CHEMICAL SCIENCE. 2ol 



into water of 86 the bromide alone gains the upper part of the tube, care 

 having been taken to keep it conveniently cool. 



We do not insist upon the chemical composition of these combinations ; 

 it suffices us to say that it is easy to determine with certainty their nature in 

 obtaining with them the principal reactions which characterize iodine and 

 bromine. Proper care should be taken in preparing the chlorine for this 

 process, that the substances should be pure, and, above all, contain neither 

 iodine, nor bromine; besides this precaution, after having mixed the cyanide 

 of silver with the other silver salts, the whole should be heated for some 

 time, and if there are no traces of iodide or bromide of cyanogen, the pro- 

 cess is proceeded with in the ordinary way. 



IMPROVED METHOD OF PURIFYING WATER. 



The following plan of purifying water has recently been patented by 

 Henry Medlock, of England, the specification reading as follows: 



I suspend, in a tank or reservoir containing the water to be purified, by 

 means of iron rods passing across it, iron wire, of about one-sixteenth of 

 an inch in diameter, loosely packed in bundles or coils, and in the proportion 

 of about one pound weight of such wire to every one hundred gallons of 

 water. I allow the water to remain in contact with the iron wire from 

 twenty-four to forty-eight hours, according to the rapidity with which the 

 precipitation of organic matter occasioned by such contact takes place, and 

 I then pass the water through any kind of filtering medium now in use 

 which is capable of retaining the precipitate formed. For the filtration of 

 water in large bulk, I have found the ordinary sand-filter sufficient. The 

 effect of the contact of the water with the solid bodies above described, 

 when the water contains nitrogen in any form, is to decompose or oxidize the 

 organic matter and the ammonia contained in the water, whereby a certain 

 part of the organic matter and ammonia is converted into nitrous or nitric 

 acids, or both of them, by which the rest of the organic matter is rendered 

 insoluble. The nitrous and nitric acids finally combine with the iron, or with 

 some of the inorganic bases, if any, contained in the water, and the organic 

 matter rendered insoluble is precipitated, together with some part of the 

 inorganic matter, if any, contained in the water. 



FILTRATION THROUGH SAND. 



Experiments by Mr. H. M. Witt, at the Chelsea waterworks (England), have 

 proved that by simple filtration of water through sand, soluble salts, as well 

 :as suspended matters, are separated. Out of 65*527 of solid residue dissolved 

 and suspended, 24*237 were separated, including 7'559 of soluble salts (TS20 

 of these chlorid of sodium). From water containing 55'60 of solid residue, 

 32 g 75 were thus separated, 3'404 of which were dissolved salts. Hence mere 

 percolation through sandy strata for long periods may separate dissolved 

 saline ingredients from waters. Mr. Witt applies the facts to explain the 

 occurrence of fresh-water springs on coral islands, that ebb with the tide. 

 But here Mr. Darwin's explanation seems most reasonable, that the fresh 

 waters are from the rains; and that, being detained upon the coral rock, and 

 at the same time being lighter than the suit water of the ocean, the setting 

 in of the tides pushes the fresh waters before them. Silliman's Journal. 



