SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 157 



The paper scale, so convenient in the closed hydrometer, is inadmissible when the 

 internal load has to be shifted. Consequently the scale (millimetres) is etched on the 

 outside of the stem. When prepared for use the top of the stem is loosely closed by 

 a thin rod of white enamel glass which passes down the stem so far as the engraved 

 scale extends, and it is kept suspended in this position by being thickened to a 

 button at the top. This addition was made in order to provide an opaque white 

 surface behind the scale. I was unable at the time to procure suitable tubing with 

 white slip let into it. It is an integral part of the instrument, to the mass of which it 

 contributes its share. 



The mass of the hydrometer is equal to the sum of the masses of its parts, namely, 

 the glass, the ballast, and the air respectively. 



§ 81. In order fully to appreciate the importance of each of these masses in furnishing 

 the effective weight of the instrument, let us imagine that we are actually working in a 

 vacuum, and at sea-level in latitude 45° ; thus the air both outside and inside falls 

 away, and we have only the glass and the ballast remaining. Let us assume that 

 the load is of shot and has been so adjusted that the instrument, when immersed in 

 distilled water, of the temperature which is to be maintained uniform during the 

 experiment, floats with only a small portion of the stem immersed. 



The first operation performed in the vacuum is to weigh the instrument ; let its 

 weight be W grams (true). Experiment No. 1. — Let it then be floated in distilled 

 water of the fixed constant temperature, and let the surface of the water cut the stem in 

 a line at C, fig. 6, next page. Then the weight of the water displaced by the instrument 

 below the line C is W grams (true). If we replace the water in the cylinder by a liquid 

 of greater density, such as a saline solution, and float the instrument in it, we find 

 that less of the instrument is immersed, and we are obliged to add weights at the top 

 of the stem in order to immerse it until the surface of the liquid cuts the stem in 

 the line C. Let the weight so added be w grams (true). The weight of the liquid dis- 

 placed by the hydrometer when immersed in it up to the line C is then (W 4- w) grams 

 (true). From this it follows that the weights of equal volumes of distilled water and 

 of the experimental liquid, at the particular fixed temperature, are in the proportion of 



W : W -h tf, and the specific gravity of the liquid is — ^ — , referred to that of distilled 



water of the same temperature as unity. 



Experiment No. 2. — Let us now replace the experimental liquid in the cylinder 

 by distilled water and float the hydrometer in it. As before, the surface of the water 

 will cut the stem at C. Let us add a small weight to the stem, so that the instru- 

 ment is depressed until the surface of the water cuts the stem at D. Let the weight 

 of this small weight be u^ : then the weight of water so displaced up to D is (W-|-Mj) 

 grams (true) ; let us now replace the water in the cylinder by the same experi- 

 mental liquid as before ; and let small weights be added to the top of the stem 

 until the surface of the liquid cuts it at D. Let the weight of these small weights 



