ON CONGELATION. 251 



XV. 



Observations on the Congelation of Water. By M, Dis- 



PAN, Professor of Chemistry at Toulouse, 



jnLBOUT the close of the winter of the year XI. we Remarkable 



had at Toulouse, after several days of a temperature re- factofcongela- 



markably mild, a return of cold very sudden and strong; ing water, 



the canal was frozen in one or two nights, and there was which did not 

 , , . , , - XL • 1. freeze under » 



skating, a spectacle very uncommon la this country, bridge. 



The ice remained for eight hours without thawing; but 

 notwithstanding this, the water under the bridges was 

 never frozen, not even slightly. This singularity was no- 

 ticed with surprize by every one, and I was for a long 

 time at a loss to discover the cause. I think I now un- 

 derstand it. 



The earliest and the latter frosts are called white frosts. Explanation. 

 and their cause is well understood. The white appear- Black and 

 ance is formed by the dew, which crystallizes as it falls. ^"^'^ ^°*''' 

 The hard frosts in the depth of winter are, on the con- 

 trary, called black frosts, and this expression is. equally 

 a,pplicable to the appearance of the ground in that cir- 

 cumstance. For this effect it is requisite that the cold 

 should have previoisly deprived the atmosphere of the 

 moisture it contained. No hirg is precipitated; but the 

 water upon the ground or soaked info it becomes solid. 



In fact, when a cold night suddenly follows a succession Their cause, 

 of warm days, as iiappens at Toulouse at the times I men- 

 tion, an abundant hoar frost succeeds. The still waters 

 receive such a quantity, that their caloric, already in part 

 absorbed at the surface by the coldness of the air, can no 

 longer keep up the fluid state. The hoar frost, or pre- 

 cipitated ice, then forms a pellicle at the surface of the 

 water, and by its contact determines the congelation 

 from one part to the other to a certain thickness. 



This is not the case with running waters. These hy 

 their continual motion prevent the hoar frost from form- which running 

 ing a coating to the surface. The frozen particles as they water congeals, 

 fall from the atmosphere are immersed and mixed with 

 the stream ; and when the coldness of the atmosphere 



itself 



