§ 141. WATEE. 275 



The Standard Solution of Soap. — Mix together 150 

 parts of lead plaster and 40 parts of potassic carbonate, 

 exhaust the mass with alcohol, filter the liquid, evaporate 

 the filtrate on the v/ater-bath, and dissolve the residue in 

 50 parts of 56" I „ alcohol. Dissolve, also, 0.523 grm. of 

 pure and dry baric chloride in water, and dilute the solu- 

 tion to one litre. 



Provide a bottle of about 200 c.c. capacity, with a well- 

 fitting glass stopper, and a mark on the side to indicate a 

 capacity of 100 c.c, put 100 c.c. of the solution of baric 

 chloride in this bottle, and add the solution of soap from 

 a burette or a graduated pipette, with frequent agitation, 

 until an abundant delicate lather appears that lasts Jive 

 minutes. The shaking of the bottle should always be 

 performed in the same manner ; the best way is to grasp 

 the stopper and the neck with one hand, and the bottom 

 wuth the other, and shake it up and down. 



Having tested the strength of the solution of soap, 

 dilute it with 56° j^ alcohol to such an extent that 45 c.c. 

 are necessary to produce the lather with 100 c.c. of the 

 solution of baric chloride ; this quantity of the latter 

 solution produces the same degree of hardness as 12 

 mgrs. of lime. 



Exammation of a Sample of Water.— If a hard spring 

 water is to be examined, put 10 c.c. of it into the bottle 

 described above, and add distilled water up to the 100 c.c. 

 mark ; if it is a soft river water, fill the bottle up to this 

 mark with the water alone ; then add the solution of soap 

 from the burette, as above. Add at first larger quantities of 

 the standard solution at a time, but, towards the close, 

 the agitaiion should be repeated -after each 0.5 or 1 c.c. 

 that is added, and finally between each drop or two. 



On repeating the experiment, in case but little of the 

 standard solution was used, take 25 to 50 c.c. of the water, 

 but not a quantity that will require more than 45 c.c. of 

 the standard solution ; in this second trial add, at once, 



