SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 101 



hydrometer when immersed in them, are identical, and are really the temperature 

 shown by the thermometer, and that this temperature is exactly that chosen as the 

 standard for the series of experiments. It requires much study and practice in a 

 suitable room before even an experienced chemist or physicist can feel confident that 

 he can produce this combination of equalities when required. 



It is to the failure to perceive the necessity of this preliminary education that, 

 though the method has been the property of science for forty years, it has been used 

 practically by none except myself and those whom I have personally instructed. 



§ 34. In the tables of Class A all the facts of observation are to be found. In all the 

 solutions the quantity of water is the same, namely, 1000 grams; the quantity of salt 

 dissolved in this mass of water is specified for each solution of the same salt in the 

 first column under m, in terms of the gram-molecule. In the second column, under W, 

 we have the weight in grams of each solution ; it is given by the sum 1000 + m. MR = W ; 

 where MR represents the molecular weight of the salt. The symbol used to express the 

 weight of salt dissolved in 1000 grams of water is tv, whence iv = mM^. In the third 

 column, under 8, we have the specific gravity of the solution. The experimental data on 

 which it is founded are the weight, H', in grams, of the hydrometer when it floats at 

 a given division of the stem in the solution, and its weight, H, when it floats at the same 

 division in distilled water, both of these liquids having the same temperature, T. The 

 quotient H'/H is the specific gravity, S, of the solution, as entered in the third column 

 of the tables. If we divide the weight of the solution, W, by its specific gravity, 8, we 

 obtain the displacement of the solution, which is entered in the fourth column under 

 A. It is the expression of the proportion H : H' : : A : W, in which H and A are weights 

 of distilled water, and H' and W are weights of the solution. It may be expressed 

 in words as follows : — The displacement of the solution. A, bears to its weight, W, the 

 same relation as the weight H of distilled water displaced by the hydrometer bears 

 to H', that of the solution displaced by the same portion of the same hydrometer 

 at the same temperature, T. Therefore, the unit of displacement used in the tables 

 is the space occupied by 1 gram of distilled water having the temperature T. 



Generally, our measure of displacement of a body having the temperature T is the 

 weight of distilled water having the same temperature which the body displaces 

 when totally immersed in the water. The body in question may be solid, liquid, or 

 gaseous. The unit of displacement is then the unit of weight, gram or kilogram, of 

 water having the particular temperature, T, which is chosen to suit the conditions 

 of the experiment, and it must be the common temperature of the body and the 

 water. Under this convention the unit of displacement is the space occupied by, say, 

 1 gram water at T, whatever value T may have. Thus, in our experiments the value of 

 T is in some 15°, in some 19-5°, and in others 23° ; but whichever temperature is used 

 as that of the distilled water, it is also that of the salt or saline solution which is supposed 

 to displace it when its specific gravity is being determined. The use of displacement 

 instead of volume to specify the amount of space occupied by a body is advantageous 



