394 DR DAVY ON THE SPECIFIC GRAVITY OF SUBSTANCES 



are not excited, remain stationary, or almost so, showing only a very slight ten- 

 dency to ascend. 



The result of these experiments admits of some practical applications, and 

 may aid to explain some phenomena of an obscure kind, in certain processes. 



Were the substances treated of lighter than water, it might be expected that 

 in every instance, when mixed with water, whether precipitated from an alco- 

 holic, or obtained from an etherial solution, or mechanically detached, as in the 

 operation of boiling, that they would of necessity find their place of rest, and be 

 collected at the surface. But, on the contrary, if their specific gravity is either 

 the same, or in the smallest degree superior to that of water, then the same could 

 not be expected ; all that could be expected would be, that no more of each 

 substance would rise to the surface on admixture with water, than is buoyed up 

 by the adhering particles of air : and no confidence would be placed in the cir- 

 cumstance of specific gravity in an operation of analysis, for collecting the whole 

 of the substance sought. In illustration, cholesterine may be specially mentioned 

 a substance of common occurrence in animal concretions and morbid deposi- 

 tions ; indeed, as I have satisfied myself by recent inquiry, much more common 

 than is generally supposed. If a concretion containing cholesterine be digested 

 in hot alcohol, and the alcoholic solution be precipitated by water, a portion of 

 the cholesterine will rise to the surface, and appear there as a pearly film ;* 

 whilst another portion, not rendered buoyant, will subside, and, on careful 

 inspection, will be found at the bottom. Or if the concretion be broken up, and 

 boiled in water, as cholesterine is not fusible at the boiling temperature of 

 water, its crystalline plates, on rest, will form a sediment, and may be separated 

 by decantation ; or if extremely minute, and they are sometimes met with 

 not more than go^oth of an inch in width, they may be suspended for a consi- 

 derable time, imparting a milky opaqueness to the fluid. 



The raising the cream of milk may be mentioned as another instance of the 

 influence of disengaged air. It is well known, that in cold weather, cream rises 

 slowly. Is not this owing chiefly to the milk, at a low temperature, resisting 

 that change to which it is so prone in warm weather, the fermentation of its 

 saccharine part, and the formation of carbonic acid ? Milk, the instant it is 

 drawn from the cow, is, I find, destitute of air : I have been able to obtain none 

 from it, when collected with proper precautions and subjected to the air-pump. f 



* Oleine is a frequent accompaniment of cholesterine in animal concretions, and when present, 

 being considerably lighter than water, may be looked for in the film alluded to in the text. Mixed with 

 cholesterine and air, its appearance is very like that of cream on milk. 



t Researches, Physiological and Anatomical, vol. ii. p. 221. I find that, when milk fresh from the 

 cow is subjected to the air-pump, a small portion of cream soon collects at the surface ; and farther, that 

 it may be kept many days (I have kept it twelve days) without any sensible increase in the quantity of 

 cream, or distinct diminution of the opaque whiteness of the milk, seeming to indicate, that a part of 



