296 PREPARATION OF ALDEHYDE AND ACETIC ACID. 
parent rhombohedral crystals. From this, as is well known, 
perfectly pure aldehyde is prepared by the reaction of di- 
lute sulphuric acid. 
In some experiments made to determine the delicacy of 
aldehyde as a test for oxide of silver, we obtained the fol- 
lowing results: — 
1. A solution of 1 part of nitrate of silver in 1000 of dis- 
tilled water, when heated gently in a test tube with a drop 
or two of aldehydite of ammonia, formed a brilliant metallic 
pellicle on the inner surface of the glass. 
2. A solution containing 1 part in 2000 produced the 
pellicle in distinct spots and not continuous as in the former 
case. At the same time the liquid became of a deep green- 
ish purple colour, and although only one quarter of an inch 
thick was nearly opaque. 
3. A solution of 1 part in 10,000 gave no adherent pel- 
licle, but on continuing the heat for two or three minutes 
became strongly coloured, presenting a deep greenish 
purple by transmitted, and a dull olive by reflected light. 
4. In a solution of 1 part in 20,000, the peculiar greenish 
purple tint was still quite decided, and even when the so- 
lution was diluted so as to contain only _^^.-th of nitrate 
of silver this colour was very distinctly manifested after 
heating it sometime with aldehydite. Compared with the 
faint opalescence caused by the addition of chloride of so- 
dium to the same solution, this effect of the aldehyde ap- 
peared to be the more striking of the two. 
The peculiar purplish tint of the liquid, remarked in 
these experiments, is evidently due to the finely divided 
metallic silver held in suspension, and affords therefore a 
striking confirmation of the statement of Dupasquier, that 
not only gold, but silver and other opaque bodies, present 
this hue when greatly subdivided and viewed through a 
clear suspending medium, (Comp. Rend., No. 1, July,. 
1845.) It may be added that the same hue is developed 
when a very dilute solution of nitrate of silver is subjected 
to the reducing action of a formiate. 
