256 Mr. A. B. Northcote on the Water of 



iron or aluminium compound, the organic matter itself contri- 

 butes to the opalescence. The October water, after a lapse of 

 six months, lost more than half its organic matter, as determined 

 by loss on ignition, and became clear and bright, although still 

 slightly coloured ; but the water of April, after standing for a 

 year, became colourless, as well as perfectly transparent, whilst 

 its organic matter was reduced to one-fourth of its original 

 amount. In this water I determined the silicic acid, and also 

 the ferric and aluminic precipitate which ammonia produces after 

 removal of silica ; and the comparison which follows exhibits the 

 diminution of these constituents : — 



Silica. Alumina. 



Water of April 1866 . . -0102 -0040, means per 1000. 



„ 1867. . -0070 -0020 



This, however, is but a single experiment and may err on the 

 side of deficiency; at all events, to show how slowly any such 

 deposition takes place, I may observe that the determinations of 

 silica made in the October water, a month after it was received, 

 amounted to -0140 and "0121 in the 1000, whilst in two bottles 

 which had remained unopened for three months *0130 and *0150 

 in the 1000 were the quantities obtained. I may add that the 

 determinations of these substances were always made after the 

 waters had been allowed to stand for at least a month; and the 

 silicic residues were in every case boiled twice with moderately 

 strong chlorhydric acid before weighing. 



The presence of so much silicic acid, part of which probably 

 exists free or in unsaturated combination, must have a consider- 

 able effect on the determination of the solid residue of the water. 

 I have made many of these determinations both with and with- 

 out addition of disodic carbonate, and have found that although 

 the former are invariably less than the solid contents of the 

 water as deduced from analysis, yet the latter always pre- 

 sent a still greater discrepancy. It is possible that by the influ- 

 ence of disodic carbonate decomposition and loss of the organic 

 constituents of the water may go on more rapidly than by sim- 

 ple evaporation; and indeed the loss on ignition after addition 

 of this salt I found to be rather less than when it had not been 

 added ; but I apprehend that the main cause of the difference 

 between these determinations and the analytical results consists 

 in the formation of silicates during the evaporation at the ex- 

 pense of previously existing carbonates. 



The attempts to determine the amount of " organic matter " 

 present in the water were necessarily confined to ascertaining 

 the loss on ignition; for circumstances prevented my working 

 on the spot, or receiving the waters sufficiently soon after collec- 



