216 Mr. Graham on the Influence of the Air in determining 



the receiver long before the upper part had fallen to the tem- 

 perature of the atmosphere. In such cases, crystallization 

 beginning on the surface of the mercury, advanced slowly and 

 regularly through the solution. Above, there always remained 

 a portion of the solution too weak to crystallize, being im- 

 poverished by the dense formation of crystals below. It was 

 also necessary to clean the lower and external part of the 

 receivers, when placed in the trough, from any adhering so- 

 lution, as a communication of saline matter was sometimes 

 formed between the solution in the receiver and the atmo- 

 sphere without. When these precautions were attended to, 

 saline solutions over mercury remained as long without cry- 

 stallizing as when separated from the atmosphere in the usual 

 mode. 



Solutions which completely filled the receivers when placed 

 in the trough, allowed a portion of mercury to enter, by con- 

 tracting materially as they cooled. A bubble of air could thus 

 be thrown up, without expelling any of the solution from the 

 receiver, and the crystallization determined, without exposing 

 the solution directly to the atmosphere. 



The first observation made was, that solutions of sulphate 

 of soda sometimes did not crystallize at all upon the intro- 

 duction of a bubble of air, or at least for a considerable time. 

 This irregularity was chiefly observed in solutions formed at 

 temperatures not exceeding 150° or 170°, although water dis- 

 solves more of the sulphate of soda at these inferior tempera- 

 tures than at a boiling heat. Brisk ebullition for a few se- 

 conds, however, rendered the solution upon cooling amenable 

 to the usual influence of the air. In all successful cases, cry- 

 stallization commenced in the upper part of the receiver around 

 the bubble of air, but pervaded the whole solution in a very 

 few seconds. A light glass bead was thrown up into a solu- 

 tion without disturbing it. 



It occurred to me, that, since the effect of air could not be 

 accounted for on mechanical principles, it might arise from a 

 certain chemical action upon the solution. Water always holds 

 in solution a certain portion of air at the temperature of the 

 atmosphere, which it parts with upon boiling. Cooled in a 

 close vessel after boiling, and then exposed to the atmosphere, 

 it reabsorbs its usual proportion of air with great avidity. 

 Now, this absorbed air appears to affect in a minute degree 

 the power of water to dissolve other bodies ; at least a con- 

 siderable part of it is extricated upon the solution of salts. 

 When a bubble of air is thrown up into a solution of sulphate 

 of soda, which has previously been boiled and deprived of all 

 its air, a small quantity of air will certainly be absorbed by 



the 



