LIGHTHOUSE ILLUMINATION BY MAGNETO-ELECTEICITY. 
67 
sufficient proportion to neutralize the alkali, and then placing the resulting liquid on a 
dialyser. In about three days the acid is found pure, with the loss of about 20 per cent., 
the salts having diffused entirely away. It is remarkable that the purified acid is not 
pectized by acids or salts even at the boiling temperature. Evaporated to dryness, it 
forms vitreous scales, like gum or gelatin, which sometimes adhere so strongly to the 
surface of the evaporating dish as to detach portions of it. It may be heated to 200° C. 
without losing its solubility or passing into the pectous state, but at a temperature near 
redness it undergoes a molecular change, losing at the same time 2-42 per cent, of water. 
When water is added to unchanged tungstic acid, it becomes pasty and adhesive, like 
gum ; and it forms a liquid with about one-fourth its weight of water, which is so dense- 
as to float glass. The solution effervesces with carbonate of soda, and tungstic acid is 
evidently associated with silicic and molybdic acids. The taste of tungstic acid dis¬ 
solved in water is not metallic or acid, but rather bitter and astringent. Solutions 
of tungstic acid containing 5, 20, 50, 66-5, and 79-8 per cent, of dry acid, possess the 
following densities at 19°:—1-0475, 1*2168, 1-8001, 2-396, and 3-243. Evaporated in 
vacuo liquid tungstic acid is colourless, but becomes green in air from the deoxidating 
action of organic matter. Liquid silicic acid is protected from pectizing when mixed 
with tungstic acid ; a circumstance probably connected with the formation of the double 
compounds of these acids which M. Marignac has lately described. 
Molybdic Acid has hitherto been known (like tungstic acid) only in the insoluble 
form. Crystallized molybdate of soda dissolved in water is decomposed by the addition 
of hydrochloric acid in excess without any immediate precipitation. The acid liquid is 
thrown upon a dialyser and more hydrochloric acid occasionally added to it. After a. 
diffusion of three days, about 60 per cent, of the molybdic acid remains behind in a pure 
condition. The solution of pure molybdic acid is yellow, astringent to the taste, acid to 
test-paper, effervesces with carbonates, and possesses much stability. Soluble molybdic 
acid, when dry, has the same gummy aspect as soluble tungstic acid, and absorbs mois¬ 
ture if exposed to damp air. Both acids lose their colloidality when combined with soda, 
and give a variety of crystallizable salts. 
LIGHTHOUSE ILLUMINATION BY MAGNETO-ELECTRICITY. 
BY J. H. GLADSTONE, ESQ., PH.D., F.ILS. 
Any one who, on a tolerably clear night, has crossed the Channel between Folkestone 
and Boulogne, and remained on deck, must have noticed on the French coast what ap¬ 
peared a brilliant star, now waxing, now waning. It was the light of the far-famed 
Pharos, on Cape Grisnez. But if he has made the passage within the last eighteen 
months, his gaze will have beeu attracted by a still brighter star on the British coast, 
of a bluish tint, steady and brilliant. This is the Magneto-electric Light at Dungeness, 
the brightest spark in the world, and one which unites a rare scientific with a practical 
interest, and may prove only the first lighted of a multitude of similar beacons. I pro¬ 
pose to say a few words on the history, production, and merits of this light. 
History .—If we ask the parentage of the Magneto-electric Light, Mr. Frederick 
Hales Holmes is certainly its father, but, like other beings, it has had two grandfathers 
—the philosopher who first showed the conducting power of charcoal, and the brilliancy 
of the light between charcoal terminals of an interrupted galvanic current; and Professor 
Faraday', who discovered that when a piece of soft iron, surrounded by a coil of metallic 
wire, was made to pass by the poles of a magnet, an electric current was produced in the 
Avire, which revealed its existence by effecting chemical decompositions, or by giving a 
spark. This spark, it is true, was barely visible as at first obtained, but it has been ex¬ 
alted into the present Magneto-electric Light. 
It appears that in 1853 some large magneto-electric machines were erected in Paris 
for producing gas by the decomposition of water, the object of the proprietor being to 
use this gas for the purposes of combustion; but the scheme failed, the Company that 
was being formed came to nothing, and the machines were pronounced by leading scien¬ 
tific men to be only expensive toys. Mr. Holmes, however, who was one of the referees, 
proposed to turn them to account for electro-plating and gilding, and thought it possible 
that the Electric Light might be produced advantageously by their means. “ My pro- 
