20 



Transactions of the 



The plan usually adopted is, to make a saturated solution of the salt at the boiling 

 point of the solvent (or of the saturated solution), and to allow it to cool gradually, with 

 frequent or occasional agitation by the hand to prevent super-saturation, to the raspective 

 temperatures at which the determinations are made; at which temperatures portions of 

 •the liquid are withdrawn, and the amounts of dry salt contained therein, determined. 



By such a method, where the temperature is being continually lowered, and there are 

 no means of constant agitation, there is evidently a possibility, if not a probability, that 

 super-saturation will occur. It was the object of the work herein described, to ascertain 

 if it were not possible, by a simple mechanical contrivance and a different method of 

 work, to avoid the errors arising from this cause; and to that end was employed the ap- 

 paratus figured below. It consists of a copper tank A (shown here in section), 

 inside of which revolves a bottle B, containing the salt to be dissolved and the solvent. 

 This bottle is perforated at the base, and at the side. Through the perforation at the 

 base, fitted tightly with a rubber stopper, passes a glass tube cc, having its outer end 

 closed by a rubber tube and a pinch-clamp /. This tube serves as an axis of revolution, 

 the power being applie<l by means of a cord passing over the grooved wheel e e, which is 

 firmly attached to the tube c c, just outside the tank. The same tube also furnishes a 

 means of producing pressure in the bottle sufficient to force out the solution through an- 

 other tube d d, passing through a tight stopper in the neck of the bottle; which tube also 

 serves as the axis of revolution. This tube is furnished with a tightly fitting piston p; 

 and at a short distance outside the tank it has an orifice o, through which, when the pis- 

 ton is drawn out beyond that point, the solution is forced. Within the bottle this tube is 

 bent so as to allow its end h, to be under the surface of the liquid in the bottle, even when 

 but little remains. 



The perforation g at the side, fitted tightly with a stopper, is for the purpose of intro- 

 ducing crystals of the salt as often as necessary, to keep the latter in excess of the solvent. 



The lamp L, beneath the tank, serves to regulate the temperature, which is indicated 

 by the two thermometers, t t. 



For maintaining the bottle in revolution, any constant power may of coui-se be used; 

 but in the work already done with this apparatus, the motor employed has been a small 

 electro-magnetic engine, excited by two Bunsen elements. 



Our method of work is as follows: A large amount of the salt is introduced into the 

 bottle (the amount depending of course upon the solubility of the salt under examina- 

 tion), the bottle nearly filled with distilled water, and put firmly in place. The tank is 

 filled with water to a level above the top of the bottle, the latter set in revolution by the 

 motor, and the lamp lighted. The temperature of the water in the tank, indicated by 

 two trusty tliermometers, is slowly raised to the lowest temperature at which a determina- 

 tion is to be made, and carefully kept at that point for one half-hour (or longer, if that 

 is found insufficient for obtaining a saturated solution), special care being taken to avoid 



