43S URINE. 



structed by Alexander,* of Munich, yields, according to my expe- 

 rience, much more accurate results than one might be disposed 

 to expect, a priori, from its construction. It is arranged in the 

 following manner : Two parallel graduated glass tubes, both open 

 at one end, and communicating with each other at their other 

 ends, at which is a small syringe, are introduced, the one into 

 water, and the other into the liquid to be examined. The air in 

 the tubes is now slightly rarefied by means of the syringe, when, 

 by comparing the elevation of the water and of the other liquid in 

 the tubes, the ratio of the specific gravities is given. This is the 

 best of all the instruments for rapidly determining the density, as 

 the influences of the temperature and of atmospheric pressure are 

 here almost eliminated. 



The hydrostatic balance with a glass sinking-ball generally 

 yields more accurate results than the areometer ; but yet, notwith- 

 standing every precaution, it does not admit of the exactness pre- 

 sented by the direct weighing of volumes. The defects in this 

 method depend principally upon the irremediable loss of a portion 

 of the water of the animal fluid by evaporation, and more especially 

 upon the circumstance that the balance gives a much less accurate 

 result when the glass ball is weighed in water or in an animal 

 fluid than when it is weighed in the air; and on this account 

 fluids that are at all viscid, such as blood-serum, should not be 

 treated by this method : defibrinated blood cannot be examined in 

 this manner, for we often find that even the addition of one or 

 two centigrammes does not affect the beam of the balance. Even 

 if the unavoidable adhesion of vesicles of air to the glass did not 

 render this mode of determination unsuitable for the blood, its 

 employment in the case of a fluid in which solid particles are 

 irregularly distributed, appears, from well-known physical grounds, 

 to be wholly irrational. 



The ordinary method of determining the specific gravity by 

 the direct weighing of equal volumes in glass flasks is the best, but 

 its value may unfortunately be very considerably diminished if it 

 be not conducted with a care and attention which many medical 

 chemists scarcely seem to think necessary, excepting in the case of 

 elementary analyses. It is not sufficient in this method to weigh 

 the empty and carefully dried flask, to determine its weight when 

 filled with water, and finally with the fluid to be examined, for 

 several calculations will be required to make the necessary correc- 

 tions, on account of differences in the thermometric and barometric;. 

 * Polyteclm. Centralb. 1847. Heft. C, S. 30 i. 



