202 
POPULAR SCIENCE REVIEW. 
amount of nitrogen is made up of tlie nitrogen of the ammonia and the or- 
ganic nitrogen ; the former must therefore he subtracted. 
How to Prepare Urea. — The following account of a process for this pur- 
pose is given in a paper read before the Chemical Society (Dec. 5) by Mr. 
John Williams. The author proceeds^ in the first instance, to prepare 
cyanate of potassium by fusion of the cyanide (best commercial quality, con- 
taining 90 per cent.) with red oxide of lead, keeping the temperature as low 
as possible. This product is dissolved in cold water, mixed with nitrate of 
barium to precipitate the carbonate which it usually contains, then thrown 
down as lead-salt by adding a solution of the nitrate. The cyanate of lead 
is easily purified by washing, and is then dried at a gentle heat. For the 
preparation of artificial urea equivalent amounts of sulphate of ammonia and 
cyanate of lead are digested together in warm water, the insoluble sulphate 
filtered oflt, and the solution when evaporated yields a product of unusually 
good quality, and of larger amount than by the ordinary plan. Mr. Williams 
finds that the process is applicable to the preparation of the compound-ureas, 
using the corresponding sulphate instead of the simple ammonia-salt. 
Water-Filtration. — The Silicated Carbon Company, of whose filters we 
some time since spoke in high terms of praise, have just devised and ofiered 
for sale a piece of filtering apparatus which we have no doubt will be found 
very useful by the poor. It is termed the Silicated Carbon Filtering Tap. 
In many of the poverty-stricken districts of the metropolis, where water is 
often stored in water-butts that are perhaps not cleaned once a year, this 
piece of apparatus will, if used, be found highly beneficial. All the water 
must travel through the filtering medium placed in front of the tap, and as 
the tap is made to unscrew into two portions, the carbon ” may occasionally 
be taken out for cleansing without letting the water run away.” 
Action of Light on Chloride of Silver. — M. Morren, of the University of 
Sciences of Marseilles, has published an interesting note on this phenomenon . 
Take, he says, a glass tube 3 centimetres in diameter, and from 45 to 50 
centimetres in length, close one end and introduce two bulbs, one containing 
nitrate of silver, the other chloride of potassium, in equal equivalents ; fill 
the tube with a concentrated solution of chlorine in water, then carefully 
seal it before the blowpipe. Break the bulbs by agitation, when the result 
will be chloride of silver deposited in an excess of chlorine water. If the 
tube be exposed for several days to the rays of the sun, the following facts 
may be observed. 1st. As long as the liquid preserves the yellow colour 
given to it by the chlorine, the chloride of silver remains white. 2nd. 
When this yellow colour disappears by the action of the chlorine on the 
water under the influence of light, the chloride of silver slowly assumes, not 
the very deep violet which we see in the reactions of photography, but a red 
brown, which at first only appears gradually and on the surface, but in time 
penetrates the entire white mass, provided the tube is sufficiently agitated, 
and submitted to the action of a bright sun. 3rd. The tube being placed, if 
not in obscurity, at least in the diffused light of the laboratory, the brown 
colour disappears gradually, and the chloride of silver reassumes, in all its 
intensity, its original white aspect. Beplace the tube in the sunlight, and 
the coloration takes place afresh, to disappear again when the tube is re- 
turned to the shade, and so on indefinitely. — Vide Chemical News, vol. xvi. 
No. 419. 
