COMMUNICATION FROM PROF. GEO. H. COOK.

ROB'T GERE, Esq.,

Superintendent Onondaga Salt Springs:

Sir, The following is a report of the results of my experiments and observations made upon the Onondaga brines, as directed by you. It includes experiments upon the brines directly from the wells, and upon those taken in different stages of the manufacture; the general object in view being the improvement of the quality of salt made from said brines.

Under this general subject, the inquiry first in importance, is, whether the Onondaga brines are more impure than others from which salt of acknowledged excellence is made. To determine this, the numerous analyses of the former, heretofore made, have been verified, and the results compared with published analyses of sea-water, and of various brine springs. The bittern, or mother-water, from which salt has been crystalized, has also been analysed, and compared with bittern from other salt works. (For a full statement of these and other results, see Appendix.)

The comparison leaves no doubt that if salt of an inferior quality is made, it is the fault of the manufacturer and not of the brine. The salt from sea-water has the highest reputation of any in the market ; and yet it will be observed that both the water and the bittern of it, are highly charged with impurities.

2. Numerous analyses of salt, both foreign and domestic, have been made, and the comparison shows very favorably for much of the salt made at Onondaga.

3. A careful examination has been made of the differences between salt made by boiling and that made by solar evaporation. If common boiled salt, which comprises most fine salt, be dissolved in water, the

solution is generally milky; and if allowed to stand till it becomes clear, a white or dirty white precipitate will be found deposited. This is common to Onondaga and Liverpool salt. Its amount is very variable, in some specimens, being scarcely perceptible, and in others nearly one per cent. The largest quantity is found in the salt which has been boiled the most violently. It is made up of sulphate of lime, (gypsum,) which has lost its water of combination, and sometimes contains small quantities of magnesia and of oxide of iron.

The sulphate of lime in the sediment comes from bad panning; and this in boiling with the salt and brine, loses its combined water.+

The magnesia comes from the chloride of magnesium, a constituent of the brine, which is decomposed by a moderate heat. The kettles being suspended by their rims are entirely exposed to the fire. When the brine in the kettle is diminished by boiling, it leaves uncovered a portion of the kettle which is exposed to the direct action of the fire, and of course becomes very hot. The effect of this is to decompose any chloride of magnesium which may be in the scale on the inside of the kettle and above the brine. The fact of this decomposition going on, is known from the escape of hydrochloric acid into the air where it is easily detected by its odor or by its action on litmus paper, the common acid test; and the magnesia is always to be found in the scale which collects on the inside of the kettle, and, as above mentioned, sometimes in the salt.

The oxide of iron is also a constituent of the brine, and it is sometimes produced by the rusting of the kettles.

In salt prepared without boiling no sediment of this kind is ever • Panning is a technical term applied by salt boilers to the process of removing the impurities which precipitate in the kettles before the salt crystalizes. The kettles, common potash kettles, containing about 100 gallons, being hemispherical in form and the heat applied to the bottom and sides, the heated brine rises along their inner surface and descends in the centre, carrying with it the particles of solid matter which may be in the liquid. At the filling of the kettles, pans similar in form to large frying pans, are set in the bottoms of each. The solid particles descending in the centre, fall directly into these pans. When the salt begins to crystalize, the pans with their contents are drawn out. If the boiling is gentle and the pans drawn with care, but very little of this sediment or "bitterings," as it is called, is left in the kettle with the salt.

Common gypsum, after it has been dried at the heat of boiling water, (212°,) contains nearly twenty-one per cent of water. This is true of the crystals from the solar salt fields. The pannings and the sediment when dried at this temperature retain scarcely any.

found; whatever sulphate of lime it contains, is unchanged and dissolves with the salt; no magnesia is found in it, and but very rarely a trace of oxide of iron.

4. A peculiarity in the manufacture of fine salt, adopted in order to remove oxid of iron from the brine more effectually and rapidly, has also been examined. A very minute portion of this oxide gives to salt a reddish yellow hue, and diminishes its market value, although the oxide of iron is quite harmless in its nature. If the brine is exposed to the air a sufficient length of time, the oxide of iron in it is precipitated. The addition of a little quick-lime to the brine is generally said to hasten this process. Its use is allowed by law, and for some years past, all the manufacturers of fine salt on the Onondaga reservation have used it. The amount to be used for this purpose is very small, only one quart to brine enough to make one hundred bushels of salt, and if this quantity were never exceeded, it would be a difficult matter to assign any injurious results arising from it. But it is liable to be used in excess, and it is so used, as is proved by finding recently made salt which was slightly alkaline; by finding salt containing a large amount of carbonate of lime by finding brine in which most of the chloride of magnesium is wanting and its place supplied by an equivalent portion of chloride of calcium; and by the remarkable effect produced on the color and hardness of the salt; the color being of a chalky white, and the grain or crystal of the salt soft and spongy. The last character is a matter of common observation; those who are accustomed to handling salt will detect the smallest amount of lime in it by the feel. This excess sometimes gets in by carelessness, at others by drawing the brine into kettles before the lime which has been properly added in the reservoirs has had time to settle; lime is used for stopping cracks in kettles, and it is used for stopping leaks in the reservoirs. Whenever kettles are allowed to stand empty for a few days, or when during work they are filled too full, they rust. This rust discolors the salt and renders it unsaleable, and the boilers who are generally paid according to the amount of merchantable salt, iose their labor in making it. A small qnantity of lime mixed with the salt hides this color, and it is sometimes used for this purpose.

(Note.)—It should be stated that whenever salt containing an excess of lime is found by the inspectors, it is at once condemned and destroyed.)

If lime as is generally conceded, makes the grain of the salt soft it must affect its draining, causing it to lose its bittern or motherwater more slowly than other salt. As salt is sold by the weighed bushel the use of lime may in this manner become a fraudulent operation.

In comparing salt made with lime and that made without its use, unusual facilities have been enjoyed in the manufactory of Thomas Spencer, Esq. late superintendent. In the Spring of 1849 this gentleman put in operation a block, or fine salt manufactory, in which he proposed to dispense entirely with the use of lime. Through the whole of the seasons of 1849 and 1850 his plan has been carried out, and the results have equalled his most sanguine expectations, producing salt which compares well with the best Liverpool salt, as is shown by the analysis, and by the experience of dairymen and others who have tried it carefully and thoroughly. His manufactory is large and admirably arranged for producing salt of good quality and at a cheap rate. A material difference between his arrangement and the common one is that by the waste heat of his block the brine is warmed and made to precipitate its iron before being drawn into kettles.

5. Experiments have been made to find some substitute for lime, which would be practicable in the blocks as they are now arranged. For this purpose it was proposed to try the effect of alum, adding a pound to brine enough to make one hundred bushels of salt. Through your efficient co-operation this experiment has been thoroughly and practically tried. The results are of the most satisfactory character, the salt being of good color, firm grain, and draining almost perfectly dry in a short time. The success may be judged of by the testimonials which you have procured for me of the several manufacturers who have tried it with special care the past season.*

In addition to the experience of the manufacturers on the effects of alum, the following facts may be stated.

Alum has long been used in different countries to clear turbid water; and at sea to percipitate flocculent matter which forms in it. Its use in the manufacture of salt is by no means new. Dr. Charles Leigh, in his "History of Lancashire and Cheshire," Eng., written in the year 1700, says that salt boilers have long used it in those counties to improve the quality of salt, (p. 44.) Thomas Lowndes, Esq., of

• See Letter on p. 19.