SODIUM CHLORIDE BERTHOLLETS LAWS 411 



of Aral of the Caspian), and their water has evaporated and formed 

 (if the mass of the inflowing fresh water were less than that of the 

 mass evaporated) deposits of rock salt. It is always accompanied 

 by gypsum, because the latter is separated from sea water before the 

 sodium chloride. For this reason rock salt may always be looked for 

 in those localities where there are deposits of gypsum. But inasmuch 

 as the gypsum may remain on the spot where it has been deposited (as 

 it is a sparingly soluble salt), whilst the rock salt (as one which is very 

 soluble) may be washed away by rain or fresh running water, it may 

 sometimes happen that although gypsum is found still there may be 

 no salt ; but, on the other hand, where there is rock salt there will 

 always be gypsum. As the geological changes of the earth's surface 

 are still proceeding to the present clay, so in the midst of the dry land 

 salt lakes are met with, which are sometimes scattered over vast dis- 

 tricts formerly covered by seas now dried up. Such is the origin of 

 many of the salt lakes about the lower portions of the Volga and in the 

 Kirghiz steppes, where at a geological epoch preceding the present the 

 Aralo- Caspian Sea extended. Such are the Baskunchaksky (in the 

 Government of Astrakhan, 112 square kilometres superficial area), the 

 Eltousky (140 versts from the left bank of the Volga, and 200 square 

 kilometres in superficial area), and upwards of 700 other salt lakes 

 lying about the lower portions of the Volga. In those in which the 

 inflow of fresh water is less than that yearly evaporated, and in which 

 the concentration of the solution has reached saturation, the self- 

 deposited salt is found already deposited on their beds, or is being yearly 

 deposited during the summer months. Certain limans, or sea-side lakes, 

 of the Azoff Sea are essentially of the same character as, for instance, 

 those in the neighbourhood of Henichesk and Berdiansk. The saline 

 soils of certain Central Asian steppes, which suffer from a want of 

 atmospheric fresh water, are of the same origin. Their salt originally 

 proceeded from the salt of seas which previously covered these localities, 

 and has not yet been washed away by fresh water. The main result of 

 the above described process of nature is the formation of masses of rock 

 salt, which are, however, being gradually washed away by the subsoil 

 waters flowing in their neighbourhood, and afterwards rising to the 

 surface in certain places as saline springs, which indicate the presence 

 of masses of deposited rock salt in the depths of the earth. If the sub- 

 soil water flows along a strata of salt for a sufficient length of time it 

 becomes saturated ; but in flowing in its further course along an im- 

 pervious strata (clay) it becomes diluted by the fresh water leaking 

 through the upper soil, and therefore the greater the distance of source 

 of a saline spring from the deposit of rock salt, the poorer will it be in 



