ITS SPECIFIC GRAVITY. 439 



relations. It must further be borne in mind that the weighing is 

 not conducted in a vacuum, and that the specific gravity alone 

 possesses any value when it has been reduced for a vacuum. This 

 is easily effected when the specific gravity of the glass and the co- 

 efficients of expansion of the air and water are known ; and the 

 calculation may be very considerably shortened by the use of 

 logarithms, or of a couple of algebraic equations.* 



But in how few of the numerous determinations of the density 

 of animal fluids has it been thought necessary to employ all these 

 precautions ! No one, however, who compares the results obtained 

 with and without these corrections can deny their necessity. Then, 

 moreover, we very rarely find the mode of determination indicated 

 in the notice of the specific gravity, although the knowledge of the 

 method employed is quite as important here as in the case of the 

 numerical results of the analysis. How can we place entire confi- 

 dence in the technical mode of conducting such a determination, 

 when this essential part of the calculation of the specific gravity 

 has been neglected ? We can hardly expect that an experimentalist 

 who neglects to attend to the influences of temperature and the 

 degree of expansion of the different media employed in these 

 measurements, should regard all the other necessary precautions ; 

 amongst which, we may enumerate the following as points worthy 

 of attention. In every experiment for the determination of volume, 

 we should use freshly boiled distilled water ; the glass should be 

 held and dried with some non-conducting substance, and care 

 should be taken to avoid all contact with the heated or perspiring 

 hands ; all vesicles of air should be excluded as far as possible, 

 and the glass cover or plate should be moistened before it is 

 placed upon the flat surface, in order to remove any adhering 

 air- and the flask should be dried with some cleaner substance 

 than ordinary linen or strips of paper, which may give rise to 

 great inaccuracy. 



The practice of drying the flask by means of a wire wrapped 

 in linen or paper, is not only laborious and tedious, but may, at 

 the same time, give rise to slight errors; on which account, it 

 is better to place the flask over sulphuric acid in a vacuum, 

 which accomplishes the proposed purpose very rapidly and effect, 

 ally or, after the flask has been placed in the sand-bath, the air 

 may be suffered to pass through it, as in smoking, oy me 



* We would refer those who may be ignorant of the mode of -nstruetirjg 



einer Untersuchungens methods thienscher Safte. 



