BRITISH ASSOCIATION. 
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commerce. Of indium, too, the latest of the newly-discovered metals revealed by the 
spectrum, it must suffice to say that it has been obtained in quantity which places its 
existence as a distinct metal beyond question. I am indebted to my friend Prof. Roscoe 
for the small specimens of the metal and its sulphide now upon the table. An exten¬ 
sive branch of industry is now springing up in the improved methods of voltaic deposi¬ 
tion of the metals. Weil has, by the use of an alkaline solution of tartrate of copper, 
contrived to coat iron and steel with a tough, closely adherent sheathing of copper, by 
simply suspending the articles to be coated by means of a wire of zinc in the metallic 
bath. No battery is required. Lead and tin may in a similar manner be deposited on 
copper, iron, or steel, if the oxide of tin or of lead be dissolved in a bath of strong solu¬ 
tion of caustic soda. I must, before I conclude, advert to one or two interesting addi¬ 
tions to our knowledge upon the side where chemistry and physics meet. Few results, 
perhaps, were more unexpected than those obtained by Deville and Troost upon the per¬ 
meability to gases of certain dense metals at elevated temperatures. They have proved 
that platinum and iron, when white-hot, become for the time porous, and are rapidly 
permeated by hydrogen, which will even pass out under the pressure of the atmosphere 
and leave a vacuum almost perfect within the tube. In one form of these experiments, 
tubes of hammered and of cast platinum (which in one case was as much as a twelfth 
of an inch in thickness) were fitted by means of corks into the axis of a shorter and 
wider tube of glazed porcelain ; a slow current of pure and dry hydrogen was then main¬ 
tained through the porcelain tube, whilst a current of dry air was transmitted through 
the platinum tube. At ordinary temperatures no change was observed in either gas. A 
fire was then lighted around the outside of the porcelain tube, and gradually raised until 
the heat became very intense. At 2000° Fahr. the oxygen contained in the air had en¬ 
tirely disappeared ; nothing but nitrogen mixed with steam passed out of the platinum 
tube, hydrogen had passed through the pores of the platinum and entered into combina¬ 
tion with the oxygen of the air within; whilst at still higher temperatures the moist 
nitrogen became mixed with hydrogen. As the tube cooled, the same phenomena oc¬ 
curred in the inverse order, till, when the ordinary temperature had been regained, no 
diffusion of hydrogen was perceptible, and unaltered air was collected from the platinum 
tube. Analogous results were obtained when a tube of soft cast steel was substituted 
for that of platinum, though the thickness of the steel tube was an eighth, or in some 
cases as much as a sixth of an inch. From these experiments one practical conclusion 
deducible is, that air-pyrometers, the bulbs of which are formed of iron or platinum, 
cannot be relied on when employed for measuring elevated temperatures; glazed porce¬ 
lain, however, was found to coniine the gases completely. Curious as these results are, 
they are but parenthetical in another series of more general bearing, in which Deville 
has for some time been engaged, viz. the phenomena of dissociation , as he has termed 
the partial decomposition which compound gases experience under the influence of a 
temperature more or less elevated. A very striking result was obtained by the use of an 
apparatus similar to that employed in the experiments just described, but in which a 
brass or silvered tube was substituted for the platinum or iron tube. A rapid flow of 
water was maintained through the metallic tube, so that it was kept quite cool, whilst 
the outer porcelain tube was gradually raised to an intense hea.t as before. On trans¬ 
mitting a current of pure and dry carbonic oxide through the porcelain tube, the lower 
part of the surface of the. cold metallic tube became covered with deposited carbon, 
whilst a portion of the carbonic oxide, by combining with the oxygen previously united 
with this carbon, became converted into carbonic anhydride. Sulphurous anhydride was 
by similar treatment resolved into sulphur and sulphuric anhydride; and even hydro¬ 
chloric acid was partially separated into hydrogen and chlorine. These experiments 
are intimately connected with the attempts made to explain the cause of certain excep¬ 
tions to Ampere’s law, that equal volumes of gases or vapours contain the same number 
of molecules of each. Chemists now generally assume that the molecule, both of simple 
and of compound bodies, forms two volumes of vapour, and consequently that the mo¬ 
lecular weight of any substance corresponds with the number which represents twice its 
density, when referred to the density of hydrogen, if this be taken as unity. But there 
are exceptions to this law: pentachloride of phosphorus, hydrochlorate of ammonia, 
hydriodate of phosphuretted hydrogen, and various other bodies, instead of forming two 
volumes when one molecule of each is converted into vapour, yield four volumes. In 
order to explain these anomalies, Kopp and Cannizzaro suppose that, at the temperature 
