and CQutraction of Building Stmies. 609 



marble this crack would have a width of .03264, nearly twice the 

 thickness of common pasteboard ; and for sandstone .054914, 

 nearly three times the thickness of pasteboard. These cracks are 

 not only distinctly visible, but they allow water to pass freely into 

 the heart of the wall. The' mischief does not stop here: by this 

 constant motion back and forth in the coping, the cement, of 

 whatever kind the joints might be made., would be crushed to 

 powder, and in a short time be totally washed by the rains from 

 its place, leaving the whole joint open. 



Observatimis on Saline Crystallization. By Henry Ogden, 

 M.D., Extraordinary Member of the Royal Medical Society 

 of Edinburgh. Communicated by the Author. 



In crystallizing small quantities of saline substances, it has fre- 

 quently occurred to me to observe, that concentrated warm solu- 

 tions may be cooled down to the ordinary temperature of the 

 atmosphere, without depositing crystals. This property, as 

 appertaining to sulphate of soda, is noticed in most of the sys- 

 tematic works on chemistry ; but it is usually stated that it is 

 necessary to defend the solution from the pressure of the atmo- 

 sphere, by removing the vessel containing it from the fire, whilst 

 in a state of ebullition, and immediately corking it up. 



Gay-Lussac*, however, has shewn, that atmospheric pressure 

 does not necessarily induce crystallization ; he found that if the 

 liquid was covered with a film of oil of turpentine, it might be 

 cooled without crystallizing. And more lately, Professor Tur- 

 n6r"f* has succeeded in the same experiment, without the use of 

 oil, ** by causing the air of the flask to communicate with the 

 atmosphere by means of a moderately narrow tube. 



I have made a number of experiments on this subject, with 

 many different salts ; but I have never observed any relation to 

 exist between the pressure of the atmosphere and the occurrence 

 of crystallization in supersaturated solutions. The solutions 

 have been cooled in vessels of all forms, from a narrow-necked 

 flask, corked or not corked, to an open jar, or even a wide basin, 

 without their crystallizing. 



• Aunales de Chimie, vol. IxxxviL f Elem. Chem. 



