TRANSACTIONS OF THK SECTIONS. 



67 



On the Action of Low Temperatures on Supersaturated Saline Solutions. 

 By C. ToMLiNsoN, F.B.S. 



It is known that when a saline solution, saturated at a certain temperature, is 

 reduced in temperature, salt is deposited ; but in the case of certain salts their su- 

 persaturated solutions, contained in clean vessels, and protected from the action of 

 nuclei, do not by reduction of temperature deposit the normal salt, but a salt of a 

 modified character, and of a lower degree of hydration, as in the case of sodic sul- 

 phate, when a seven-watered salt is formed instead of the usual ten-atom hydi'ate. 

 Mr. Tomlinson, in some recent experiments, has shown that the supersaturated 

 solutions of certain salts may be reduced to near the temperature of zero without 

 any deposit of salt ; but below this they form tetrahedral crystals, which increase 

 until the whole of the solution becomes solid. K, now, the tube be put into snow 

 and water at 32°, the solid melts rapidly, and the solution becomes clear, bright, and 

 supersatiirated as before. This effect may be produced any number of times, pro- 

 vided the solution be preserved from the action of nuclei, or carriers of nuclei, such 

 as the air. The only precautions to this end are to use clean filtered solutions in 

 clean tubes, kept plugged with cotton-wool. 



A supersaturated solution of the double salt, formed by the sulphates of zinc and 

 magnesia in atomic proportions, became solid at —10° F., that of the double sulphate 

 of copper and magnesia at — 4°, that of sulphate of zinc and potash alum at 0°, that 

 of sulphate of zinc and ammonia at 4°. Other examples are given in the paper, with 

 cases in which modified salts are formed and remain peimanent when the tempera- 

 ture is raised. 



If the cotton-wool be removed only for a few seconds, while the solution is solid, 

 it crystallizes dm-iug the melting into the normal salt, thus showing the action of a 

 nucleus. 



At these low temperatures the water of the solutions does not separate and 

 freeze, but combines with the saline molecules so as to form unstable hydrates. It 

 is remarkable that in so many cases the crystalline form of these hydrates should 

 be tetrahedral. 



On a Salt invisible in its Mother Liquor. By C, Tomliitson, F.B.S. 



Sir David Brewster has pointed out a method of examining precious stones in the 

 rough, for the purpose of determining their refi-active density and freedom from 

 flaws without the trouble and expense of grinding and polishing them. For this 

 purpose the rough stone is put into a mixture of oil of cassia and olive-oil, so ad- 

 justed as to be of about the same refractive density as the stone, when the latter 

 becomes invisible, or nearly so, while the flaws and defects start into view. 



When a supersaturated solution of the double salt formed by mixing the sul- 

 phates of zinc and soda in atomic proportions with a small quantity of water, boil- 

 ing and filtering into clean tubes, is exposed to the temperature of about 0° F., a 

 salt is formed, and is permanent for some days at ordinaiy temperatures, but it is 

 invisible on account of its having the same index of refraction as its mother liquor. 

 The latter is now only a satm-ated solution. On closing the tube vdth the thumb, 

 and inverting it so as to allow the mother liquor to drain off", air enters into the 

 cavities in the crystals ; and on allowing the mother liquor to flow back, these 

 air-filled cavities, having a different refractive index, become -visible, and have a 

 massive appearance. 



On the Eledro-deposition of Copper and Brass. 

 By W. H. Walenn, F.C.S. 



The present condition of the electro-deposition of copper and brass is put forward 

 in this paper with sufEcient reference to the history of the subject to enable com- 

 paratively recent improvements to be well understood, but treating the process in a 

 practical manner, and with reference to some improvements and manipulations that 

 are adopted by the author. 



Ordinarily, a solution containing the cyanides of copper and zinc, respectively 

 dissolved in a " solvent solution," consisting of a mixture of potassic cyanide with 



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