32 CHEMISTRY FOR AGRICULTURAL STUDENTS 



weighed flask and weigh. Subtract the weight of the flask 

 to obtain the weight of the salt solution. Since 20 c.c. of 

 water (at 15° C, see page 13) as run from 

 the burette will weigh 20 grams, the relative 

 weights of equal volumes of the salt solution 

 and water can thus be ascertained. Taking 

 the density of water as i, what is the relative 

 density of the salt solution ? 



The density of a solid or liquid is always compared 



with water as unity. The number thus obtained is the 



Fig. 17. relative density (or specific gravity) of the substance. 



A customary method for taking specific gravities is by 



the specific gravity bottle, constructed to hold exactly 50 or 100 grams 



of water at 15° C. (Fig. 17). An improvement on this method is 



described in the next paragraph. 



In repeating the determination to obtain concordant results 

 employ a U tube (Fig. 18), by the use of which the tempera- 

 ture of the liquid can be controlled. Construct 

 such a tube out of a piece of soft glass tubing 

 having a 5 or 6 m.m. bore, and weigh it. Now 

 fill with water by attaching an india-rubber tube 

 and sucking up the water through the drawn-out 

 end. Having removed the india-rubber tube, 

 the drawn-out end of the U tube should remain 

 filled with water, while the water in the other 

 end will sink to a certain point. Mark this 

 point with a file. Immerse the tube in a 

 beaker of water at 15° C. for a few minutes; 

 then, if necessary, run a little water in, or suck 

 a little out by pressing a bit of filter paper to 

 the drawn-out end, until the height of the water in the one 

 limb exactly coincides with the mark when the tube is quite 

 upright, and, having dried the outside of the tube, weigh. 

 Empty the tube, and having first rinsed it out with the salt 



Fig. 18, 



