CONSIDERED LIGHTER THAN WATER. 395 



But cream, whenever separated, however fresh, is found to abound in ah*. Were 

 it not for ah" attached to the cream globules, it seems questionable that any sepa- 

 ration of them would take place, as then- albuminous envelope (adopting the 

 inference of MULLER and HENLE, that they are so provided)* seems to give them a 

 specific gravity about the same as that of the medium in which they are sus- 

 pended.f 



In pharmaceutical processes, in which it is required to suspend powders in 

 fluid mixtures, the efficacy is well known, of subjecting the powder to attrition 

 with a very little water or alcohol. This seems to be owing entirely to the 

 separation of air, either contained in the substance of the powders, or adhering 

 to their surface in minute globules, and is well illustrated by the effect of attri- 

 tion, as described in a preceding page, on cork. 



On the contrary, whenever great lightness and buoyancy are necessary, 

 whether for raising bodies in the atmosphere, or floating them in water, or making 

 them run or press lightly on the earth's surface, air is in some way included. 

 The hollow bones of birds, filled with air the swimming-bladders of fishes, dis- 

 tended with air the elastic cellular structure of cork, the rigid cellular and 

 tubular structure of pumice, full of air or, in the instance of the latter, nearly 

 so filled, are examples of the kind, and may be deserving of being studied, par- 

 ticularly the last mentioned substances, with wax and caoutchouc, in relation to 

 the principles on which vessels, and implements, and edifices should be made, 

 requiring an unusual degree of buoyancy or lightness, especially if intended for 

 a permanency. 



EDINBURGH, 3d April 1843. 



the cream globules, those which rise, may be without an albuminous envelope, and that another part, 

 those which do not ascend, may be provided with such a membrane. 



* Histoire des Tissus et de la Composition Chimique du Corps Humain, par J. HENLE, p. 165. 



t Butter, I find, when as pure as it can be rendered by melting, is, at the boiling point of water, of 

 the sp. gr. .902 ; at 100, apparently .913 ; and at 48, .932, employing the method used in the instances 

 of wax and spermaceti. The lightness of the substance of butter increasing with its temperature, must 

 necessarily expedite the raising of cream, as when the " scalding" process is employed. 



VOL. XV. PART III. 5 O 



