60 
ANALYSIS OF POTABLE WATERS. 
in which the evaporation is being effected. This evaporation is best performed in a 
platinum crucible capable of containing at least three ounces of water, and provided with 
a platinum cover. If it be intended to evaporate down two deci-gallons of the water, a 
known quantity, say four or five grains, of carbonate of soda should be added (or about 
03 grm. of ihe carbonate to a litre of water). When the weight of the dry residue is 
ascertained, the amount of this carbonate is simply deducted from the weight obtained. 
Where the examination of several waters is in progress at once, the evaporations may be 
conveniently conducted in a copper steam-bath, in which they are carried to apparent 
dryness. The drying must be completed in an air-bath, the temperature of which is main¬ 
tained for one hour at about 284°.or not exceeding 302° F. (140° to 150° C.). The residue 
is then carefully weighed. The organic matter is afterwards burnt off at a red heat barely 
visible in daylight, and the residue when cold is again weighed. These weighings are 
more liable to error than any others during the analysis, and nowhere is an error of one or 
two hundredths of a grain more important. It is almost needless to say that the crucible 
must in each case be covered, and allowed to cool completely over a dish of sulphuric 
acid, and that the crucible must be introduced into the balance by means of tongs, as, if 
it be touched with the fingers, the warmth of the hand, by expanding the air con¬ 
tained within the covered crucible, would be liable to sensibly alter its weight. After 
the residue has been ignited, the saline mass must be moistened with a saturated solu¬ 
tion of carbonic acid in distilled water, as first recommended by Dr. T. Clark. From two 
to three ounces of the carbonic acid solution should then be poured into the crucible. 
This solution must again be evaporated to dryness, and the residue maintained for 
one hour in the air-bath at the same temperature as that employed in the original 
desiccation. If any sensible amount of magnesian salts be contained in the water, the 
residue, after this treatment, will be found to have increased in weight by two or three 
tenths of a grain, when compared with the weight after simple incineration. 
In this way, we shall have ascertained the amount of the soluble constituents (both 
fixed and organic), as well as an approximative estimate of the amount of organic 
matter. 
Nearly all waters, however, contain ammoniacal salts in minute quantity, and a vari¬ 
able amount of nitrates. The ammonia would of course escape as carbonate during the 
evaporation, but as the quantity of ammonia seldom exceeds a few thousandths 0 of a 
grain per gallon, this introduces no error of importance. The nitrates would also remain 
nearly unchanged, even after ignition, unless the quantity of organic matter were con¬ 
siderable, when they would necessarily undergo decomposition more or less complete on 
incineration, and the loss of weight then observed would be due partly to this cause. 
(8.) Use of Permanganate of Potash .—In the year 1850 Professor G. Forchhammer, of 
Copenhagen, proposed to employ a solution of permanganate of potash for the determi¬ 
nation of the amount of organic matter in water (Trans. Roy. Danish Society, 5th series, 
Physical and Mathem. section, vol. 2nd). At the time, the method attracted but little 
attention, but recently many chemists have been induced to attempt to modify the pro¬ 
cess, and substitute its indications for the ambiguous results obtained by incineration. 
As a substitute it will probably fail, but as an accessory test it appears to be likely to 
render good service in assisting the judgment as to the salubrity of a water. 
I have made a large number of experiments with the view of testing this method, and 
will now describe the plan which has succeeded in my hands. 
A solution of permanganate of potash of convenient strength is first prepared ; for this, 
purpose the liquor may contain an amount of permanganate such that 10,000 water-grain 
measures shall in an acidulated solution exert an oxidizing power equal to that of one 
grain of oxygen, or 1 c.c. of liquid shall be equivalent to OT milligramme of oxygen. 
For this purpose 3-95 grains of pure crystallized permanganate in 10,000 grains of water 
should suffice. This will be equivalent to 0-395 gramme of the salt in a litre of water ; 
but.its power may be exactly measured by means of a freshly prepared solution of crys^ 
tallized oxalic acid containing 7'875 grains in 10,000 measured grains of water, or 0-7875 
gramme of oxalic acid in 1 litre of water. 100 measures of this solution warmed with 
a very dilute solution of sulphuric acid should decolorize exactly 100 measures of the 
permanganate. 0-05 gall. (8 oz.) of each of the several waters for trial are then placed 
symbols used by the author as he has adopted the new chemical nomenclature and notation 
winch would be unintelligible to many of our readers.— Ed. Pir. Jouux. 
