ITS SPECIFIC GRAVITY. 437 



progression in the specific gravity which may be expressed nu- 

 merically, cannot correspond with that of the increase of the solid 

 constituents ; and that in the analysis of the urine, Schmidt's mode 

 of determining the specific gravity, as a volumetric check on the 

 chemical determinations, possesses only a fictitious accuracy. The 

 reasons of this uncertainty consist partly in our ignorance of the 

 coefficient of condensation of many of the constituents of the 

 urine, which are present in very variable quantities, and partly on 

 the utter impossibility of determining the quantity of some of the 

 substances contained in the urine, even with a moderate degree of 

 accuracy. 



Although we regard it as entirely out of place in a work on 

 physiological chemistry, to enter more fully into the methods of 

 determining densities, or to pass an opinion upon their value, 

 since these are subjects which should be learnt from physics, or at 

 all events from practical chemistry, we cannot forbear making a 

 few remarks, which may prove serviceable to those who have been 

 unable to form any opinion regarding the numerous determinations 

 of densities with which pathologico-chemical literature is over- 

 burdened. The ordinary means employed for the determination 

 of the specific gravity of animal fluids are, the areometer, the 

 hydrostatic balance, and the direct weighing of equally large 

 volumes of distilled water and of the fluid in question. We need 

 hardly repeat an observation which we have already made more 

 than once, that the areometer gives only approximately correct 

 results, even when it has been graduated for a definite temperature, 

 and is in other respects well made. It would, however, be wholly 

 at variance with the principles of areometry, if we were to expect 

 to arrive at even a tolerably accurate result, if we applied the 

 areometer to fluids containing any solid particles in suspension. 

 Even if such approximate determinations may suffice in the case 

 of analyses of the urine, they should be discarded in all other 

 animal fluids ; for if the specific gravity is to be anything beyond a 

 mere appendage to the analysis, its approximate determination 

 will simply furnish a means of error. Our remarks naturally 

 apply to all the other methods in use for determining the densities 

 of fluids, and even with greater force, in so far as they justify us 

 in expecting more accurate results than those which can be fur- 

 nished by the areometer. . 



Among the different areometers, there is only one whicl 

 deserves any special notice; but this instrument, which is con-. 



