To ascertain this point, the temperature of the solution of sea salt was made successively to be 0°, 5°, 10°, 15°, 20°, 25°, and 30° C.; and three pipettes of the solution were weighed exactly at each of these temperatures. The third of these weighings gave the mean weight of a pipette. The corresponding weights of a pipette of the solution, were afterwards graphically interpolated from degree to degree. These weights form the second column of the following table, intitled, Table of Correction for the Variations in the Temperature of the Normal Solution of the Sea Salt. They enable us to correct any temperature between 0 and 30 degrees centigrade (32° and 86° Fahr.) when the solution of sea salt has been prepared in the same limits.
Let us suppose, for example, that the solution has been made standard at 15°, and that at the time of using it, the temperature has become 18°. We see by the second column of the table, that the weight of a measure of the solution is 100·099 gr. at 15°, and 100·065 at 18°; the difference 0·034 gr., is the quantity of solution less which has been really taken; and of course we must add it to the normal measure, in order to make it equal to one thousand millièmes. If the temperature of the solution had fallen to 10 degrees, the difference of the weight of a measure from 10 to 15 degrees would be 0·019 gr. which we must on the contrary deduct from the measure, since it had been taken too large. These differences of weight of a measure of solution at 15°, from that of a measure at any other temperature, form the column 15° of the table, where they are expressed in thousandths; they are inscribed on the same horizontal lines as the temperatures to which each of them relates with the sign + plus, when they must be added, and with the sign - minus, when they must be subtracted. The columns 5°, 10°, 20°, 25°, 35°, have been calculated in the same manner for the cases in which the normal solution may have been graduated to each of these temperatures. Thus, to calculate the column 10, the number 100·118 has been taken of the column of weights for a term of departure, and its difference from all the numbers of the same column has been sought.
Table of Correction for the Variations in the Temperature of the Normal Solution of the Sea Salt.
| Tem- pera- ture. | Weight. | 5° | 10° | 15° | 20° | 25° | 30° |
|---|---|---|---|---|---|---|---|
| gram. | mill. | mill. | mill. | mill. | mill. | mill. | |
| 4 | 100,109 | 0·0 | - 0·1 | + 0·1 | + 0·7 | + 1·7 | + 2·7 |
| 5 | 100,113 | 0·0 | - 0·1 | + 0·1 | + 0·7 | + 1·7 | + 2·8 |
| 6 | 100,115 | 0·0 | 0·0 | + 0·2 | + 0·8 | + 1·7 | + 2·8 |
| 7 | 110,118 | + 0·1 | 0·0 | + 0·2 | + 0·8 | + 1·7 | + 2·8 |
| 8 | 100,120 | + 0·1 | 0·0 | + 0·2 | + 0·8 | + 1·8 | + 2·8 |
| 9 | 100,120 | + 0·1 | 0·0 | + 0·2 | + 0·8 | + 1·8 | + 2·8 |
| 10 | 100,118 | + 0·1 | 0·0 | + 0·2 | + 0·8 | + 1·7 | + 2·8 |
| 11 | 100,116 | 0·0 | 0·0 | + 0·2 | + 0·8 | + 1·7 | + 2·8 |
| 12 | 100,114 | 0·0 | 0·0 | + 0·2 | + 0·8 | + 1·7 | + 2·8 |
| 13 | 100,110 | 0·0 | - 0·1 | + 0·1 | + 0·7 | + 1·7 | + 2·7 |
| 14 | 100,106 | - 0·1 | - 0·1 | + 0·1 | + 0·7 | + 1·6 | + 2·7 |
| 15 | 100,099 | - 0·1 | - 0·2 | - 0·0 | + 0·6 | + 1·6 | + 2·6 |
| 16 | 100,090 | - 0·2 | - 0·3 | - 0·1 | + 0·5 | + 1·5 | + 2·5 |
| 17 | 100,078 | - 0·4 | - 0·4 | - 0·2 | + 0·4 | + 1·3 | + 2·4 |
| 18 | 100,065 | - 0·5 | - 0·5 | - 0·3 | + 0·3 | + 1·2 | + 2·3 |
| 19 | 100,053 | - 0·6 | - 0·7 | - 0·5 | + 0·1 | + 1·1 | + 2·2 |
| 20 | 100,039 | - 0·7 | - 0·8 | - 0·6 | 0·0 | + 1·0 | + 2·0 |
| 21 | 100,021 | - 0·9 | - 1·0 | - 0·8 | - 0·2 | + 0·8 | + 1·9 |
| 22 | 100,001 | - 1·1 | - 1·2 | - 1·0 | - 0·4 | + 0·6 | + 1·7 |
| 23 | 99,983 | - 1·3 | - 1·4 | - 1·2 | - 0·6 | + 0·4 | + 1·5 |
| 24 | 99,964 | - 1·5 | - 1·5 | - 1·4 | - 0·8 | + 0·2 | + 1·3 |
| 25 | 99,944 | - 1·7 | - 1·7 | - 1·6 | - 1·0 | 0·0 | + 1·1 |
| 26 | 99,924 | - 1·9 | - 1·9 | - 1·8 | - 1·2 | - 0·2 | + 0·9 |
| 27 | 99,902 | - 2·1 | - 2·2 | - 2·0 | - 1·4 | - 0·4 | + 0·7 |
| 28 | 99,879 | - 2·3 | - 2·4 | - 2·2 | - 1·6 | - 0·7 | + 0·4 |
| 29 | 99,858 | - 2·6 | - 2·6 | - 2·4 | - 1·8 | - 0·9 | + 0·2 |
| 30 | 99,836 | - 2·8 | - 2·8 | - 2·6 | - 2·0 | - 1·1 | 0·0 |
Several expedients have been employed to facilitate and abridge the manipulations. In the first place, the phials for testing or assaying the specimens of silver should all be of the same height and of the same diameter. They should be numbered at their top, as well as on their stoppers, in the order 1, 2, 3, &c. They may be ranged successively in tens; the stoppers of the same series being placed on a support in their proper order. Each two phials should, in their turn, be placed in a japanned tin case ([fig. 90.]) with ten compartments duly numbered. These compartments are cut out anteriorly to about half their height, to allow the bottoms of the bottles to be seen. When each phial has received its portion of alloy, through a wide-beaked funnel, there must be poured into it about 10 grammes of nitric acid, of specific gravity 1·28, with a pipette, containing that quantity; it is then exposed to the heat of a water bath, in order to facilitate the solution of the alloy. The water bath is an oblong vessel made of tin plate, intended to receive the phials. It has a moveable double bottom, pierced with small holes, for the purpose of preventing the phials being broken, as it insulates them from the bottom to which the heat is applied. The solution is rapid; and, since it emits nitrous vapours in abundance, it ought to be carried on under a chimney.
The agitator.—[Fig. 91.] gives a sufficiently exact idea of it, and may dispense with a lengthened description. It has ten cylindrical compartments, numbered from 1 to 10. The phials, after the solution of the alloy, are arranged in it in the order of their numbers. The agitator is then placed within reach of the pipette, intended to measure out the normal solution of sea salt, and a pipette full of this solution is put into each phial. Each is then closed with its glass stopper, previously dipped in pure water. They are fixed in the cells of the agitator by wooden wedges. The agitator is then suspended to a spring R, and, seizing it with the two hands, the operator gives an alternating rapid movement, which agitates the solution, and makes it, in less than a minute, as limpid as water. This movement is promoted by a spiral spring, B, fixed to the agitator and the ground; but this is seldom made use of, because it is convenient to be able to transport the agitator from one place to another. When the agitation is finished, the wedges are to be taken out, and the phials are placed in order upon a table furnished with round cells destined to receive them, and to screen them from too free a light.
When we place the phials upon this table, we must give them a brisk circular motion, to collect the chloride of silver scattered round their sides; we must lift out their stoppers, and suspend them in wire rings, or pincers. We next pour a thousandth of the decime solution into each phial; and before this operation is terminated, there is formed in the first phials, when there should be a precipitate, a nebulous stratum, very well marked, of about a centimetre in thickness.
At the back of the table there is a black board divided into compartments numbered from 1 to 10, upon each of which we mark, with chalk, the thousandths of the decime liquor put into the correspondent phial. The thousandths of sea salt, which indicate an augmentation of standard, are preceded by the sign +, and the thousandths of nitrate of silver by the sign -.