| Gold terchloride | 1 | grain. |
| Sodium acetate | 10 | " |
| "chloride | 10 | " |
| Hot water | 20 | ounces. |
Mix twenty-four hours before use. Neutralize with chalk or whitening (carbonate of lime).
The name of M. Le Gray must be mentioned as the originator of the lime and gold toning bath; although the original formulæ differ somewhat from the one now used, the results are identical. The original formulæ consisted first in washing away a portion of the free silver by soaking the proofs for a few minutes in two changes of water, then submitting them to the action of an auriferous bath, composed of
| Terchloride of gold, 1 per cent. solution | 1 | part. |
| Hyperchloride of lime (white powder) | 3 | " |
| Distilled water | 1,000 | " |
The action was complete in ten to fifteen minutes, when the prints required washing in two changes of water to free them from the chloride of lime remaining in the fibers previous to fixing in one to six of hypo. If the tone were satisfactory at the expiration of fifteen minutes, the ordinary washing could be proceeded with; if not, the proofs were submitted to a final bath composed of:
| Gold terchloride | 2 | parts. |
| Hypo. | 200 | " |
| Distilled water | 1,200 | " |
M. Le Gray says: "The proof ought not to be left in this bath less than fifteen minutes, as that is the minimum time necessary to insure the permanency of the picture; but it may be allowed to remain in it for as much longer as is requisite for obtaining the desired tone." Efficient washing in warm and cold waters completed the operation. Should any of our provincial members experience a difficulty in obtaining calcium chloride for their experiments, it can be easily made by causing dilute 7 to 3 hydrochloric acid to react on common whitening, and when neutral filter and set aside for the crystals to separate out.
Experiment 6.—The uranium and gold toning bath has many friends. The tones are said to be richer and to economize gold, while it is very easy to work. I am unable to give the author's name, but I can present a formula which has worked well in my hands. After washing away the free silver tone in the following mixture:
No. 1.
| One grain acid solution of gold terchloride | 1 | ounce. |
| Water | 7 | ounces. |