the action was very rapid. 



10. Specimens of experiments (prospective at first) . It was found 

 convenient to use a solution of 2 grms. of iodide of potassium, after- 

 wards changed to 1 grm., in 100 of water, and to use half of this for 

 an experiment, i.e., 50 cub. centims. of the solution, which may be 

 called A. 



A nitric acid solution having an acidity equal to 1 per cent, of 

 sulphuric anhydride was made ; this may be called B. Only very 

 small portions of B were added to A. 



The following experiments were made in wide loosely corked and 

 half filled test tubes, and here are early trials : — 



13th February. 

 50 c.c. of A with 0*2 c.c. of B ; no action in three hours. 

 50 „ 0*4 „ action; | not looked at before 



50 „ # 8 „ decided action ; J the end of 3 hours. 



No colour in a shaded part of the room in 420 hours in liquids of 

 the same strength. 



