The solution of gold, when kept in the dark, will keep a long time.
For fixed negative the following intensifiers have been used with good results:—
1. Intensifier with pyrogallol.
| (a.) Silver nitrate | 5 | parts by weight |
| Distilled water | 75 | parts by weight |
To this solution is added—
| (b.) Citric acid | 5 | parts by weight |
| Distilled water | 75 | parts by weight |
| (c.) Pyrogallol | 1·25 | parts by weight |
| Distilled water | 150 | parts by weight |
2. The hydrochinone intensifier recommended by Baron Hübl:—
| Hydrochinone | 10 | parts |
| Water | 1000 | parts |
To this as much acid is added as will keep the solution clear for {39} five minutes, when it is mixed with a third of its quantity of 1 : 30 solution of silver nitrate. The addition of a few drops of sulphuric acid or 5 parts of citric acid is generally enough. This intensifier acts somewhat more delicately than the silver and iron solution, and has the advantage over the pyrogallol that it can be applied as frequently as desired in full daylight to the fixed negatives without any danger.
3. The metol intensifier.